![38
Subscript for K1c denotes that the plane strain fracture toughness for mode I crack
displacement. Brittle materials, for which appreciable plastic deformation is not possible in
front of an advancing crack, have low K1c values and are vulnerable to catastrophic failure.
On the other hand, K1c values are relatively large for ductile materials. Fracture mechanics is
especially useful in predicting catastrophic failure in materials.
The compact tension (CT) and the three point loaded bend specimen have been
standardized by ASTM (Dieter, 1986). After the notch is machined in the specimen, the
sharpest possible crack is produced at the notch root by fatiguing the specimen in a low cycle
high strain mode. The initial crack length a includes both the depth of the notch and the length
of the fatigue crack. Plain strain toughness test is generally carried out in a tensile testing
machine which provides a continous record of load P and relative displacement. A typical
load displacemnet curve for a brittle material is as shown in the figure 3.14 .The curve shows
a complete pop in instabilty where the initial crack movement continously propagates towards
failure.
Fig 3.14: Load displacement plot for a brittle material
The value of PQ determined from load displacement curve is used to calculate a conditional
value of fracture toughness denoted by KQ (Dieter, 1986).
KQ = (PQ S/BW3/2
) [ 2.9 (a/W)1/2
– 4.6(a/W)3/2
+ 21.8(a/W)5/2
– 37.6 (a/W)7/2
+ 38.7 (a/W)9/2
]](https://image.slidesharecdn.com/dff80ee5-ba31-4a1a-94dd-c7a881611830-160908134021/85/Master-Thesis_Ganesh-40-320.jpg)
![68
1 2 3
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
FractureToughness(MPa(m)
1/2
)
Sample position
Dry Condition
Wet Condition
Table 2: Fracture toughness calculations for Elliptio crassidens in dry and wet conditions
The dehydrated specimen, however, showed lower fracture toughness 0.83 MPa√m as
compared to for the hydrated specimen 1.37 MPa√m. This difference is significant as the
conditional fracture toughness (Kq) in hydrated condition is almost twice compared to the
value in dry condition as shown below in the fig 4.30.
Fig 4.30: Calculated values of fracture toughness sample of Elliptio crassidens in wet
and dry condition
Name S (m) a (m) B (m) W (m) Pq (N) a/W
Elliptio (Dry) 0.0200 0.0016 0.00255 0.0051 69.69 0.628
Elliptio (Wet) 0.0200 0.0020 0.00250 0.00485 58.27 0.8
Name
[2.9 *(a/W)1/2
– 4.6*(a/W)3/2
+21.8*(a/W)5/2
–
37.6*(a/W)7/2
+ 38.7*(a/W)9/2
]
(Pq*S)/(B*W3/2
)
MPa√m
Kq
MPa√m
Elliptio (Dry) 4.20 0.199 0.83
Elliptio (Wet) 8.74 0.157 1.37
Name P0 (N) σ0 (MPa) 2.5*( Kq/σ0)2
Elliptio (Dry) 69.69 31.52 0.0017
Elliptio (Wet) 58.27 29.72 0.0053](https://image.slidesharecdn.com/dff80ee5-ba31-4a1a-94dd-c7a881611830-160908134021/85/Master-Thesis_Ganesh-70-320.jpg)
Master Thesis_Ganesh
- 1. A MASTER THESIS REPORT O MECHA ICAL CHARACTERIZATIO OF HYBRID BIOCOMPOSITES Master thesis report submitted to the Department of Chemistry at the Technical University of Munich, the Faculty of Chemistry and Pharmacy of the Ludwigs-Maximilians-University of Munich and the Department of Mathematics and atural Science of the University of Augsburg in the partial fulfillment of Masters Degree in Advanced Materials Science Under the guidance of: Prof. Dr. Wolfgang W. Schmahl Department of Earth and Environmental Sciences Ludwig Maxmilians University of Munich, Munich, Germany Submitted by Ganesh Kumar Tirumalasetty Advanced Materials Science Technical University of Munich, Ludwig Maxmilians University of Munich, University of Augsburg ovember 2007-July 2008
- 2. CO TE TS Acknowledgement 1 Abstract 3 Chapter: 1 – Introduction 4 Chapter: 2 – Literature review 7 2.1 – Importance of nanocomposites 7 2.2 – Shell structures and their importance 9 2.3 – Influence of shell design 11 2.4 – Shell composition and microstructure 12 2.5 – Shell mechanics 14 Chapter: 3 – Experiments and Testing Procedures 17 3.1 – Materials 17 3.2 – Specimen preparation 20 3.3 – Mechanical characterization tests 22 3.4 – Cost effective device for measuring mechanical properties of intricately shaped bio-composites 40 Chapter: 4 – Results and Discussions 46 4.1 – Results 46 4.2 – Discussions 72 Conclusions 77 Engineering Perspective 78 Future Scope 79 References 80
- 3. 1 Acknowledgement I am extremely pleased to present before you my Master thesis report titled Mechanical Characterization of Hybrid Biocomposites. I consider it a privilege to express a few words of gratitude and respect to all those who guided and inspired me in completing this thesis. I am extremely thankful to Prof. Dr. Wolfgang W. Schmahl, Lehrstuhl für Anorganische und Biogene Geomaterialien, Ludwig Maxmilians University of Munich for being kind enough for permitting me to carry out this work and for providing me with necessary facilities for my project. His invaluable guidance and enduring association throughout the course of this internship has encouraged me a lot. I would like to take this opportunity to extend my gratitude to Dr. Erika Griesshaber, Ludwig Maxmilians University of Munich for the SEM work and for giving her valuable suggestions during the entire course of project. My heartfelt thanks are due to Mrs. Enders and Prof. Dr. P. Gille, Ludwig Maxmilians University of Munich for their guidance and keen interest in sample preparation. I would like to take this opportunity to extend my gratitude to Prof. Dr. Steinhauser and Mrs. Christine Hausner Henzel of Hochschule Muenchen for their impact testing equipments, Mr. Thomas Wehlus of University of Augsburg for the Nanoindentation testing, Mr Pondicherry Shanmugham Kartik of University of Ulm for the Microindentation testing, Dr. Stefan Eichhorn and Mr. Tobias Winkler of Technical University of Munich for their three point bending tests and Fracture Toughness measurements and Dr. Enrico Schwabe of Bavarian State Collection of Zoology (ZSM) for the mollusc samples and also for his help in finding the names and literature of the molluscs from New Zealand . My cordial thanks are due to Mr. Casjen Merkel, Ludwig Maxmilians University of Munich and Mr. Srihari Subramanian, Technical university of Munich for their valuable guidance and suggestions course of project.
- 4. 2 On my personal behalf, I would also like to thank Mr. Markus Sieber and his group members from the mechanical workshop and Mr. Max Haeberle and Mr Deblef Koerner from electronic workshop, Ludwig Maxmilians University of Munich for constructing the tensile testing machine. My project work would remain incomplete without acknowledging the moral support of all the research group members of Prof. Dr. Wolfgang W. Schmahl, as the knowledge delivered from them has been a stepping stone and a moral rider for this project. Finally it is said that behind every successful man there is a women and in my case it happens to be my mother. I would like to express my deepest thanks for the support provided to me by my mother Mrs. Jayalakshmi Tirumalsetty and my father Mr. Lava Kumar Tirumalsetty without which my work would not have moved so far.
- 5. 3 Abstract Structural composite materials found in nature like the mollusc shells posses a unique combination of mechanical properties to protect soft tissues from the mechanical aggressions of outside environment. These shells are composed of calcium carbonate crystals interleaved with layers of viscoelastic proteins, having dense, tailored structures that yield excellent mechanical properties. However, mechanical testing of these samples in a natural condition hasn’t been done before owing to the intricate shape of these materials. Studies have been done till date, using micro and nanoindentation in dry conditions. Although there are few studies on elasticity and strength, yet characterizing these materials in wet conditions hasn’t been performed so far. The main basis for entire research is that the molluscs originally live in water and the mechanical properties of them could be influenced by water. Initially a cost effective tensile testing machine was designed and constructed indigenously to serve the purpose of testing these intricately shaped biological composites. The tensile testing equipment is relatively inexpensive, requires little skill to construct and is easy to use and further yields sufficiently accurate data on mechanical properties and its use for studies of shells of different sizes requires only minor modification. For the first time, three-point bending tests were performed to determine the elastic and ultimate properties of two different kinds of mollusc in wet and dry conditions. While dry specimens exhibit brittle behaviour, hydrated specimens exhibited matrix-reinforcement-pullout ductility. The flexural modulus, bending strength, fracture strain and nominal work-to-fracture properties were higher for the hydrated samples rather than the corresponding dehydrated samples. Also impact toughness and fracture toughness measurements were carried out on the shell samples in wet and dry conditions and the samples show relatively higher toughness in wet conditions than their dry counterparts. This implies that it is the structure itself that is mainly responsible for these mechanical properties. The fracture behaviour and morphologies of the fracture surfaces varied significantly with the influence of water on these shell materials. Key words: Molluscs, Wet, Dry, mollusc, three point bending, Impact, Fracture Toughness.
- 6. 4 Chapter-1 Introduction Billions of years of evolution have produced extremely efficient natural materials, which are increasingly becoming a source of inspiration for engineers. Biomimetics the science of imitating nature is a growing multidisciplinary field which is now leading to the fabrication of novel materials with remarkable mechanical properties. These high- performance natural composites are made up of relatively weak components arranged in intricate ways to achieve specific combinations of stiffness, strength and toughness. If one is able to determine the features which control the performance of these materials one could introduce these features into artificially bio-inspired synthetic materials, using innovative techniques such as layer-by-layer assembly or ice-templated crystallization. The most promising approaches, however, are self-assembly and biomineralization because they will enable tight control of structures at the nanoscale (Zhiyong et al., 2003). In this 'bottom-up' fabrication, also inspired from nature, molecular structures and crystals are assembled with a little or no external intervention. The resulting materials will offer new combinations of low weight, stiffness and toughness, with added functionalities such as self-healing. Recent investigations have shown that mollusc shells are natural nanocomposite materials, composed of layers of nanocrystalline inorganic aragonite (CaCO3) surrounded by an organic biopolymer matrix in an arrangement commonly described as a ceramic plywood. Their laminated structure achieves a thousand fold increases in toughness over its constituent materials (Kamat et al., 2000). The current methods for synthesis are not very practical, because the micro architectures and toughening mechanisms have not been explored and understood completely (Kessler et al., 1996). This deficiency opens up this field of nanocomposites for further research and development. Therefore these inexpensive light weight structures have been inspiring material scientists to develop synthetic, biomimetic nanocomposite assemblies that attempt to reproduce nature’s achievements (Sellinger et al., 1998). Investigations in this field can also gain considerable commercial importance owing to the substantial usage of nanocomposites in aerospace and automobile industries and their constant search for composites of light weight, high strength and low cost still remains to be explored.
- 7. 5 Although these materials exhibit extraordinary combinations of mechanical properties, macroscopic mechanical measurements like tensile tests have been rarely performed on these materials due to their intricate shapes. Better understanding of the know-how of bulk properties of these biomaterials in relation to the microstructure can be had by measuring the mechanical properties at micron and nanometer scale (Li and Nardi, 2004). Several studies have been done till date, using microindentation and nanoindentation to estimate the hardness of the sea shells in dry and embedded conditions ( Griesshaber et al., 2006, Perez-Huerta et al., 2007, Merkel et al., 2007). Nevertheless, characterizing these materials in wet conditions hasn’t been performed so far due to the difficulty in characterizing the indentations on the sample surface. In addition, measurements taken in wet conditions can cause damage to the sample surface. Consequently micro and nanoindentations were carried out on dry and embedded samples. Previous studies on bones and teeth signify that testing in dry and embedded condition is not ideal and wet condition would be closer to the in vivo conditions experienced by these materials (Blackburn et al., 1992, Weaver 1996). Biological samples show a significant change in mechanical properties in a fluid environment compared to the usual ambient testing conditions. Hardness and elastic stiffness values usually increase with decreasing water content and the degree of change is considerable (Schöberl and Jaeger 2006, Dall’Ara et al., 2007, Bell et al., 2008). Unlike the other biomaterials, mollusc shells with their hierarchical structure also should show similar behavior in terms of their exposure to the humid environments. Further, testing of these samples in a natural condition hasn’t been done before owing to the difficulty of sample preparation and area determination for standard mechanical testing. Three-point bending tests were performed to determine the elastic and ultimate properties of two different kinds of molluscs in different positions of the shell in wet and dry conditions. Additionally classical toughness measurements like the Charpy Impact toughness and fracture toughness have also been carried out in wet and dry condition to confirm the influence of water on the mechanical properties of these shell materials. In this study, three mollusc shells were tested with nanoindentation, microindentation and three point bending techniques. The aim of this study is to investigate the differences in
- 8. 6 mechanical properties of the shell materials in wet and dry conditions, evaluated by nanoindentation, three point bend tests, impact and fracture toughness testing. Several complex and rather expensive electronic instruments have been designed by scientists until now to measure the strength of general engineering materials like metals, alloys, ceramics, polymers and composites. Generally these instruments are not readily available to biologists studying the mechanical properties of these shells. Further installing tensile testing equipment or testing samples statistically large number of samples is relatively costly. Keeping the above points in view, a tensile testing machine is indigenously built which is relatively inexpensive, requires little skill to construct and is easy to use and further yields sufficiently accurate data on mechanical properties and its use for studies of shells of different sizes requires only minor modification.
- 9. 7 Chapter-2 Literature Survey 2.1- Importance of nanocomposites Nanocomposites of organic and inorganic materials are a fast growing area of research focused mainly on the ability to obtain control of the nanoscale structures through innovative synthetic approaches. The properties of nano-composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics (Nanocomposites, internet, 2008). Fig 2.1: anocomposites and its commercial applicability in the field of automobile, space, ocean, medical, agricultural, automotive, building construction, railway, etc Experimental work has generally shown that virtually all types and classes of nanocomposite materials lead to new and improved properties when compared to their macrocomposite counterparts. This rapidly developing field is generating many exciting new materials with novel properties by not only combining properties from the parent constituents into a single material but also to produce properties which are entirely different from those of parent materials. Therefore, nanocomposites promise new applications in many fields such as mechanically reinforced lightweight components.
- 10. 8 The general class of organic/inorganic nanocomposites may also be of relevance to issues of bio-ceramics and biomineralization in which in-situ growth and polymerization of biopolymer and inorganic matrix is occurring. Finally, lamellar nanocomposites represent an extreme case of a composite in which interface interactions between the two phases are maximized. Since the remarkable properties of conventional composites are mainly due to interface interactions, the materials dealt here could provide good model systems in which such interactions can be studied in detail using conventional bulk sample as opposed to surface techniques. By judiciously engineering the polymer-host interactions, nanocomposites may be produced with a broad range of properties. Inorganic layered materials exist in great variety. They possess well defined, ordered intralamellar space potentially accessible by foreign species. This ability enables them to act as matrices or hosts for polymers, yielding interesting hybrid nano-composite materials. Lamellar nano-composites can be divided into two distinct classes, intercalated and exfoliated. In the former, the polymer chains alternate with the inorganic layers in a fixed compositional ratio and have a well defined number of polymer layers in the intralamellar space. In exfoliated nano-composites the number of polymer chains between the layers is almost continuously variable and the layers stand greater than 100 Å apart. The intercalated nano-composites are also more compound-like because of the fixed polymer/layer ratio, and they are interesting for their electronic and charge transport properties. On the other hand, exfoliated nano-composites are more interesting for their superior mechanical properties. Presently, most of the work is focused on the lamellar class of intercalated organic/inorganic nanocomposites and namely those systems that exhibit electronic properties in at least one of the components. This subclass of nano-composites offers the possibility of obtaining well ordered systems some of which may lead to unusual electrical and mechanical properties. Selected members of this class may be amenable to direct structural characterization by standard crystallographic methods. An important issue in this area is that there is very less information available regarding the structural details, and therefore, any system that is subjected to such analysis is of great interest. Nanocomposites also offer the possibility to combine diverse properties which are impossible within a single material, e.g. flexible mechanical properties and superconducting properties. Another exciting aspect is the possibility of creating heterostructures composed of different kinds of inorganic layers, which could lead to entirely new area of multifuntional materials
- 11. 9 2.2- Shell structures and their importance Natural selection provides a tool by which nature can process, improve, and refine biologically based organisms over millions of years. Scientists can learn from these evolutionary refinements and develop technologies based on natural designs. At present, even the simplest bio-mineralized structures cannot be synthesized in the laboratory without the use of living organisms. Hence studies are being carried out with the intention to contribute to the ongoing development of the next generation of synthetic materials (Sarikaya, 1994, Srinivasan et al., 1991), based on biomimickry. Many natural materials exhibit extraordinary combinations of mechanical properties which are achieved through highly tailored and organized hierarchical microstructures. In particular materials which function as natural body armor such as mollusc shells, possess a structure with important features and properties at a variety of length scales, from the various constituent blocks to the overall integrated and synergistic mechanical model of their complex assemblies. Basic inorganic materials used in nature are, on their own, very weak. However, when combined with proteins, self-organized into highly ordered structures, and refined over long periods, these basic materials make very strong composites, sometimes increasing their strength by orders of magnitude (Kamat et al., 2001). Examples of such biological materials include bone, teeth, sponge spicules, diatoms, and mollusc shells where their nanometric dimension play an important role in their superior mechanical behavior (Vincent, 1991, Weiner, 1997). These complex composites contain both inorganic and organic components in their macro-, micro-, and nanostructures (Baer et al., 1992, Lowenstam, 1989). The complexity of these structures and their ability to self-assemble has drawn considerable attention (Nicolis and Prigogine, 1997, Sarikaya 1994, Whitesides, 2002). An increase in strength due to structure can be seen in other laminates as they form stronger materials from weak base materials, however, the relative strength gain found in these biocomposites remains unparalleled in synthetic materials. By investigating various shells with similar composition, but dissimilar structural organization, one can observe the role of macro-, micro-, and nanostructures in the mechanical response of such biocomposites. Such unique structures have been widely studied, stimulating the creativity of materials scientist for developing new synthetic materials exhibiting high feat of properties.
- 12. 10 One of the most biogenic composite studied is the mollusc shell, especially the nacreous structure. It is composed by platelets of aragonite (calcium carbonate) stacked in a brick-and-mortar fashion. The crystals are glued by a fibrous 0.1–5% organic phase (Srinivasan et al., 1991). Such a layered structure has been on the basis of the high bending strength and toughness of nacre. Fig 2.2: Schematic drawing of the cross-lamellar structure of Stombas gigas. Each layer also consists of first-, second-, and third-order lamella (Lin et al., 2006). It is assumed that these materials possess extraordinary mechanical properties because of extensive delamination at large distances ahead of the crack tip, and by the adhesive properties of the organic matrix. The nacreous organic matrix is complex in nature and is just one of the seven different microarchitectural structures found in molluscs’ shells (Weiner, 1997). The most common is the crossed lamellar structure that may present four hierarchical levels of organization (Lowenstam, 1989). Those various levels may be a way found by nature to reduce the anisotropy that exists at the lowest ordering stages (Nicolis and Prigogine, 1997). The tendency of reducing mechanical anisotropy has been found in other mineralized tissues like the skeletons of sea urchins, lamellar bone or biogenic silica and has been linked to the fact that the material must perform satisfactorily under many different loading conditions (Nicolis and Prigogine, 1997).
- 13. 11 2.3- Influence of shell design (Ketchum, internet, 2008) 1. There are only a few basic structural systems for post and beam structures, but for shell structures, there are thousands, each requiring a unique approach to design. 2. The supports for a shell are more important than the shell. Stiffest path concepts are useful in understanding shell structures. 3. Shell structures are very complex and carry forces by many paths. Shell structures are usually understood as a set of beams, arches and catenaries and can be analyzed by that approach. Fig 2.3 : a) Arch Bridge b) Fuselage of aircraft 4. Shell structures can carry relatively large point loads. For any shell structure, there will be a simple method of analysis that can be used to check the more precise analysis. 5. Shell structures get their strength by shape and not by high strength of materials. A typical example would be that of an egg shell which is easier to compress and break in the horizontal direction but in the vertical direction it takes relatively higher compressive load to break it into fragments. Fig 2.4: Egg shell in compression 6. Shell structures, because of their complexity and unfamiliarity require a large lead time for developing the design. Shell structures are also light weight constructions and find applications in structural engineering deigns like fuselages in aeroplanes, ship and boat hulls and also the roofing’s of buildings a b
- 14. 12 2.4- Shell composition and microstructure (Prezant, 1998) The molluscan shells are comprised of calcium carbonate mineral which is either deposited or embedded within an organic matrix. The organic matrix constitutes to less than 10 % of the shell and is mainly composed of glycoprotiens. Inorganics like aragonite and carbonate constitute towards 90 % of the shell and form various microstuctural layers in the bivalvia. Fig 2.5: a) Fibrous prismatic b) acreous (malacsoc, internet, 2008) The most primitive shell types had prismatic nacreous microstructure. In this shell type, a prismatic layer composed of either calcitic or aragonitic prisms of variable thickness underlies the periostracum. This, in turn is underlain by a nacreous or mother of pearl layer that forms the inner shell surface. The prisms, often polygonal in cross section, are arranged perpendicular to the shell surface and the flat nacreous tablets lie in the plane as the shells surface. Various types of prismatic layers can be discerned including simple, fibrous, composite and spherulitic prismatic layers. A specialized simple prismatic layer, the myostracum, forms the distinct shell structure involved in the muscle attachment. As such myostracum lies underneath most of the muscle attachment areas and is surficially revealed as muscle scars of the shell interior surface. The nacreous layer of aragonite forms the lustrous inner layer of many mollusk shells having the composition similar to that of pearl. Nacre can be deposited either polygonal or round tablets that merge with growth within and upon the organic matrix. In general, nacre grows as • thin flat sheets called sheet nacre • tall pyramidal columns called columnar nacre • Elongated stacked tablets called row stacked nacre. a b
- 15. 13 In many pteriomorphia, a foliated calcite layer is present composing of flat elongated lathes or blades that represent roofing shingles. Most bivalves have shells with some crossed lamellar microstructure. As such there could be variations on the theme of rectangular lamellae rods or blades crossing each other. Small third order lamellae, essentially elongate crystals compose the larger second order lamellae which in turn contain still larger first order lamellae. The first order lamellae, often rectangular in form are deposited parallel to the shell surface. Many pteriomorphs have first order lamellae’s arranged either concentric to or parallel to the shell edge with alternating blocks arranged obliquely to each other. Complex cross lamellar microstructures can have first order lamellae arranged in cones that lie perpendicular to the shell surface. Most hetrodonts have complex cross lamellar layer often found below a foliated calcitic layer. Fig 2.6: a) Simple prismatic b) Crossed lamellae (malacsoc, internet, 2008) Small spherules of aragonite may compose a distinct homogeneous layer found in the protobrachia and some heterodonta. A homogeneous layer sensu stricto is composed of small spherules whereas a granular homogeneous layer is composed of larger granules. Although shell microstructure is conservative for the most part, significant alterations in basic microstructures could take place with the shifts in the environment. a b
- 16. 14 2.5- Shell mechanics The mathematically regular spirals of mollusc shells (Thompson, 1942, Cortie 1992, Cortie 1993) and their relatively higher resistance to fracture (Wang, et al., 2001, Li, et al., 2004) have been of interest since long time. The mechanical strength has resulted from the evolutionary interplay between predation on the species and its need to ensure survival and productiveness (Vermeij, 1993). The fracture toughness of Inorganic CaCO3 is 0.9 MPa√m, which is similar to ordinary glass (Ashby and Jones, 1980). However, natural materials like mollusc shells are vastly more durable and tougher than man made materials like glass. Research has slowly revealed the reasons. Firstly, shells are composite structures of aragonite, other calcareous materials such as calcite, where 95 volume % of the composite is the inorganic material and up to 5% by volume of protein ‘glue’ known as conchiolin. A large component of the toughening derives from the unique aragonitic microstructure of the nacre layer which facilitates crack tip blunting, deflection, closure and bifurcation—concepts well known from the field of fracture mechanics. Moreover, it has been found that the nacreous structure can also undergo inelastic deformation (Wang et al., 2001), which is helpful. Finally, it has recently been claimed that aragonite platelets themselves are nanostructured in such a way as to permit some internal plastic deformation (Li et al., 2004). When a crack travels perpendicular to the layers of bricks, it deflects around the aragonite crystals, which then must pull out as the crack widens (Okumura and de Gennes, 2001) Nacre, the inner lustrous layer of many mollusc seashells is a material that has intrigued scientists for many years. The structure of this material has evolved through millions of years to a level of optimization not currently achieved in engineered composites. Materials scientists have spent many years analyzing the structure of the aragonitic calcium carbonate layers, their crystallography, defect structure and also growth mechanisms (Katti, Katti, 2006). Likewise, much work has also been done on analyzing the nature of the protein rich 20–30 nm layers that separate the aragonitic platelets (Katti, Katti, 2006). The organic matrix has been known to contain aspartic acid rich macromolecules. These macromolecules have been linked to control of crystal nucleation and growth, texture and morphology (Weiner and Addadi, 1997, Mann et al., 1989). The role of the constituents of the organic phase: h-chitin, silk-like proteins and acidic glycoproteins are to control the mineralization of aragonite.
- 17. 15 Recently, cryo-transmission electron microscopy has been used to evaluate the structure of organic layers in bivalve Atrina embedded in vitrified ice. These experiments have indicated that the organic layers are composed of highly ordered and aligned h-chitin fibrils (Kalisman et al., 2001). Over several decades, many spectroscopy techniques have been used to study the crystallographic, molecular and electronic structure of nacre. Fourier transform infrared spectroscopy has been used to analyze the organic and inorganic layers of nacre (Katti, Katti, 2006). Considerable work has also been done on the experimental determination of mechanical properties of nacre. Measurements based on careful experiments were first done by Currey in 1977. Mean values of tensile strength, compressive strength, bending strength and modulus of elasticity were obtained for several species. Currey’s paper from 1977 also gave a stress– strain curve of nacre in tension. This curve indicated that nacre shows a linear (elastic) region until a sharp yield point at about 0.2% strain followed by failure at about 0.6% strain. Further Jackson et al. obtained the mechanical properties of nacre for species Pintada umbricata . They reported a modulus of about 70 GPa and a tensile strength of about 170 GPa. They also reported a work of fracture of 350–1240 J/m2 . Sarikaya et al. 1992, measured fracture strength of red abalone (Haliotis rufescens) to be about 185 MPa and fracture toughness of 8 MPa√m. Recently, we have reported three-point bend tests on samples of nacre from red abalone shells. SEM micrographs were obtained across the cross-section of the samples from the compression to tension faces. Distinctly different microstructures were observed for the compression and tension faces. It is found from micrographs that the interface where the transition appears to occur from compression to tension is about a third from the top of the sample. These biocomposites displays a high fracture toughness which is comparable to those of some high-technology structural ceramics (Jackson et al., 1988, Currey, 1977, Sarikaya et al., 1995). Several toughening mechanisms of the nacre have been examined in the literature and exploited to produce strong materials like (Sarikaya et al., 1995, Cleg, 1999, Rao et al., 1999) 1) crack blunting/branching, 2) micro crack formation, 3) internal stresses
- 18. 16 4) mechanical properties of proteins on dissipating the fracture energy (Smith et al., 1999) Fig 2.7: A model of biocomposites. (a) A schematic diagram of staggered mineral crystals embedded in protein matrix. (b) A simplified model showing the load-carrying structure of the mineral–protein composites. Most of the load is carried by the mineral platelets whereas the protein transfers load through the high shear zones between mineral platelets (Gao et al., 2003). The crossed lamellar microarchitecture of the shell provides for channelled cracking in the outer layers and uncracked structural features that bridge crack surfaces, thereby significantly increasing the work of fracture, and hence the toughness, of the material ( Kamat et al., 2000). The coarsest structures are the inner, middle, and outer layers, which are oriented in either weak or tough orientations with respect to the direction of potential catastrophic crack propagation. Sequential cracking of the weak and strong layers of the shell occur during crack propagation, and it is necessary to partition the energy dissipated during fracture into these two processes. This is done here through quantitative micromechanics modelling (Kamat et al., 2004). These nanocomposites also exhibit a generic mechanical structure in which the nanometer size of mineral particles is selected to ensure optimum strength and maximum tolerance of flaws (Gao et al., 2003).
- 19. 17 Chapter-3 Experimental and Testing Procedures 3.1- Materials This study involves testing and analysis performed on a three shell specimens. The samples are from three different genus of mollusc called Dosina anus, Gari strangeri and Elliptio. Dosina anus and Gari strangeri was collected in Paihia, New Zealand. Elliptio is a fresh water mollusc collected from the Ohio River, Illinois USA. 3.1.1- Dosina anus (Powell, 1979) Fig 3.1: Dosina anus from ew Zealand. Description: These shells are relatively large, solid and moderately inflated and winged posteriorly as shown in the fig 3.1. The ridges are flat-topped, fine and concentric. There are number of these ridges which are approx 19-20 per cm and there is no lamellose at the dorsal margins. Pallial sinus is broadly V shaped and its apex is slightly beyond the middle of the shell. They are mostly white in colour with some having a light yellowish brown colour externally. Habitat These shells are found in Northern Islands and Cook Straight in shallow waters especially on the coastal beaches.
- 20. 18 3.1.2- Gari strangeri (Powell, 1979) Fig 3.2: Gari stangari from ew Zealand. Description: These shells are elongated in shape with a distict posterior flexure. They are slightly rostrate with the lower edge being slightly pointed out. They have a sculpture of closely spaced sharp and crisp concentric lamellae of about 30 per cm distance. There colour ranges from cream to buff with a slight iridescence as shown in the fig 3.2. The interior of the shell has colour ranging from cream to chrome yellow. Habiatat These shells are mainly found on the north and south islands of Australia and New Zealand. They are commonly seen in shallow water off ocean beaches.
- 21. 19 3.1.3- Elliptio Crassidens (Mussel Manual, internet, 2008) Elliptio is a fresh water mollusc locally abundant in some parts of the Ohio, Illinois and Indiana. Because of its shape, it is commonly referred to as Elephants ear, Mule's ear or blue ham. Fig 3.3: Elliptio crassidens from Ohio River, Pulaski County, Illinois. Description The shell in general is heavy, solid, and triangular with dark brown to black periostracum. The Anterior end of the shell is rounded and the posterior end is pointed with dorsal margin having a slight curvature (fig 3.3). In general younger shells have a curved ventral margin whereas in older shells it is mostly straight. Umbos are low and usually not elevated above the hinge line. The Posterior ridge is prominent and is angled sharply and the surface of the shell is mostly smooth. Periostracum is reddish brown with faint green rays in small shells, becoming dark brown towards the left valve. The lateral teeth are short, rough, and straight and the beak cavity is very shallow. The colour of nacre usually varies from purple to pink or white. Habitat Elliptio shells are mainly found in large rivers in mud, sand, or fine gravel. Present Status These shells are relatively widespread in other parts of the world but are a rarity in the Midwest. They are locally abundant in some parts of the Ohio and White rivers of Illinois and Indiana. Although they are available now, there is a growing concern about the dangers of extinction in Missouri, Ohio, Wisconsin and Illinois.
- 22. 20 3.2- Specimen Preparation anoindentation sample preparation Samples were prepared by cutting macroscopic fragments of 5 mm thick and 25 mm in length with a diamond saw. Further the samples were polished with increasingly fine sand paper number up to 4000 grit along the transverse direction. Thereafter the surface was polished by means of a napped cloth impregnated with 1 µm diamond paste. A scratch was created on the sample surface from one end of the sample to the other end. The intersection of edge of the scratch and the outer edge of the sample is taken as the reference point and the indentations are taken at a distance of 1500 µm in Y direction and 1000 µm in X direction. The distance between the indents is fixed at 15 µm. Rewetting the samples was performed by placing the samples for 60 hrs in tap water. To avoid the scatter in the hardness for dry and wet conditions, the indentations in wet conditions were taken in the close proximity (30 µm) of the indentations in dry condition. Fig 3.4: Sample preparation for three point bending test a) Wire saw used for cutting the samples b) sample cutting and numbering Specimen Preparation for Three point bending test The shell specimens were initially cut into small fragments using a High-speed water-cooled diamond saw or a wire saw along the longitudinal1 direction. All the four specimen surfaces except the top and the bottom faces were ground and polished to 2.5 mm breadth and 25 mm length. The polished samples are numbered in ascending order from anterior end to posterior end (i.e from left to right) so that one could remember the positions of the samples easily as shown in the fig 3.4 b. The actual lengths of the samples are 25 mm but the guage length (i.e length between the supports in three point bending test) is 15 mm. 1 Note: longitudinal direction here means that the cutting is done along the direction of layers and transverse direction here refers to the cutting being done along the direction perpendicular to the direction of layers of the shell. a b Anterior end Posterior end
- 23. 21 Specimen Preparation for impact toughness tests For classical impact tests one requires samples which are relatively flat and long in order to place the samples in between the grips of the impact testing machine. Consequently, Elliptio crassidens was used in impact toughness tests. The shell specimens were initially cut into small fragments using a wire saw along the longitudinal direction. The samples are then grinded and polished to the dimensions of 6*4*50 mm3 (i.e. 6 mm breadth 4 mm height and 50 mm length). All the samples are finely polished to 10 µm surface finish, so that the crack does not initiate from the surface of the sample. In general for impact toughness measurements a notch is introduced into the sample, but in the case of our testing since most of the tests are for comparative purpose, the samples were tested without creating a notch. Fig 3.5: Sample preparation for Fracture toughness tests Specimen Preparation for Fracture toughness tests In the case of fracture toughness tests one requires samples which are relatively flat and thick in order to test the samples in between the three point bending supports of the tensile testing machine. Thus, Elliptio crassidens was used in impact toughness tests. The shell specimens were initially cut into small fragments using a wire saw along the longitudinal direction. The samples are then grinded and polished to the dimensions of 2.5*5*35 mm3 (i.e. 2.5 mm breadth 5 mm height and 35 mm length2 ). All the samples are finely polished to 1 µm surface finish by polishing it with a diamond paste, so that the crack does not initiate from the surface of the sample. A notch is introduced into the sample by means of a High speed cutting saw. At the tip of the notch, a fatigue crack was initiated by tapping a single edged razor blade into the notch with a hammer while the specimen was constrained in a bench vise to avoid the crack propagation. The schematic diagram of fracture toughness samples and their dimensions are shown in fig 3.5 2 Note: Although the sample has the length of 35mm the guage length here was 20 mm.
- 24. 22 3.3- Mechanical Characterization tests 3.3.1- anoindentation Nanoindentation is the method of measuring hardness and other micromechanical properties of metals and materials and Nanoindenter is a tool which has an appreciable contribution in enabling us in the understanding of material properties at nano length scales. The device is used to measure mechanical properties like Elastic Modulus, hardness, toughness, fatigue, etc. of materials, by means of indentation and scratch tests. The basic setup of the Nanoindenter consists of an indenter tip usually made of diamond formed into a sharp pyramidal shape like the three sided Berkovich indenter, an arrangement of sensors and actuators to apply and measure the indenter load and displacement. In the advanced testing equipment, the nanoindenter is integrated with high resolution microscopes for qualitative analysis as well as electrical testing units for measuring micro electrical properties of materials. There are different kinds of setups even based on the testing methods viz. direct testing and differential testing with the latter being quicker in measuring apart from the fact that it avoids the need of a lot of correction factors (Fischer-Cripps, 2004). Working of a anoindenter Basically, indentation is made by applying a small load (less than 100 mN, and using Hysitron instruments, we can get to loads in the range of micro newtons (µN)) with a diamond indenter of a known geometry and the change in displacement as a function of load as the indenter is pressed into the specimen surface is recorded. A schematic of the load-displacement curve is given below in Fig.3.6. Once a nanoindentation machine has collected load-displacement data, a number of different analyses can be used to determine the mechanical properties of the sample from the data. Fig 3.6: Schematic of a typical load-displacement curve
- 25. 23 Nanoindentation tests were performed using NanoTest 600 testing system. This system records the dynamic load and also displacement of the indenter with a Berkovich three sided pyramid with a force resolution of 100 nN and displacement resolution of 0.06 nm. A significant advantage of using the "depth sensing indentation" approach is that one can obtain much more information than just hardness. However the results obtained depend on the analysis model chosen and can be very sensitive to the details of the analysis. The slope of the curve, dP / dh, upon unloading is indicative of the stiffness S of the contact. This value generally includes a contribution from both the material being tested and the response of the test device itself. The stiffness of the contact can be used to calculate the reduced modulus of elasticity Er as where A(hc) is the area of the indentation at the contact depth hc (the depth of the residual indentation), and β is a geometrical constant on the order of unity. The reduced modulus Er is related to the modulus of elasticity Es of the test specimen through the following relationship from contact mechanics. Here, the subscript i indicates a property of the indenter material and ν is Poisson's ratio. For a diamond indentor tip, Ei is 1140 GPa and νi is 0.07. Poisson’s ratio varies between 0 and 0.5 for most materials (though it can be negative) and is typically around 0.3. Nanoindentation hardness is calculated using the indented peak load (Pmax) and projected area of contact (A) of the indentation. From the load–displacement curve, hardness can be obtained at the peak load as H =Pmax /A The displacements and loads can be calculated in various ways, for instance, the former using a differential capacitor gauge or an LVDT (Linear Variable Differential Transducer), and some advanced equipment use Lasers to measure the displacement (similar to that in AFM), S = dP / dh
- 26. 24 and loads on the other hand, can be controlled using electromagnetic or controlled using electromagnetic or electrostatic attenuation or actuation using springs. Modes of operation of the anoindenter There are many modes of operation of the Nanoindenter based on the property of interest we are looking for, for instance: • Basic Hardness Mode • Fatigue Test Mode • Creep Mode • CSM Mode (for Fracture toughness) • Scratch Test Mode and so on and so forth (Fischer-Cripps, 2004)… Basic Setup of anoindenter Given under in Fig 3.7 is a photograph of a Nanoindenter. Fig 3.7: Photograph of a nanoindenter ( anoTest 600) Models in anoindentation Models give us an idea and an understanding of the theory and assumptions behind the working and functioning of the system. Nanoindentation too is based on models, and given under is a brief description of the most basic model.
- 27. 25 Oliver and Pharr Model (Oliver and Pharr, 1992) Oliver and Pharr (O&P) made a critical improvement to the method proposed by Doerner and Nix (D&N). Sneddon's (Sneddon, 1965) contact solution predicts that the unloading data for an elastic contact for many simple indenter geometries (sphere, cone, flat punch and paraboloids of revolution) follows a power law that can be written as follows P = α hm In this equation P is the indenter load, h is the elastic displacement of the indenter and α and m are constants. Oliver and Pharr apply this formulation to determine the contact area at maximum load as it is valid even if the contact area changes during unloading. To do this, they derive the following relationship for the contact depth from Sneddon's solutions hc = hend – θ*(Pmax/ S) where q = 0.72, 0.75 and 1, for cone-, sphere- and flat-punch-geometry respectively. The procedure for O&P analysis is to then fit a power law function to the unloading segment. This yields the contact stiffness as slope of this function at maximum load. This slope in addition to the appropriate value of q is used in order to determine the actual contact depth so that it is finally possible to derive the indentation modulus (2) and the indentation hardness (3) from the measurement. Figure 3.8 shows a schematic sketch of such an analysis. Fig 3.8: a) Schematic plot of the Oliver and Pharr model b) AFM image of the nanoindent made by a berkowich indenter a b
- 28. 26 Both values, indentation modulus as well as indentation hardness, depend strongly on the area function A (hc) and the accuracy with which it is determined. It is possible to improve the O&P model further by introducing a correction factor accounting for the radial surface displacement in Sneddon's solutions (Hay et al.). In case of materials which show a pile-up or sink-in behaviour it is furthermore possible to supplement the model with a parameter accounting for the deviation from the ideal or calibrated area function with respect to the real one (McElhaney et al.). There are other models describing Dynamic contact method, CSM method, Fatigue test method and numerous others which are a special case of the above methods. Applications Nanoindentation is far and wide being used in the fields of Material Science as well as Medicine. Its applications range from measuring of basic hardness of materials to measuring the micromechanical properties of Cobra fangs, to synthesis of nanocrystalline phases in a glassy material. Biomedical Applications Applications include measurement of mechanical properties of microstructural features in biomaterials and mapping of mechanical properties in tissues with complex microstructures. Nanoindentation can serve as a complementary characterization tool to other techniques that assess composition or structure with high spatial resolution, such as Raman spectroscopy, magnetic resonance imaging, micro-computed tomography, histology, or infrared spectroscopy. In this regard, nanoindentation has played a pivotal role in defining structure- property relationships for tissues and their constituents. Limitations This tool has its own limitations, like the dependence of the results on the tip geometry and the efficiency of the capacitors and the attenuators as well as the surface of the specimen to be indented, which has to be smooth enough.
- 29. 27 3.3.2-Microhardness testing Some of the biological samples require the determination of hardness over small areas. Determination of hardness of individual constituents of a microstructure or checking the hardness of a delicate sample in such samples offers a great challenge for materials scientists. For this reason a Vickers hardness test with a square base diamond pyramid is used as a standard for measuring the hardness of such samples. The indenter has an included angle of 1360 between the opposite faces of the pyramid. This angle was chosen because it approximates the most desirable ratio of the indentation diameter to the ball diameter (Dieter, 1986). Applied loads are much smaller than for Rockwell and Brinell, ranging between 1 and 1000 g. The resulting impression is observed under a microscope and measured. This measurement is then converted into a hardness number. Careful specimen surface preparation like grinding and polishing are necessary to ensure a well-defined indentation that may be accurately measured. The Vickers hardness numbers are designated by HV. In general, Vickers hardness testing is also referred to as microhardness testing on the basis of load and indenter size. Both are well suited for measuring the hardness of small, selected specimen regions (Callister, 1999). Microhardness tests were performed on the same samples using a Buhler microhardness tester attached to a light microscope equipped with a video imaging system as shown in fig 3.9. Fig 3.9: Photograph of vickers microhardness testing equipment
- 30. 28 Indents were produced using a Vickers indenter by applying a load of 10 ponds for a period of 10 sec. Under these conditions the indents produced had diagonals between 15 and 30 µm long. Fig 3.10: Image of a vickers microindent The diagonal length was measured within 30 sec after the indentation, using a Buhler hardness measuring digital eyepiece. The microhardness values were calculated using the following equation (Dieter, 1986): HV= 1.854 F/d2 Where HV stands for Vickers Hardness and is expressed in kg/mm2 , F is the applied force in Newtons, and d the mean value of the diagonals in mm. The indentations were made on each of the surfaces starting from one end of the sample to the other end.
- 31. 29 3.3.3- Three point bending test A materials response to the three major forms of stress i.e. Tension, compression and shear could be measured on a Universal testing machine commonly called as tensile testing machine. The engineering tensile testing generally provides basic design information on the strength of materials and is widely used as an acceptance test for the specification of materials. The stress–strain behaviour of hard and brittle materials like shells is not usually ascertained by a tensile test for three reasons (Callister, 1999). • Firstly, it is difficult to prepare and test specimens having the required geometry. • Secondly, it is difficult to grip brittle materials without fracturing them • Thirdly, brittle materials fail after about 0.1% strain, which necessitates that tensile specimens be perfectly aligned in order to avoid the presence of bending stresses, which are not easily calculated. Therefore, a more suitable transverse bending test is most frequently employed, in which a rod specimen having either a circular or rectangular cross section is bent until fracture using a three- or four-point loading technique. The three-point loading scheme is illustrated in Figure. At the point of loading, the top surface of the specimen is placed in a state of compression, whereas the bottom surface is in tension. Stress is computed from the specimen thickness, the bending moment, and the moment of inertia of the cross section; these parameters are noted in Figure for rectangular and circular cross sections. Fig 3.11: A three-point loading scheme for measuring the stress–strain behavior and flexural strength of brittle materials
- 32. 30 The maximum tensile stress (as determined using these stress expressions) exists at the bottom specimen surface directly below the point of load application. Since the tensile strengths of ceramics are about one-tenth of their compressive strengths, and since fracture occurs on the tensile specimen face, the flexure test is a reasonable substitute for the tensile test. The stress at fracture using this flexure test is known as the flexural strength, modulus of rupture, fracture strength, or the bend strength, an important mechanical parameter for brittle ceramics. For a rectangular cross section, the flexural strength Where P is the load at fracture, L is the distance between support points, b is the breadth and d is the depth of the sample (Callister, 1999). For a circular cross section of radius R, the flexural strength is calculated by Since, during bending, a specimen is subjected to both compressive and tensile stresses, the magnitude of its flexural strength is greater than the tensile fracture strength. Furthermore, P will depend on specimen size; with increasing specimen volume (under stress) there is an increase in the probability of the existence of a crack-producing flaw and, consequently, a decrease in flexural strength. Table 3.1 shows the expressions for computing stress for rectangular and circular cross sections (Callister, 1999). σ = Mc/I M = maximum bending moment c = distance from center of specimen to outer fibers I = moment of inertia of cross section P = applied load Table 3.1 M c I σ Rectangular FL/4 d/2 bd3 /12 3PL/2bd2 Circular FL/4 R ∏R4 /4 PL/∏R3 ** Note: ∏ = Pi = 22/7
- 33. 31 The three point bending test was used to provide the values for the modulus of elasticity in bending EB, flexural stress σf, flexural strain εf and the flexural stress-strain response of the material. For this study the three point bend tests were carried out on two different equipments. The preliminary tests were carried out on Zwick tensile testing machine with a loading rate of 2 mm/sec and the second set of measurements were carried out on indigenously constructed tensile testing equipment at a loading rate of 1 µm/ sec. The gauge length of 15 mm was used for testing the samples in both the testing equipments. Fig 3.12: Photograph of three point bending testing equipment For each load- displacement curve as shown below, flexural stress, flexural strain, and bending modulus can be calculated using the formulas. σf = Stress at the midpoint in the material, (MPa) εf = Strain in the material, (%) Eb = Modulus of elasticity in bending, (MPa) P = load at a given point on the load deflection curve, (N) L = Support span, (mm) b = Width of test beam, (mm) d = Depth of tested beam, (mm) D = maximum deflection of the center of the beam, (mm) m = Slope of the tangent to the initial straight-line portion of the load deflection curve, (N/mm)
- 34. 32 From a load displacement curve one can obtain the stress-strain curve in three point bending by inserting the values of load and displacement in the above equations. Since, three point bending is a pure state of bending; maximum load is concentrated always at the center. Hence for brittle materials like shells it is sufficient to measure the dimensions of the fractured surface by vernier callipers to calculate the values of stress and strain. In addition one can also measure the mechanical properties as shown in the fig 3.14. Fig 3.13: Load displacement plot of ductile material Fig 3.14: Plot showing stress- strain behavior of ductile material and the mechanical properties that are obtained through this diagram
- 35. 33 Measures of yielding (Dieter, 1986) The stress at which the plastic deformation begins depends on the sensitivity of the strain measurements. Various criteria for the initiation of yielding are used depending on the sensitivity of the strain measurements and the intended use of data. True elastic limit is calculated based on microstrain measurements at strains in the orders of 2*10-6 . This elastic limit is a very low value and is related to the motion of a few hundreds of dislocations. Proportional limit is calculated as the highest stress at which stress is directly proportional to strain. It is obtained by observing the deviation from the straight line portion of the stress- strain curve. Elastic limit is the greatest stress the material can withstand without permanent strain and regains its original shape on the complete release of load. With the sensitivity of strain usually employed (10-4 ), the elastic limit is greater than the proportional limit. Exact determination of elastic limit requires a tedious loading-unloading test procedure. Modulus of elasticity in bending The degree to which a structure deforms or strains depends on the magnitude of an imposed stress. For most materials that are stressed, at relatively low levels, stress and strain are directly proportional to each other through the relationship σ = E € This is known as Hooke’s law, and the constant of proportionality E (GPa or psi) is the modulus of elasticity, or Young’s modulus. Yield strength It is the stress required to produce a small amount of plastic deformation inside the material. It is obtained by the stress corresponding to the intersection of the stress- strain curve and a line parallel to the elastic part of the curve drawn from a strain of 0.2 or 0.1 percent. Tensile Strength in bending After yielding, the stress necessary to continue plastic deformation in materials increases to a maximum point and then decreases to the eventual fracture. The tensile strength (MPa or psi) is the stress at the maximum on the engineering stress–strain curve. This corresponds to the
- 36. 34 maximum stress that can be sustained by a structure in bending. If this stress is applied and maintained, fracture will result. Fracture Strength in bending The Fracture strength (MPa or psi) is the stress at the fracture on the engineering stress–strain curve. In the case of brittle materials like shells this value mostly corresponds to the tensile stress as there is very less of plastic deformation inside a brittle material. Ductility Ductility is another important mechanical property. It is a measure of the degree of plastic deformation that has been sustained at fracture. A material that experiences very little or no plastic deformation upon fracture is termed brittle. Resilience Resilience is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. The associated property is the modulus of resilience, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding. Computationally, the modulus of resilience for a specimen subjected to a bending test is just the area under the engineering stress–strain curve taken up to the yield strength Toughness Toughness is a measure of the ability of a material to absorb energy up to fracture. Specimen geometry as well as the manner of load application is important in toughness determinations. For dynamic (high strain rate) loading conditions and when a notch (or point of stress concentration) is present, notch toughness is assessed by using an impact test. Furthermore, fracture toughness is a property indicative of a material’s resistance to fracture when a crack is present. For the static (low strain rate) situation, toughness may be ascertained from the results of a tensile stress–strain test. It is the area under the stress–strain curve up to the point of fracture. The units for toughness are the same as for resilience (i.e., energy per unit volume of material). For a material to be tough it must display both strength and ductility and that’s the reason behind the ductile materials which are relatively tougher than their brittle counterparts.
- 37. 35 3.3.4- Impact Testing Prior to the advent of fracture mechanics as a scientific discipline, impact testing techniques were established so as to ascertain the fracture characteristics of materials. It was realized that the results of laboratory tensile tests could not be extrapolated to predict fracture behavior. For example, under some circumstances normally ductile metals fracture abruptly and with very little plastic deformation. Impact test conditions were chosen to represent those most severe relative to the potential for fracture, namely (Dieter, 1986), (1) deformation at a relatively low temperature, (2) high strain rate (i.e., rate of deformation), and (3) triaxial stress state (which may be introduced by the presence of a notch). Fig 3.15: Schematic of loading procedure and the principle for Charpy V-notch impact testing Impact Testing Techniques (Callister, 1999). Two standardized tests, Charpy and Izod, were designed and are still used to measure the impact energy, sometimes also termed notch toughness. The Charpy V-notch (CVN) technique is most commonly used in the United States. For both Charpy and Izod, the specimen is in the shape of a bar of square cross section, into which a V-notch is machined. In case of our Biocomposites the specimen is of flat rectangular crossection without the
- 38. 36 introduction of a notch. The load is applied as an impact blow from a weighted pendulum hammer that is released from a cocked position at a fixed height h. The specimen is positioned at the base as shown in fig 3.15 . Upon release, a knife edge mounted on the pendulum strikes and fractures the specimen at the notch, which acts as a point of stress concentration for this high velocity impact blow. The pendulum continues its swing, rising to a maximum height h*, which is lower than h. The energy absorption, computed from the difference between h and h*, is a measure of the impact energy. The primary difference between the Charpy and Izod techniques lies in the manner of specimen support, as illustrated in Figure 3.15. Furthermore, these are termed impact tests in light of the manner of load application. Variables including specimen size and shape as well as notch configuration and depth influence the test results. Both plane strain fracture toughness and these impact tests determine the fracture properties of materials. The former are quantitative in nature, in that a specific property of the material is determined (i.e., K1c ). The results of the impact tests, on the other hand, are more qualitative and are of little use for design purposes. Impact energies are of interest mainly in a relative sense and for making comparisons—absolute values are of little significance. Plane strain fracture toughness tests are not as simple to perform as impact tests. Furthermore, equipment and specimens are more expensive. Fig 3.12: Photograph of Zwick 5102 impact testing equipment Zwick 5102 impact tester was employed in this study to investigate the impact behavior of these biocomposites. The compact tester is designed for determination of the impact energy up to 5 Joules and works according to Charpy testing standards.
- 39. 37 3.3.5-Fracture Toughness From fracture mechanical principles, an expression has been developed that relates this critical stress for crack propagation (σ) to crack length (a) as ( William D. Callister ) Kc =Y* σ* √ (∏*a) In this expression Kc is the fracture toughness, a property that is a measure of a material’s resistance to brittle fracture when a crack is present. Kc has the unusual units of MPa√m or psi√in. (alternatively ksi√in.). Furthermore, Y is a dimensionless parameter or function that depends on both crack and specimen sizes and geometries, as well as the manner of load application. Relative to this Y parameter, for planar specimens containing cracks that are much shorter than the specimen width, Y has a value of approximately unity. Mathematical expressions for Y have been determined for a variety of crack-specimen geometries and these expressions are often relatively complex. For relatively thin specimens, the value of Kc will depend on specimen thickness. Fig 3.13: Sample preparation and loading in fracture toughness testing However, when specimen thickness is much greater than the crack dimensions, Kc becomes independent of thickness; under these conditions a condition of plane strain exists. By plane strain we mean that when a load operates on a crack in the manner represented in Figure 3.13, there is no strain component perpendicular to the front and back faces. The Kc value for this thick-specimen situation is known as the plane strain fracture toughness K1c and it is also defined by ( William D. Callister ) K1c=Y* σ* √ (∏*a)
- 40. 38 Subscript for K1c denotes that the plane strain fracture toughness for mode I crack displacement. Brittle materials, for which appreciable plastic deformation is not possible in front of an advancing crack, have low K1c values and are vulnerable to catastrophic failure. On the other hand, K1c values are relatively large for ductile materials. Fracture mechanics is especially useful in predicting catastrophic failure in materials. The compact tension (CT) and the three point loaded bend specimen have been standardized by ASTM (Dieter, 1986). After the notch is machined in the specimen, the sharpest possible crack is produced at the notch root by fatiguing the specimen in a low cycle high strain mode. The initial crack length a includes both the depth of the notch and the length of the fatigue crack. Plain strain toughness test is generally carried out in a tensile testing machine which provides a continous record of load P and relative displacement. A typical load displacemnet curve for a brittle material is as shown in the figure 3.14 .The curve shows a complete pop in instabilty where the initial crack movement continously propagates towards failure. Fig 3.14: Load displacement plot for a brittle material The value of PQ determined from load displacement curve is used to calculate a conditional value of fracture toughness denoted by KQ (Dieter, 1986). KQ = (PQ S/BW3/2 ) [ 2.9 (a/W)1/2 – 4.6(a/W)3/2 + 21.8(a/W)5/2 – 37.6 (a/W)7/2 + 38.7 (a/W)9/2 ]
- 41. 39 The crack length (a) used is measured after the fracture with the help of vernier callipers. The factor 2.5(KQ /σ0)2 is calculated and if this factor is less than both the thickness and crack length of the specimen, then KQ is equal to K1c. Else a thicker specimen is necessary to determine K1c. 3.3.6- Scanning Electron Microscopy Impacted fracture surfaces, three point bending fractured samples in wet and dry conditions and fracture toughness samples in wet and dry conditions were examined using scanning electron microscope. Samples were cut from the fractured samples and then they were cleaned dried and sputter coated with thinnest possible carbon coating for SEM study. Fig 3.15: Scanning electron microscope Scanning electron micrographs were obtained using LEO Gemini 1530 SEM equipped with HKL technology. SEM images were generated using an accelerating voltage of 20 KV and beam current of 3.0 nA.
- 42. 40 3.4-Cost effective Device for measuring mechanical properties of intricately shaped Bio- Composites The maximum load which a shell can withstand before it breaks depends generally on the various physical properties of shells such as shell thickness, density, porosity, shape. Tests of shell strength include • Compression resistance (i.e the strength required to crush the shell between a fixed and a moving surface ) • Puncture resistance (i.e. the force required to penetrate a shell with a small punch like the hardness) • Impact resistance (i.e shells resistance to force generated by a sudden impact) Description of the tensile testing equipment Several complex and rather expensive electronic instruments have been designed by scientists until now to measure the strength of general engineering materials like metals, alloys, ceramics, polymers and composites. Generally these instruments are not readily available to biologists studying the mechanical properties of these shells. Further installing tensile testing equipment or testing samples statistically large number of samples is relatively costly. For this reason a simple tensile testing equipment was designed that will yield accurate data on mechanical properties of these shells. The instrument consists of the following components. 1. Stepper motor 2. Load sensor 3. Displacement Sensor 4. Adjustable crosshead This tensile testing machine is displacement controlled in which the operator adjusts the displacement and the load adjusts itself to the displacement. This machine represents a screw driven machine in which the crosshead moves at a predetermined velocity. In the case of this tensile testing equipment the strain rate of the equipment is fixed at 1 µm/s. A constant crosshead velocity testing machine applies a constant total strain rate that is the sum of • elastic strain rate in the specimen • plastic stain rate in the specimen, and • the stain rate resulting from the elasticity of the testing machine
- 43. 41 Fig 3.16: Tensile testing equipment The heavy Iron crosshead and its supports of the machine are cut from a wissenberg camera and are used in the construction of this equipment, as it allows for a slow and smooth increase in pressure as the crosshead is raised gradually. The crosshead of the machine moves between two steel rods of about 12 mm in diameter and is driven over a circular screw of 15 mm in diameter. A control knob along with a butterfly nut is also provided which allows for both coarse and fine adjustments in height. A stepper motor is attached to the moving crosshead of the machine and the electronics of the motor are constructed such that the crosshead moves at a strain rate of 1µm/sec. The electronic board is also constructed so that the cross head rotates in both in clock wise and anticlockwise direction. This allows the user to test the samples both in tension as well as compression. For high degree of accuracy in measurements a load sensor of 5 KN from Bruster was used in the construction of this equipment. Also a displacement sensor from Mitutyo which has accuracy of measuring 1µm change was chosen as shown in the fig 3.16. Since the equipment works in tension and Displacement Sensor Load Sensor Stepper motor Moving crosshead Stationary base Control knob Butterfly nut
- 44. 42 compression, different types of mechanical tests can be carried out on this equipment by changing the grips/ supports like (Fig 3.17 a , 3.17 b) 1. Tensile testing 3. Compression testing 2. Three Point bending 4. Four point bending This tensile testing equipment is not only used for testing the samples but also to check the crack propagation and fracture in these materials. As such the equipment can also be placed in the horizontal direction under a light microscope to observe the fracture. Fig 3.17: Tensile testing machine with a) three point bending grips and b) tensile testing grips Since in brittle materials the crack propagation rates are faster the stepper motor is also adjusted so that the cross head of the equipment moves at a strain rate of 1µm/sec. The equipment also has a flexible base plate where one can insert the sample holder of their choice to do the testing of these biological samples. Operating the tensile testing equipment The movable crosshead is lowered by turning the coarse control knob of the stand. As a result, the crosshead is raised well above the shell support leaving enough space for inserting a shell. A shell to be tested is placed in between the supports of the machine. The a b
- 45. 43 crosshead has to be lowered now and the supports should be adjusted such that the supports just touch the shell surface. It should however be ensured that the supports do not exert any measurable pressure on the shell by checking the reading of the load sensor. The height of the movable crosshead is now fixed in position by a butterfly nut provided at the top of control knob. The shell specimen is carefully positioned so that the sample is placed exactly at the center of the supports. The butterfly nut is placed and the equipment is switched on. The stepper motor now starts to rotate the crosshead slowly at a rate of 1µm/sec. The displacement sensor has one of its ends fixed with the moving crosshead of the machine and the free end is touching the base plate. If either displacement or the load is not showing zero then one can also manually adjust both the load and displacement to zero by pressing the zero button on the displacement sensor. Once the shell is in position the movement of the crosshead introduces the pressure on the shell which is transmitted through the support. The load sensor which is fixed to the support reads the pressure measurements manually. The displacement sensor which is also fixed to the movable crosshead shows the displacement. Following the fracture of the shell, the system should be switched off and the crosshead has to be manually raised up to remove the fractured pieces of the sample. The supports must be cleaned and the next set samples could be tested in a similar procedure. As an initial step towards the automation and efficient data collection, both the load sensor and the displacement sensors are connected externally to a computer. The individual load and displacement measurements are read and saved into two different excel files inside the computer. The obtained values of load and displacement are put into one excel sheet and they are plotted to observe the load displacement behavior of the material. From the obtained values of load and displacement one can determine the stress- strain behavior by measuring the sample dimensions and by substituting them in the equations of stress and strain. With the help of stress strain graph one can determine various mechanical properties like E- modulus, yield strength, tensile strength, ductility, resilience and toughness This instrument can be used with minor modifications to study the fracture propagation in shells. This type of in-situ observation can be observed by mounting the testing equipment in the horizontal position under a light microscope. If one requires the testing of
- 46. 44 0.0 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 Load(N) Displacement (mm) Glass Linear fit of Data for Glass 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 50 100 150 200 250 Load(N) Displacement (mm) Polystyrene 1 Polystyrene 2 shell as a whole without cutting into smaller fragments then one could load the whole shell upon the three supports and could examine the mechanical properties in four point bending. . Fig 3.18: Load- Displacement behavior of glass in tensile testing machine in three point bending. Fig 3.19: Load- Displacement behavior of polystyrene in tensile testing machine in three point bending.
- 47. 45 To check the effectiveness of our equipment we have calibrated by using glass as the standard material. Since the shell specimens are mostly brittle a material which is having almost similar characteristics like glass was chosen as a standard for calibration of the equipment. The load-displacement plot for glass shows almost straight line behavior. In order to verify the efficiency of the whole testing a theoretical linear fit of data was projected on the experimental data obtained from the equipment. The plot in the fig 3.18 shows that the linear data fit almost matches with that of the experimental fit proving that the equipment works fine with brittle materials like shells. For checking the consistency of the results produced by the equipment two polystyrene samples of similar dimensions were tested in three point bending. From the results shown in the fig 3.19, it is clear that the results obtained from the equipment are consistent and very much reliable.
- 48. 46 2.0 2.5 3.0 3.5 0 20 40 60 80 100 E-Modulus(GPa) Hardness (GPa) Dry Condition Wet Condition Chapter-4 Results and Discussions 4.1 Results Illustrative indentation plots for Shell samples of Gari stangari and Elliptio crassidens in dry and wet are shown in figure 4.1 and figure 4.2. Significant differences were observed in mechanical properties during the hold periods on both the wet samples. Fig 4.1: Variation of E- modulus and hardness in dry and wet conditions for the Gari stangari. The appreciable changes in the mechanical properties of these samples according to the position of the indent indicate that these shells have anisotropic behaviour. Hardness and elastic modulus were determined from unloading curves using the procedure of Oliver and Pharr. Although the reliability of absolute values has been questionable due to the accuracy in determining the contact depth (and hence area) on anisotropic materials, the unloading slope approach should be sufficient to determine the relative changes which we are concerned with here. Testing wet shows some difference between the two materials with greater decreases in hardness and modulus for Gari stangari than Elliptio crassidens, particularly, for the same indentation depths. The data implies that Gari stangari is more effectively plasticized than Elliptio and water is more effective in plasticizing the surface layers of Gari stangari.
- 49. 47 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 50 55 60 65 70 75 80 85 90 E-Modulus(GPa) Hardness(GPa) Dry Condition Wet Condition Fig 4.2: Variation of E- modulus and hardness in dry and wet conditions for Elliptio. Fig 4.1 and fig 4.2 shows how the hardness and reduced modulus vary with position of the indent and test medium. Hardness and elastic modulus determined from the Oliver and Pharr procedure are almost invariant with load, increasing by ~10% for the lowest indentation load compared with the largest. In case of Gari stangari, the average hardness in dry condition was 3.14 +/- 0.48 GPa and in wet condition it was found out to be 1.97 +/- 0.32 GPa. The average E- Modulus in dry condition was 73.60 +/- 11 GPa and in wet condition was 33.8 +/- 3.7 GPa. The change is very significant and it accounts about 37% change in hardness and about 54.1% change in E- Modulus between the wet and dry conditions and is shown in the Fig 4.1. The ratio of Hardness to E- Modulus was found out to be 0.043 in dry condition and is increased to 0.058 in wet conditions. For Elliptio crassidens, the average hardness in dry condition was 3.15 +/- 0.35 and in wet condition it was found out to be 2.69 +/- 0.25. The average E- Modulus in dry condition was 71 +/- 9 GPa and in wet condition was 54 +/- 3.9. The change here also is significant and it accounts about 15% change in hardness and about 24.1% change in E-Modulus between the wet and dry conditions as shown in the Fig 4.2. The ratio of Hardness to E- Modulus was found out to be 0.044in dry condition and is increased to 0.05 in wet conditions.
- 50. 48 3.6 4.0 4.4 4.8 60 64 68 72 76 80 E-Modulus(GPa) Hardness (GPa) Dry Condition Wet Condition 2.8 3.2 3.6 4.0 4.4 4.8 60 70 80 90 100 110 E-Modulus(GPa) Hardness (GPa) Dry Condition Wet Condition Fig 4.3: Variation of E- Modulus and hardness in dry and wet conditions for Dosina anus in the first measurement. Fig 4.4: Variation of E- modulus and hardness in dry and wet conditions for Dosina anus in the second measurement.
- 51. 49 In case of Dosina anus, the Average hardness in dry condition was 3.77 +/- 0.5 GPa and in wet condition it was found out to be 3.6 +/- 0.56 GPa. The average E- Modulus in dry condition was 72.3 +/- 5 GPa and in wet condition was 88.5 +/- 14 GPa. The ratio of Hardness to E- Modulus was found out to be 0.052 in dry condition and is decreased to 0.042 in wet conditions. There is a slight reduction in hardness of 3.2% but at the same time there is a significant increase in the E-modulus or the stiffness of 22.4% the sample as shown in Fig 4.3. This increase in E-Modulus is rarely seen in general engineering materials. Also in biomaterials like bones, teeth and nacre there will be a decrease in E- Modulus when they are stored in water. To confirm this peculiar behaviour, and to ensure that the difference in E- Modulus obtained by the nanoindentor is not because of the anisotropy, the same shell was tested again in dry and wet conditions. For statistical support, the samples were subjected to 15 indents in dry and 15 indents in wet conditions. In the second attempt too there was a significant increase in the E- Modulus as shown in Fig 4.4. The Average hardness in dry condition was 4.2 +/- 0.35 GPa and in wet condition it was found out to be 3.77 +/- 0.3 GPa. The average E- Modulus in dry condition was 62.5 +/- 2.7 GPa and in wet condition was 75 +/- 3.78 GPa. There is a slight reduction in hardness of 11.7% and again there is a significant increase in the E-modulus of 20% between the wet and dry conditions. The ratio of Hardness to E- Modulus was found out to be 0.067 in dry condition and is increased to 0.05 in wet conditions. The differences between the mechanical properties in dry and wet conditions of Gari stangari, Elliptio Crassidens and Dosina anus are as shown in Table 1. From the table it is clear that the hardness is lower in wet state compared to dry state in all the samples. It is also clear that the shells have lower modulus in wet conditions except in the case of Dosina anus. Also the Hardness to modulus ratio shows an increase in the case of Gari stangari and Eliiptio crassidens from dry to wet condition, but it shows a decrease in case of Dosina anus. The indentation experiments on shells in wet condition were performed in air immediately after rewetting the samples that were stored under tap water for 60 hrs. There were 10 to 20 indents taken for the comparison purpose and the total time for the indentation process varied from two to 3 hours. The equipment took approximately 30 minutes for drift correction and time for each indentation was approximately 10 minutes. The results from the previous studies on human teeth enamel show a significant change in elastic modulus and hardness between dried state and the samples constantly stored and investigated under a fluid (Guidoni et al., 2006). If the wet sample is taken out of the fluid and within a few minutes is
- 52. 50 tested by indentation, a transition from totally wet state to fairly moist state occurs. Both the hardness and E-modulus increase and reach more or less steady state within 10 min (Schöberl et al., 2006). Table 1: anoindentation values in dry and wet conditions for Gari stangari, Elliptio crassidens and Dosina anus Sample Avg Max Depth (nm) Avg Max load (m ) Avg Hardness (GPa) Avg E- Modulus (GPa) Hardness/E- modulus ratio Gari stangari (Dry) 528 18 3.13 +/- 0.48 73.6 +/- 11 0.043 Gari stangari (Wet) 531 10 1.98 +/- 0.32 33.8 +/- 4 0.058 Elliptio (Dry) 518 17 3.15 +/- 0.35 71.1 +/- 9 0.044 Elliptio (Wet) 535 15 2.7 +/- 0.25 54 +/- 4 0.050 Dosina anus 1 (Dry) 505 19 3.77 +/- 0.5 72.3 +/- 5 0.052 Dosina anus 1 (Wet) 546 22 3.65 +/- 0.56 88.6 +/- 15 0.042 Dosina anus 2 (Dry) 545 23 4.20 +/- 0.35 62.5 +/- 3 0.067 Dosina anus 2 (Wet) 563 23 3.77 +/- 0.3 75 +/- 4 0.050 It is reasonable to assume that 24 hours immersion is more than adequate for saturation of the surface layers. However, as these samples are biomaterials they are wetted for 3 days before testing. They were few questions about the reliability of such a testing procedure. Since the samples were removed from the water and were tested in air and also as the indenter takes about 30 minutes for the drift correction, there were doubts whether the sample had already reached the dry condition during the indentation process. For this reason weight loss measurements were carried out on these samples. For the weight loss measurements shell specimens were polished to similar sizes of that of standard specimens. In our case the standards like austenitic stainless steel and glass were used. The polished shell samples and standards were both weighed in dry condition. Then they were immersed in water for three days and the samples were once again weighed in regular intervals of time and the weights were noted as a function of time.
- 53. 51 0 60 120 180 0.4625 0.4630 0.4635 0.4640 0.4645 Weight(gm) Time (min) Elliptio Wet Elliptio Dry 0 5 10 15 20 25 1.3525 1.3530 1.3535 1.3540 1.3545 Weight(gm) Time (min) Austinitic Stainless Steel Wet Austinitic Stainless Steel Dry Fig 4.5: Variation of weight as a function of time in dry and wet conditions for Elliptio Crassidens. Fig 4.6: Variation of weight as a function of time in dry and wet conditions for austenitic stainless steel. 0 2000 4000 6000 8000 10000 0.4625 0.4630 0.4635 0.4640 0.4645 Weight(gm) Time (min) Elliptio Wet Elliptio Dry
- 54. 52 0 2 4 6 8 10 12 14 16 0.3035 0.3040 0.3045 0.3050 0.3055 Weight(gm) Time (min) Glass Dry Glass Wet Fig 4.7: Variation of weight as a function of time in Dry and Wet conditions for Dosina anus. Fig 4.8: Variation of weight as a function of time in Dry and Wet conditions for Glass. 0 1500 3000 4500 6000 1.312 1.316 1.320 1.324 Weight(gm) Time (min) Donina anus Dry Dosina anus Wet
- 55. 53 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0 10 20 30 40 50 Stress(MPa) Strain Reference 1 Reference 2 Reference 3 From the weight loss measurements it is clear that the phenomenon is not only taking place at the surface but the phenomenon is also predominantly taking place at the bulk level. Also we could see that, it takes approximately 3 hours to evaporate half of the water from the shells as shown in fig 4.5 and 4.7. This proves that the shells have still retained half of the water in them even after the nanoindentation measurements and the process of testing them in air in nanoindentation apparatus most likely represents the in vivo testing of the samples. Further after observing that the phenomenon is predominantly happening in the bulk of the sample, additional research was focused on the influence of wetting on the mechanical properties at the macroscopic level by using classical mechanical tests like the three point bend tests, impact tests and fracture toughness tests. Fig 4.9: Variation of E- modulus, fracture strength and toughness for a reference sample3 of Gari stangari in dry conditions. In the macroscopic bend tests, initially a reference sample1 of Gari stangari was tested for the purpose of comparison as shown in the fig 4.9. As explained previously in the specimen preparation for bending tests, the samples were cut from one particular shell with their edges being ground and polished and they were numbered in ascending order from 1, 2 and 3 according to their position from left to right. From the stress- strain plot shown in the 3 Note : Here the reference sample refers to the sample which is in dry sate
- 56. 54 fig 4.10, it is clear that these samples are stronger and tougher at the anterior end and their strength and toughness decreases gradually towards the posterior end of the shell. Fig 4.10: Variation of E- modulus, fracture strength and toughness in dry and wet conditions for Gari stangari. Since we see from the above results of the reference samples that the mechanical properties of these materials depend upon the position, samples for the comparison tests were cut very close to one another. The whole shell was cut into four parts and the samples from alternate positions were soaked in tap water for 60 hrs (approx 3 days) before testing. The samples were then tested in three point bending at a strain rate of 2 mm/sec. From the results we see that the samples in wet condition show higher stresses and higher strains compared to the ones in dry condition. Although there is not much of change in the elastic modulus between the dry and wet samples, there is a significant change in tensile strength, ductility and toughness in the hydrated samples. Also one could see a common difference in the stress-strain behavior in wet and dry samples. It is seen that in these biocomposites, there are saw tooth-like stress strain curves. Even though there is a crack initiation inside the wet sample the sample is able to take higher and higher load before it surrenders itself to fracture. Although we see a change in mechanical 0.00 0.01 0.02 0.03 0.04 0 20 40 60 80 B1- Wet B2- Dry B3- Wet B4- Dry Stress(MPa) Strain
- 57. 55 0.00 0.02 0.04 0.06 0.08 0.10 0 10 20 30 40 Stress(MPa) Strain Reference 1 Reference 2 Reference 3 Reference 4 Reference 5 Reference 6 properties between the wet and dry samples, it is difficult to comment that the change is entirely because of hydration. For this reason, tests were carried out on another species of mollusc namely Dosina anus. Fig 4.11: Variation of E- modulus, fracture strength and toughness for a reference sample of Dosina anus in dry conditions. Initially a reference sample of Dosina anus was tested for the purpose of comparison of mechanical properties in dry conditions. As explained previously, the samples were cut from one particular shell with their edges being ground and polished and they were numbered in ascending order from 1, 2, 3, 4, 5 and 6 according to their position from anterior to posterior end. From the stress- strain plot it is clear that these samples are having almost the same strength and toughness in dry conditions (Fig 4.11). Since we see from the above results of the reference sample that the mechanical properties of these shell materials are almost independent of the position, the samples for the comparison tests between wet and dry conditions were cut very close to one another. The whole shell was cut into seven parts and the samples from alternate positions were soaked in tap water for 60 hrs (approx 3 days) before testing. The samples were then tested in three point bending at a strain rate of 2 mm/sec. From the results we see that the samples in wet condition show higher stresses and higher strains compared to the ones in dry condition as shown in fig 4.12. Although there is a slight decrease in the elastic modulus between the dry and wet samples, there is a significant
- 58. 56 change in tensile strength, ductility and toughness in the hydrated samples. The wet samples show almost a two fold increase in strength. Further the ductility of the sample is also enhanced by wetting. Additionally, even the toughness is increased because of the increase in both strength and ductility in wet conditions. With an exception of sample C5 all the rest of the samples follow the same trend. Fig 4.12: Variation of E- modulus, fracture strength and toughness in dry and wet conditions for Dosina anus. The fractured surfaces of the samples were observed with SEM. The fracture observed in both the dry and wet condition is of a mixed type which is neither entirely ductile nor entirely brittle. Also, from the fractured surfaces shown in fig 4.13 and fig 4.14, it is evident that, in wet condition, more organic matrix is available for deformation, producing more fibrils. The dehydration leads to less plastic deformation of the matrix and less energy dissipation. It is seen clearly that larger matrix fibrils were pulled out from the wet sample as compared with dry samples, which indicates more plastic deformation of organics during the bend testing. SEM images in fig 4.15 and fig 4.16 at a higher magnification show that some were still wrapped inside organic matrix, showing the stronger interfacial bonding between carbonate particles and organic matrix in hydrated shells. 0.00 0.04 0.08 0.12 0.16 0.20 0 10 20 30 40 50 60 C1- Wet C2- Dry C3- Wet C4- Dry C5- Wet C6- Dry C7- Wet Stress(MPa) Strain
- 59. 57 Fig 4.13: SEM image of fractured surface of Dosina anus tested in wet condition during the bending test. Fig 4.14: SEM image of fractured surface of Dosina anus tested in dry condition during the bending test Larger matrix fibrils
- 60. 58 Fig 4.15: SEM image of fractured surface of Dosina anus tested in wet condition during the bending test. Fig 4.16: SEM image of fractured surface of Dosina anus tested in dry condition during the bending test. Carbonate particles wrapped inside the matrix
- 61. 59 All the three point bending tests so far were carried out on a Zwick tensile testing machine which operates at a strain rate of 2 mm/sec. Since there are only few shells which are tested for observing this phenomenon, few more tests were carried out on Gari stangari and Dosina anus in wet and dry conditions. In order to test the efficiency and effectiveness of the cost effective tensile testing equipment and to confirm the phenomenon taking place in these mollusc shells, supplementary tests were carried out for statistical support. As said previously the tensile testing equipment which is built indigenously has a strain rate of 1 µm/sec. Hence we observe a change in the load- displacement behaviour compared to the previous plots. Initially for the four point bend test, two shells of similar sizes and shapes are chosen. One of the samples was put in water for three days and was tested along with the other dry sample. Fig 4.17: Variation of E- modulus, fracture strength and toughness in dry and wet conditions for Dosina anus in four point bending by indigenously built tensile testing machine. From the results obtained from the equipment ( fig 4.17) it is clear that the shells are gaining their strength and toughness in water in the longitudinal direction. It is worth while to compare the shells in two conditions without normalizing the plots to stress and strain as the shells are of similar size and shape. There is about 23% increase in strength and 26% increase in ductility in wet conditions. 0.00 0.04 0.08 0.12 0.16 0.20 0 100 200 300 400 Load(N) Displacement (mm) Dry Condition Wet Condition
- 62. 60 0.00 0.02 0.04 0 20 40 60 Load(N) Displacement (mm) Dry Sample Wet Sample The second set of tests was carried out on Gari stangari with the samples being polished before to similar sizes. One of the samples was kept in water for 3 days and was tested along with the dry sample in three point bending. Fig 4.18: Variation of E- modulus, fracture strength and toughness in dry and wet conditions for Gari stangari in 3 point bending by indigenously built tensile testing machine. Also three point bending results on Gari stangari show that these shells are gaining mechanical properties in hydrated conditions (Fig 4.18). There is about 23% increase in strength and 12% increase in ductility in wet conditions. From the above two results the E- modulus in Dosina anus seems to be lesser in wet conditions compared to that of dry conditions. Further, the E- Modulus in Gari stangari is relatively higher in wet conditions compared to the dry conditions. These results contradict the results obtained from the nanoindentation tests. One of the main reasons for this sort of a behavior should be attributed to the structure. Since these shells have hierarchical structure, the structure in the longitudinal direction differs significantly from the structure in transverse direction. It is worth to note in this aspect that the three point bend tests are carried out in the longitudinal direction and the nanoindentation tests are carried out in vertical directions of the shell.
- 63. 61 0.0 0.2 0.4 0.6 0.8 0 5 10 15 20 25 Stress(MPa) Strain (%) Dry Condition Wet Condition In order to confirm that the mechanical properties depend upon the loading direction two pieces of shell of similar sizes and shapes of Elliptio crassidens were tested in transverse direction in three point bending. Fig 4.19: Variation of E- modulus, fracture strength and toughness in dry and wet conditions for Elliptio in 3 point bending in transverse direction by indigenously built tensile testing machine. Also from figure 4.19 one could observe that there is a change in E- Modulus in the macroscopic bend tests which are performed on the samples in transverse direction. These shells show an increased ductility of 15 % and 30 % decrease in fracture strength and toughness when they are stored in water. Also there is a 35% decrease in the E- Modulus which approximately coincides with the decrease in E- Modulus seen through nanoindentation testing. The above sets of tests were carried out in tap water. In view of the fact that both Gari stangari and Dosina anus are sea water molluscs instead of fresh water species, three point tests were carried out by immersing the samples in sea water for a period of three days. The samples preparation and procedures are similar to the tests explained previously except that the samples are stored in sea water. From the results in fig 4.20, 4.21, 4.22, 4.23, apart from the sample A8, it is obvious that the shells are gaining their mechanical properties even in sea
- 64. 62 water. Although the increase in mechanical properties is not that significant as fresh water, still these results set the trend for a similar phenomenon that is taking place inside these shells. Fig 4.20: Variation of E- modulus, fracture strength and toughness in dry and wet (sea water) conditions for Dosina anus in 3 point bending. Fig 4.21: Variation of E- modulus, fracture strength and toughness in dry and wet (sea water) conditions for Dosina anus in 3 point bending. 0.00 0.01 0.02 0.03 0.04 0 10 20 30 40 Stress(MPa) Strain A1 Wet A2 Dry A3 Wet A4 Dry A5 Wet 0.00 0.01 0.02 0.03 0.04 0 10 20 30 40 50 60 70 Stress(MPa) Strain A5 Wet A6 Dry A7 Wet A8 Dry A9 Wet
- 65. 63 Fig 4.22: Variation of E- modulus, fracture strength and toughness in dry and wet (sea water) conditions for Dosina anus in 3 point bending. Fig 4.23: Variation of E- modulus, fracture strength and toughness in dry and wet (Sea water) conditions for Dosina anus in 3 point bending. 0.00 0.01 0.02 0.03 0.04 0 10 20 30 40 Stress(MPa) Strain B1 Dry B2 Wet B3 Dry B4 Wet B5 Dry 0.00 0.01 0.02 0.03 0.04 0 20 40 60 80 Stress(MPa) Strain B5 Dry B6 Wet B7 Dry B8 Wet B9 Dry
- 66. 64 1 2 3 4 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Impacttoughness(J/mm 2 ) Sample position Elliptio Polished Elliptio Unpolished In order to support the results obtained from the three point bend tests and to confirm that there is an increase in toughness, impact tests were carried out on wet and dry samples of Elliptio crassidens. Initially to test whether there is any influence of sample preparation and polishing on the impact properties of shell materials, few tests were carried out between polished and unpolished samples. The unpolished samples are not only difficult to load inside the impact testing machine but also produces results which are inconsistent as shown in the fig 4.24. In addition, the area determination for an unpolished specimen is relatively difficult and laborious process. Fig 4.24: Variation of impact toughness between polished and unpolished samples of Elliptio For this reason the samples are finely polished before the testing process. Since Elliptio is a fresh water mollusc the samples are stored in tap water for 3 days before testing them. In general, Impact testing provides toughness qualitatively. Therefore, couple of samples are tested simultaneously to get an average value of fracture toughness as shown in fig 4.25. From the results obtained through impact tests the average impact toughness was calculated for hydrated and dehydrated samples. The samples which were stored in water had an average impact toughness of 2.72 J/mm2 compared to the dry samples which showed an average impact toughness of 2.37 J/mm2 (Fig 4.26). The results clearly show that the samples
- 67. 65 which were stored in water have higher toughness of approximately 15% compared to the dehydrated samples. Fig 4.25: Influence of water on impact toughness of samples of Elliptio Fig 4.26: Averaged impact toughness for samples of Elliptio in dry and wet conditions 0 1 2 3 0.0 0.5 1.0 1.5 2.0 2.5 AverageImpactToughness(KJ/mm 2 ) Sample Condition Dry Condition Wet Condition
- 68. 66 Fig 4.27: Post mortem SEM image of fractured surface of Elliptio subjected to hydration with subsequent impact testing. The fractured surfaces of the samples were observed with an SEM. The SEM image shows that some carbonate particles were still wrapped inside organic matrix, showing the stronger interfacial bonding between carbonate particles and organic matrix. . Since the impact toughness data are mostly qualitative and to check quantitatively the effect of hydration on the toughness of shell specimens, fracture toughness tests were carried out on dry and wet samples of Elliptio. Small pieces of these shells were cut from one shell just adjacent to one another and they were grinded and polished to similar sizes. The samples are then tested in three point bending to determine the load displacement behaviour for calculating the fracture toughness. Load-displacement data were analyzed to evaluate the resistance to fracture for hydrated and dehydrated shells as shown in Fig 4.28 and Fig 4.29. From the Table 2, the value of 2.5*( Kq/σ0)2 is lesser than the width of the sample, but the value is not less than the crack length a. This signifies that the sample size should be still thicker in order to determine the plane strain fracture toughness (K1c). Although the conditional fracture toughness could not approach the plane strain fracture toughness of the
- 69. 67 0.0 0.2 0.4 0.6 0.8 0 20 40 60 80 Load(N) Displacement (mm) Elliptio Dry condition 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 20 40 60 Load(N) Displacement(mm) Elliptio Wet Condition sample, the calculated results still hold a key role as these results are mainly used for comparison purpose. Fig 4.28: Load- Displacement behavior for a fracture toughness sample of Elliptio crassidens in dry condition Fig 4.29: Load- Displacement behavior for a fracture toughness sample of Elliptio crassidens in wet condition
- 70. 68 1 2 3 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 FractureToughness(MPa(m) 1/2 ) Sample position Dry Condition Wet Condition Table 2: Fracture toughness calculations for Elliptio crassidens in dry and wet conditions The dehydrated specimen, however, showed lower fracture toughness 0.83 MPa√m as compared to for the hydrated specimen 1.37 MPa√m. This difference is significant as the conditional fracture toughness (Kq) in hydrated condition is almost twice compared to the value in dry condition as shown below in the fig 4.30. Fig 4.30: Calculated values of fracture toughness sample of Elliptio crassidens in wet and dry condition Name S (m) a (m) B (m) W (m) Pq (N) a/W Elliptio (Dry) 0.0200 0.0016 0.00255 0.0051 69.69 0.628 Elliptio (Wet) 0.0200 0.0020 0.00250 0.00485 58.27 0.8 Name [2.9 *(a/W)1/2 – 4.6*(a/W)3/2 +21.8*(a/W)5/2 – 37.6*(a/W)7/2 + 38.7*(a/W)9/2 ] (Pq*S)/(B*W3/2 ) MPa√m Kq MPa√m Elliptio (Dry) 4.20 0.199 0.83 Elliptio (Wet) 8.74 0.157 1.37 Name P0 (N) σ0 (MPa) 2.5*( Kq/σ0)2 Elliptio (Dry) 69.69 31.52 0.0017 Elliptio (Wet) 58.27 29.72 0.0053
- 71. 69 Fig 4.31: SEM image of the fractured sample of Elliptio crassidens after the fracture toughness measurements in dry condition Fig 4.32: SEM image of the fractured sample of Elliptio crassidens after the fracture toughness measurements in wet condition
- 72. 70 The universal solvent water had considerable affect on the mechanical properties these biocomposites ranging from nanoscale to macroscale. As a last part of this work microindentation tests were carried out on the samples to determine the influence of water on micro mechanical properties of seashells. The indents are taken at equal distances and the values of Vickers microhardness are noted. Fig 4.33: Plot of vickers microhardness values as a function of position number. Fig 4.34: Optical micrograph of the vickers indentation on Elliptio crassidens at 100X magnification 1 2 3 4 5 6 7 8 9 1011121314151617181920 200 210 220 230 240 250 260 Vickersmicrohardness(Kg/mm 2 ) Position number Vickers microhardness of Elliptio in dry condition
- 73. 71 Fig 4.35: Plot of vickers microhardness values as a function of position number. Fig 4.36: Optical micrograph of the vickers indentation on Gari stangari at 100X magnification The microindentation experiments have been partially carried out on these shell samples. The measurements in wet conditions haven’t been conducted because of the equipment failure. Because of the paucity of time the experiments were not able to be finished on time. But however from the results of nanoindentation tests and macroscopic bend tests mentioned above, it is expected that the results microindentation tests will also show similar change in mechanical properties like the nanoindentation tests but at the microscale. 1 2 3 4 5 6 7 8 9 1011121314151617181920 260 280 300 320 340 Vickersmicrohardness(Kg/mm 2 ) Position number Vickers microhardness of Gari stangari in dry condition
- 74. 72 4.2 Disscussions Both dry and hydrated (wet) shell specimens were tested by nanoindentation and macromechanical three point bend tests along the transverse and longitudinal direction of the shell and the resulting load displacement curves are shown in the figures above Fig 4.1-4.4, 4.10,4.12, 4.17-4.23. From the figures it is evident that dry shells exhibit brittle behavior while wet shells showed slight (matrix-reinforcement-pullout) ductility. These results are explained by the assumption that water degrades the organic- inorganic interface (Soderholm, 1983) and causes a swelling of the matrix. The matrix shrinks during polymerization and will generate hoop stresses around the carbonate particles (Soderholm, 1984). These hoop stresses increase the frictional forces between the organic protein and the inorganic carbonate, thereby decreasing the strain. Fig 4.37: SEM image of Mollusc in dry condition (Barthelat and Espinosa, 2002) However, when the composite is exposed to water, three detrimental effects could occur. Firstly, the organic content in the shell with its open microstructure is readily plasticized by water. Plasticization destroys some of the bonds between the organic and inorganic phase, which results in an increase in ductility. Secondly, water could behave as a chemical plasticizer and lower the glass transition temperature of the organic interface to a lower temperature upon exposure to moisture. Once
- 75. 73 the organic phase is at or above its glass transition temperature, it will tend to creep when any applied load occurs. Thirdly, water causes the surrounding organic polymer to swell and plasticize, thus reducing the hoop stresses around the carbonate particles which will facilitate in an increase in strain. The reduced hoop stress level is a reversible process, which means that the initial hoop stress level can be recovered if the sample is allowed to shrink due to dehydration. This emphasizes the importance of the organic interface in between the carbonate tablets, which upon hydration allows sliding of layers and leads to large deformations as shown in fig 4.37. The modulus was found to be 73 GPa for the dry condition and 33 GPa for the hydrated (wet) condition for Gari stangari. This difference indicates that the hydrated organic network promotes sliding of the layers. A destruction of the organic-inorganic bonding caused by hydrolytic reactions, together with an increase in hoop stress level around wet-and-dried specimens were intermediate between the values of the samples that were stored dry and those that were tested in the wet condition. This explanation is consistent with the morphological findings and supports the hypothesis that hydrolytic degradation of biocomposites occurs when such materials are stored in a wet environment. The saw tooth-like force extension curves were observed with an increased strength and toughness after hydration during the three point bend tests. In addition, the present results confirm that the wet shells of Elliptio show higher fracture toughness than their dry counterparts suggesting three possible phenomenons taking place in these mollusc shells. Firstly the bonds between the organic matrix and the inorganic carbonate could be breaking and reforming. Each jump corresponds to local sacrificial bonds of the matrix and reinforcement, but because this process is repeated throughout large strains a high work of fracture is observed. Therefore, the organic fraction should be playing an important role in the toughness properties of mollusc shells. Secondly, the increase in strength could also be attributed with the mechanical interlocking with the water which is inside the micropores of the material. The interlocking is caused by the stress or strain fields created because of the hydration of organics with the water.
- 76. 74 Third way to understand these trends is to recognize that subcritical crack extension can be considered as the mutual competition between the classes of mechanisms: (i) Intrinsic toughening mechanisms that operate ahead of the crack tip and act to enhance the material’s inherent resistance to microstructural damage and cracking. The higher crack initiation toughness of the wet shells can be associated with the evidence of crack blunting, which is promoted by the presence of plasticity and viscoplasticity at the crack tip. Such blunting serves to lower the stresses experienced ahead of the crack tip. (ii) Extrinsic toughening mechanisms that operate primarily behind the crack tip by promoting crack-tip shielding, which reduces the local stress intensity actually experienced at the crack tip (Ritchie, 1988, Ritchie, 1999, Evans, 1990). This mechanism operates in the crack wake up and is mostly dependent on the size of the crack. It in turn leads to a rise in fracture toughness because after crack extension a more substantial shielding zone is established in the crack wake up. Intrinsic mechanisms, such as crack blunting, tend to affect the initiation toughness, whereas extrinsic mechanisms, such as crack bridging, promote crack-growth toughness. In hydrated shells, un-cracked-ligament bridging could also be one of most prevalent shielding mechanism. Since bridges can only form with crack extension, this mechanism can only affect the crack growth toughness. However, eventually a steady-state condition is reached where bridges are formed near the crack tip and destroyed in the crack wake at an equal rate. Thus, for hydrated shells, steady-state toughness is quickly reached, which appears to be a result of more extensive crack blunting leading to larger crack opening displacements. Due to the larger crack openings associated with the hydrated condition, it appears that this critical crack-opening displacement is achieved after less crack extension than in the dehydrated condition. In addition to bridging, microcracking has also been suggested as a possible toughening mechanism in hydrated biomaterials (Kahler et al., 2003, Nalla et al., in press). This mechanism is reasoned to ‘‘shield’’ the crack by creating a dilated zone that surrounds the crack and is constrained by the surrounding (un-microcracked) material.
- 77. 75 Fig 4.38: A material property chart for natural materials, plotting tensile strength rt against Young’s modulus E. The straight line with a slope of 1 represents a constant elastic strain, e = rt/E, and describes how large of a deformation is possible without failure. (Figure created using the atural Materials Selector, Wegst, 2004). To compare our experimental results with those from the literature, we have used a material property chart, plotting tensile strength against young’s modulus as shown in the Fig. 4.38 (Wegst, 2004). In comparison with other measurements of wood (green circle), the molluscs have both a higher Young’s modulus (E) and a higher Tensile strength (rt). On the chart, a constant ratio of the two, the elastic strain, rt/E, is represented by a straight line with a slope of 1. This describes that in dry conditions molluscs show very less deformation before fracture. From the Fig 4.38 it is clearly seen that the values of young’s modulus and tensile strengths obtained with our three point bend tests and nanoindentation tests in dry conditions almost fall in the range of values which are predicted by the literature. SEM examination revealed that the improvement was due to stronger interface bonding between carbonate particles and the organic matrix, which lead to more and larger matrix fibrils being pulled out during impact testing and resulted in more impact energy being absorbed. In general, voids are the main source of weak links in the sample and a crack which is initiated inside the sample propagates through these voids. These voids connect each other
- 78. 76 coalesce and then ultimately leads to the material fracture. In dry shells these voids are free from water. During impact testing, these voids or weak points initiate the formation of cracks, making carbonate particles much easier to debond from the matrix. Therefore less plastic deformation of the matrix occurs before fracture, resulting in less absorbed impact energy and lower impact toughness. SEM images (Fig. 4.13 and 4.14) also show that the interfacial bonding between carbonate particles and organic matrix has failed and which led to the separation of carbonate particles with the organic matrix. In hydrated samples, we observe from the weight loss measurements that the micro voids which are present inside the sample are generally filled with water. Therefore there were fewer holes and weak points to initiate fracture of the materials during impact test. SEM images (Fig. 4.27) show that the interface between carbonate particles and the organic matrix was not totally debonded during impact testing, with some carbonate particles still wrapped inside organic matrix. For these materials, the fracture toughness of the organic matrix plays a more important role in the impact resistance of the composite. Much longer matrix fibrils were pulled out from wet shells compared with dry shells due to stronger interfacial bonding, which indicated more matrix plastic deformation and more energy absorption during the impact testing.
- 79. 77 Conclusions The indigenously built tensile testing machine is a simple and inexpensive device that can be constructed with a minimum amount of skill. This device is easy to use and will yield reliable data on the mechanical properties of shells. Finally, the device can easily be adjusted for testing the shells of various sizes by manually adjusting the height of the movable crosshead and by modifying the shell supports. Water adsorption has a considerable impact on mechanical properties of bivalve shells at the macro and microscale. Hardness and Elastic modulus can strongly depend on the water content. Large differences can appear between the samples tested in dry and wet conditions. The high ductility achieved by tablet sliding probably dissipates a large amount of energy through friction and redistribution of the deformations which demonstrates the vital role played by organics in these materials. The higher strength of shells in water can also be attributed to the crack tip blunting which is happening inside the organics which ultimately leads to a higher toughness in these materials. Hydration can be seen to significantly increase the fracture toughness of shells. Mechanistically, this is primarily associated with extensive crack blunting, which elevates the crack-initiation toughness, and additionally from enhanced uncracked-ligament bridging, which promotes the crack-growth toughness and hence results in a substantial rise in fracture toughness. In comparison, dehydrated shells shows little blunting, which results in a lower crack initiation toughness. However, with crack extension, significant crack bridging occurs, leading to rise in fracture toughness. It was found that the total absorbed energy increased significantly during the fracture toughness tests of hydrated samples. From the fractured surfaces it is evident that, in wet condition, more organic matrix is available for deformation, producing more fibrils. SEM images shows that some carbonate particles were still wrapped inside organic matrix for hydrated samples, showing the stronger interfacial bonding between carbonate particles and organic matrix which leads to more energy dissipation and the improvement in impact resistance.
- 80. 78 Engineering perspective Fifty years from now, with the advent of global warming there would melting of polar ice caps and an increase in the level of water could lead to sinking of small islands and also places which are closer to the sea and river coasts. Reinforced Concrete which is used as a structural material in general engineering construction will weaken and fail upon longer exposure of water. Scientists around the world are already working on these lines to create structures and materials which could provide higher mechanical stability in water. So far all the engineering materials deteriorate their mechanical properties in water. Metals and alloys undergo corrosion, polymers degrade, ceramics lose their strength and composites which are combination of the above also lose their mechanical properties on long term exposure to water. From the results in dry and wet conditions, we clearly see that investigated shell materials are gaining their mechanical properties in water. If one is able to gauge out the phenomenon causing this increase, then with bio-mimicry around the corner one could artificially produce composites which gain their properties over the years when they are in water. The dream of realizing underwater structures from the lessons taught by nature is not far from reality. Surely there would be a day when these materials gain commercial importance and who knows these Sea Shells now could be She Sells in the near future.
- 81. 79 Future Scope 1. The use of a fluid cell in conjunction with a humidity control chamber inside a commercial nanoindentation system would extend the range of nanomechanical testing to cover virtually the entire range of possible moisture content (5–100% RH) and is expected to have applications in testing biological materials at highly localized scale inaccessible to other testing techniques. 2. Mechanical properties are always interlinked to the microstructure of the samples. One should investigate the microstructural changes happening inside these shells when they are stored in water. Since the phenomenon which is happening in these shells is predominantly macroscopic one could check out the structural changes by means of High resolution X ray cameras. 3. While the fracture mechanics aspects of such crack blunting are well characterized, the mechanism for this process in hydrated shells is yet to be understood, including the fluid egress near the crack tip and time dependency of the blunting.
- 82. 80 Appendix To check the stress-strain relationships in brachiopods, shells of Magellania Venosa were chosen. Samples were cut from the centre of the shell and they were named as Magellania Venosa 4 and Magellania Venosa 5 (as these samples are next to one another). Unlike molluscs these brachiopod samples have different mechanical properties for the upper part of the shell and lower part of the shell. Hence the upper part of the shell is labeled as top and lower part of the shell is labeled as bottom. The samples were prepared in the same way like the molluscs and were tested in three point bending in dry condition at a strain rate of 2mm/sec. Fig 1: Stress- Strain plots of Magellania Venosa in dry condition From fig 1 it is clear that the samples from the bottom part of the shell are stronger and tougher than the samples from the upper part of the shell. In order to compare the stress-strain relationships between the molluscs and brachiopods, the samples of Magellania Venosa, Dosina anus and Gari Stangari were chosen. The stress- strain values of these three samples in three point bending in dry conditions were taken and are plotted as shown in fig 2. From the fig 2 it is clear that the Magellania Venosa samples have higher tensile strength and toughness compared to that of Dosina anus and Gari Stangari. 0.000 0.002 0.004 0.006 0.008 0 20 40 60 80 100 Stress(MPa) Strain Magellania Venosa 4 Top Magellania Venosa 5 Top Magellania Venosa 4 Bottom Magellania Venosa 5 Bottom
- 83. 81 Fig 2: Stress- Strain plots of Magellania Venosa in dry condition It is also clear from the fig 2 that the Magellania Venosa samples have lower E-Modulus compared to those of Dosina anus and Gari Stangari. 0.00 0.01 0.02 0.03 0 20 40 60 80 Stress(MPa) Strain Magellania Venosa top Magellania Venosa bottom Dosina anus Dosina anus Gari stangari Gari stangari
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