Introduction to composite materials and application
- 2. What is a composite? ◼ A material which is composed of two or more materials at a microscopic scale and have physically / chemically distinct phases. ◼ Examples: Flesh in your leg reinforced with bones Concrete reinforced with steel Epoxy reinforced with graphite fibers.
- 3. ◼ Strictly speaking, the idea of composite materials is not a new or recent one. ◼ Nature is full of examples wherein the idea of composite materials is used. ◼ The coconut palm leaf, for example, is essentially a cantilever using the concept of fiber reinforcement. What is a composite?
- 4. Examples of Natural Composites ◼ Wood Cellulose Fibers Lignin Matrix ◼ Bones Collagen Fibers Mineral Matrix Conditions of Composites : 1.Combination of materials should result in significant property changes 2. Content of the constituents is generally more than 10% 3. In general, property of one constituent is much greater (≥ 5) than the other
- 5. Composites Demand ❖ Since the early 1960s, there has been an increasing demand for materials that are stiffer and stronger yet lighter in fields as diverse as aerospace, energy, and civil construction. ❖ It implies that, composite shall be used for the most efficient design of, say, an aerospace structure, an automobile, a boat, or an electric motor,
- 6. Historical Perspective ◼ 4000 B.C. Fibrous composites were used in Egypt in making laminated writing materials ◼ 1939: Glass fiber manufactured commercially for high temperature electrical applications ◼ 1950s: Boron and carbon fibers were produced to make ropes. ◼ 1960s: Matrix added to make polymeric matrix composites
- 7. Historical Perspectives (continued) ◼ 1970s: Cold war forces development of metal matrix composites for military aircrafts and missile guidance systems ◼ 1990s: High temperature ceramic matrix composites are being aggressively researched for use in next generation aircraft engines and power plant turbines
- 8. Advantages of composite materials ◼ Lower density (20 to 40%) ◼ Higher directional mechanical properties (specific tensile strength (ratio of material strength to density) 4 times greater than that of steel and aluminum. ◼ Higher Fatigue endurance . ◼ Higher toughness than ceramics and glasses. ◼ Versatility and tailoring by design. ◼ Easy to machine.?? ◼ Can combine other properties (damping, corrosion). ◼ Cost.
- 9. Why composites over metals? ◼ High Strength and High Stiffness ◼ Tailored Design ◼ Fatigue Life ◼ Dimensional Stability ◼ Corrosion Resistance
- 10. Why Composites over Metals? ◼ How is the mechanical advantage of composite measured? Strength Ultimate Density Modulus s Young' strength Specific modulus Specific = = = ult E where . ult = , E =
- 11. Specific Strength vs. Year
- 12. Comparative Thermal Expansion Coefficients (μin/in/oF) Material Direction-x Direction-y Steel 6.5 6.5 Aluminum 12.8 12.8 Graphite -0.02 1.1 Unidirectional Graphite/Epoxy 0.01 12.5 Cross-Ply Graphite/Epoxy 0.84 0.84 Quasi-Isotropic Graphite/Epoxy 0.84 0.84
- 13. Tailored Design ◼ Engineered to meet specific demands as choices of making the material are many more as compared to metals. ◼ Examples of choices fiber volume fraction layer orientation type of layer layer stacking sequence
- 14. Fatigue Life ◼ Fatigue life is higher than metals such as aluminum. ◼ Important consideration in applications such as aircrafts bridges structures exposed to wind
- 15. Dimensional Stability ◼ Temperature changes can result in overheating of components (example engines) thermal fatigue due to cyclic temperature changes (space structures) render structures inoperable (space antennas)
- 16. Corrosion Resistance ◼ Polymers and ceramics matrix are corrosion resistant ◼ Examples include underground storage tanks doors window frames structural members of offshore drilling platforms
- 17. Cost Considerations ◼ Compositesmay be more expensive per pound than conventional materials. Then why do we use composite materials?
- 18. Factors in Cost Estimate ◼ For Composite Materials Fewer pounds are required Fabrication cost may be lower Transportation costs are generally lower Less maintenance than conventional materials is required
- 20. Composites Classification Based on Reinforced Materials ❖Fibre - a filament with L/D very high (of the order 1000) ❖A composite with fibre-reinforcement is called Fibrous Composite ❖Particle – non fibrous with no long dimension ❖A composite with particles as reinforcement is called Particulate Composite ❖Whiskers – nearly perfect single crystal fibre ❖Short, discontinuous, polygonal cross-section
- 22. Composites Classification Polymer Matrix Composites (PMC)/ Carbon Matrix Composites or Carbon-Carbon Composites ❖Polymers are structurally much more complex than metals or ceramics. They are cheap and can be easily processed. ❖On the other hand, polymers have lower strength and modulus and lower temperature use limits ❖ Polymers are generally poor conductors of heat and electricity. Polymers, however are generally more resistant to chemicals than are metals. ❖The process of forming large molecules from small ones is called polymerization, that is, polymerization is the process of joining many monomers
- 23. Composites Classification ❖Polymers that soften or melt on heating are called thermoplastic polymers and are suitable for liquid flow forming ❖Examples include low- and high-density polyethylene, polystyrene, and polymethyl methacrylate (PMMA). ❖When the structure is amorphous, there is no apparent order among the molecules and the chains are arranged randomly; Two main kinds of polymers are thermosets and thermoplastics. amorphous
- 24. Composites Classification Two main kinds of polymers are thermosets and thermoplastics. semicrystalline ❖ When the molecules in a polymer are crosslinked in the form of a network, they do not soften on heating ❖We call such crosslinked polymers thermosetting polymers. Thermosetting polymers decompose on heating. ❖A typical example is that of rubber crosslinked with sulfur, that is, vulcanized rubber. Vulcanized rubber has ten times the strength of natural rubber. ❖Common examples of thermosetting polymers include epoxy, phenolic, unsaturated polyester, and vinyl ester.
- 25. Composites Classification Metal Matrix Composites ❖ Metals are very versatile engineering materials. ❖They are strong and tough. They can be plastically deformed, and they can be strengthened by a wide variety of Methods ❖ mostly involving obstruction of movement of lineal defects called dislocations. ❖Most metals exist in one of the following three crystalline forms: ❑ Face-centered cubic (fcc) ❑Body-centered cubic (bcc) ❑Hexagonal close packed (hcp).
- 26. Composites Classification Metal Matrix Composites Metals are crystalline materials; however, the crystalline structure is never perfect. Metals contain a variety of crystal imperfections. We can classify these as follows: 1. Point defects (zero dimensional), 2. Line defects (unidimensional), 3. Planar or interfacial defects (bidimensional), and 4. Volume defects (tridimensional).
- 27. Composites Classification Volume defects Line defects Plane defects Point defects
- 28. Composites Classification Why Fiber Reinforcement of Metals? ❖ Precipitation or dispersion hardening of a metal can result in a dramatic increase in the yield stress and/or the work hardening rate. ❖The influence of these obstacles on the elastic modulus is negligible. ❖Their only function is to impede dislocation motion in the metal. ❖The improvement in stiffness can be profitably obtained by incorporating high modulus fibers in a metal matrix. ❖ It turns out that most of these high modulus fibers are also lighter than the metallic matrix materials, the only exception being tungsten, which has a high modulus and is very heavy.
- 29. Composites Classification Ceramic Matrix Materials ❖The major disadvantage of ceramics is their extreme brittleness. Even the minutest of surface flaws (scratches or nicks) or internal flaws (inclusions, pores, or micro cracks) can have disastrous results. ❖ One important approach to toughen ceramics involves fiber reinforcement of brittle ceramics Common Ceramic Matrix Materials ❖Silicon carbide has excellent high temperature resistance. ❖Silicon nitride is also an important nonoxide ceramic matrix material. ❖Silica-based glasses and glass– ceramics are other ceramic matrices.
- 30. Composites Classification Ceramic Matrix Materials ❖ Ceramic materials are very hard and brittle. ❖They have strong covalent and ionic bonds and very few slip systems available compared to metals. ❖ High melting points, good corrosion resistance, stability at elevated temperatures and high compressive strength. ❖ Naturally, ceramic matrices are the obvious choice for high temperature applications. ❖High modulus of elasticity and low tensile strain, which most ceramics posses, have combined to cause the failure of attempts to add reinforcements to obtain strength improvement.
- 31. Fibrous Composites ◼ Generally there are two phases Fiber as a reinforcement Matrix as a binder
- 32. Advantages of Composites ◼ Specific Strength and Stiffness ◼ Tailored Design ◼ Fatigue Life ◼ Dimensional Stability ◼ Corrosion Resistance ◼ Cost-Effective Fabrication
- 33. Drawbacks of Composites ◼ High cost of fabrication of composites ◼ Complex mechanical characterization ◼ Complicated repair of composite structures ◼ High combination of all required properties may not be available
- 34. Smart composites ❖ Composites containing shape memory alloys (SMA) as an active component are a new class of advanced structural and functional materials. ❖ They utilize the unique properties of SMA which change their crystalline structure in response to the change of temperature or stress. ❖ Depending on the temperature of deformation, the SMA demonstrate two different types of mechanical behavior: superelastic and pseudoplastic. ❖ In the superelastic regime it is possible to obtain almost reversible deformations up to 10ˆ-1 with a very small effective modulus which is many orders of magnitude less than the elastic moduli of the parent material
- 35. Smart composites ❖ Superelasticity is also called “pseudoelasticity” and refers to the condition when the functional temperature is above the austenite finish temperature (Af) ❖ In this condition, NiTi is in the parent-structure austenite phase. By applying stress, the phase transformation to the sheared derivative structure/stress-induced martensite (SIM) will occur. ❖ In this situation the martensite phase is only stable in the presence of stress and by removing the stress, martensite becomes thermodynamically unstable, thus NiTi will reverse to the austenite state and the original shape will be recovered.
- 36. Smart composites ❖ Therefore, in the case of superelasticity against the SME, no thermal cycling is needed for transformation and large strains applied by loading to SMAs can be recovered by unloading
- 37. Smart composites ❖ In the pseudoplastic regime the deformation proceeds with small hardening up to very large strains but it is irreversible. ❖ The residual strain can be removed by opposite loading which recovers the microstructure to its initial state. ❖ The pseudoplastically deformed materials can be completely recovered to their initial shape by heating. ❖ This is the so-called shape memory effect which gives its name to this class of materials. ❖ Due to their special mechanical behavior, SMAs are widely used in mechanical applications.
- 38. Smart composites ❖ There are examples of using them as a component of functional and structural composite ❖ Although they are in the stage of research and development, these materials have very good potential. ❖ One of the main obstacles for their wider engineering applications is the complex nonlinear behavior of SMAs, strongly dependent on many internal and external parameters. ❖ That is the reason why the principles and characteristics of SMAs are introduced first.
- 39. Smart Materials ❖ Smart materials are materials that are manipulated to respond in a controllable and reversible way, modifying some of their properties as a result of external stimuli such as certain mechanical stress or a certain temperature, among others. ❖ Because of their responsiveness, smart materials are also known as responsive materials. These are usually translated as "active" materials although it would be more accurate to say "reactive" materials.
- 40. Smart Materials ❖ For example, we can talk about sportswear with ventilation valves that react to temperature and humidity by opening when the wearer breaks out in a sweat and closing when the body cools down, about drugs that are released into the bloodstream as soon as a viral infection is detected.
- 41. Smart Materials Piezoelectric materials ◼ They can convert mechanical energy into electrical energy and vice versa. For example, they change their shape in response to an electrical impulse or produce an electrical charge in response to an applied mechanical stress. Shape memory materials ◼ They have the ability to change the shape, even returning to their original shape, when exposed to a heat source, among other stimuli.
- 42. Smart Materials Chromoactive materials ◼ They change colour when subjected to a certain variation in temperature, light, pressure, etc. Nowadays, they are used in sectors such as optics, among others Magnetorheological materials ◼ They change their properties when exposed to a magnetic field. For example, they are currently used in shock absorbers to prevent seismic vibrations in bridges or skyscrapers. ◼ Photovoltaic materials or optoelectronics convert light to electrical current.
- 43. Applications of Smart Materials ❖An “animated lamp” designed by Romolo Stanco that uses shape- memory alloy to change its shape whenever it‟s turned on and off ❖Aircraft which will incorporate "smart materials” that will allow the wings of a craft to change shape for optimal flying conditions.
- 44. Interfaces ❖ The interface is the area where the different materials in a composite coincide. ❖ In order to have a successful, applicable composite, one must form an interface that is strong and favourable towards maximum compatibility. A good interface is imperative for a material to survive under stress.
- 45. Interfaces ❖ In viewing the above graphic of a fiber reinforced matrix, one can see the presence of a medium that has both the characteristics of the fiber and the matrix. ❖ The size of this gradient, the chemical interaction, and the number of gradients present in a composite determine the strength and application of the material. ❖ The same concept holds true for blends. The interphase created between the components must achieve some level of favourable interaction to prevent the materials from "keeping to themselves" or phase separating. ❖ If phase separation occurs in a blend, then a combination of properties is likely to be unsuccessful.
- 46. Interfaces ◼ The behaviour of a composite material is a result of the combined behaviour of the following three entities: • Fiber or the reinforcing element, • Matrix, and • Reinforcement/matrix interface.
- 47. Interfaces ◼ Wettability of the fiber or any other reinforcement by the matrix and the type of bonding between the two components constitute the primary considerations. ◼ It is important to appreciate the distinction between wettability and bonding. ◼ One can have good wettability but weak bonding between two components. ◼ On the other hand, one can also have strong bonding but poor wettability, which in effect will mean weak interface because we will have gaps or voids at the interface because of the poor wettability.
- 48. Interfaces Wettability Wettability tells us about the ability of a liquid to spread on a solid surface.
- 49. Interfaces Effect of Surface Roughness ◼ Generally, it is implicitly assumed that the substrate is perfectly smooth. This, however, is far from true in practice. ◼ More often than not, the interface between fiber and matrix is rather rough instead of the ideal planar interface
- 50. Interfaces ❖ Characterization of surface roughness of a polycrystalline alumina fiber (Nextel 610) by atomic force microscopy. ❖The three profiles on the left-hand side correspond to the two horizonal and one vertical lines on the right-hand side figure. ❖The bottom figure shows a three-dimensional perspective view of surface
- 51. Interfaces Types of Bonding at the Interface ❖ It is important to be able to control the degree of bonding between the matrix and the reinforcement. ❑ Mechanical bonding ❑Physical bonding ❑Chemical bonding ❑ Dissolution bonding ❑Reaction bonding.
- 52. Interfaces Mechanical bonding ❖ Simple mechanical keying or interlocking effects between two surfaces can lead to a considerable degree of bonding. ❖In a fiber reinforced composite, any contraction of the matrix onto a central fiber would result in a gripping of the fiber by the matrix. ❖Imagine, for example, a situation in which the matrix in a composite radially shrinks more than the fiber on cooling from a high temperature. ❖This would lead to a gripping of the fiber by the matrix even in the absence of any chemical bonding
- 53. Interfaces Mechanical bonding ❖ Mechanical gripping due to radial shrinkage of a matrix in a composite on cooling from a high temperature ❖ In general, mechanical bonding is a low-energy bond, i.e., the strength of a mechanical bond is lower than that of a chemical bond.
- 54. Interfaces Mechanical bonding (a) Good mechanical bond. (b) Lack of wettability can make a liquid polymer or metal unable to penetrate the asperities on the fiber surface, leading to interfacial voids
- 55. Interfaces Physical Bonding ❖Any bonding involving weak, secondary or van der Waals forces, dipolar interactions and hydrogen bonding can be classified as physical bonding. ❖ The bond energy in such physical bonding is very low, approximately 8–16 kJ/mol.
- 56. Interfaces Chemical Bonding ❖Atomic or molecular transport, by diffusional processes, is involved in chemical bonding. ❖ Solid solution and compound formation may occur at the interface, resulting in a reinforcement/matrix interfacial reaction zone having a certain thickness. This encompasses all types of covalent, ionic, and metallic bonding. ❖ Chemical bonding involves primary forces and the bond energy is in the range of approximately 40–400 kJ/mol.
- 57. Interfaces Types of Chemical Bonding ❖ Dissolution bonding ❖ Reaction bonding Dissolution bonding ❑ In this case, interaction between components occurs at an electronic scale. Because these interactions are of rather short range, it is important that the components come into intimate contact on an atomic scale. ❑This implies that surfaces should be appropriately treated to remove any impurities. Any contamination of fiber surfaces, or entrapped air or gas bubbles at the interface, will hinder the required intimate contact between the components.
- 58. Interfaces Types of Chemical Bonding ❖ Dissolution bonding ❖ Reaction bonding Reaction bonding ❑ In this case, a transport of molecules, atoms, or ions occurs from one or both of the components to the reaction site, that is, the interface. ❑This atomic transport is controlled by diffusional processes. Such a bonding can exist at a variety of interfaces, e.g., glass/polymer, metal/metal, metal/ceramic, or ceramic/ceramic.
- 59. Interfaces Chemical Bonding ❖ Interface zone in a metal matrix composite showing solid solution and intermetallic compound formation