Lecture 2.pptx
- 1. Mechanical Properties of Interest in Biomaterials Applications Material Technology Course Code: MET-403 Credit Hour: 3
- 2. Mechanical Properties of Material The mechanical behavior of a material reflect the relationship between its response or deformation to an applied load or force. those properties that influence the material's reaction to applied loads. Mechanical properties are used to determine how a material would behave in a given application, and are helpful during the material selection and coating specification process. Material Technology Course Code: MET-403 Credit Hour: 3
- 3. What is mechanical properties of biological materials? Biological tissues are viscoelastic materials; their behavior is both viscous, meaning time- and history-dependent, as well as elastic. A viscoelastic material possesses characteristics of stress-relaxation, creep, strain-rate sensitivity, hysteresis, and many others. The tensile strength, yield strength, elastic modulus, fatigue, strength, wear resistance, corrosion, creep, toughness, and hardness are some of the most important properties of biomaterials that should be carefully studied and evaluated before implantation. Material Technology Course Code: MET-403 Credit Hour: 3
- 4. Why are mechanical properties important for biomaterials? It is clinically important because it indicates the selected biomaterial has similar deformable properties with the material it is going to replace. These force-bearing materials require high elastic modulus with low deflection. As the elastic modulus of material increases, fracture resistance decreases. It is desirable that the biomaterial elastic modulus is similar to that of bone. This is because if it is more than bone's elastic modulus then the load is borne by the material only; while the load is borne by bone only if it is less than bone material. Material Technology Course Code: MET-403 Credit Hour: 3
- 5. Stress Stress is defined as the internal resistance of the material against deformation. Material Technology Course Code: MET-403 Credit Hour: 3 Lets Start
- 6. Strain A strain is a change in shape due to deformation per the original shape. Material Technology Course Code: MET-403 Credit Hour: 3
- 7. Tensile/Compression Stress & Strain Material Technology Course Code: MET-403 Credit Hour: 3
- 8. Shear Stress & Strain Material Technology Course Code: MET-403 Credit Hour: 3
- 9. Volumetric Stress & Strain Material Technology Course Code: MET-403 Credit Hour: 3
- 10. Stress To Strain Ratio The stress-to-strain ratio is a constant and a property of the material within the elastic limit. This ratio is known as modulus of elasticity (E) or Young's modulus. E is a measure of the stiffness of the material. The unit of the modulus of elasticity is the gigapascal (Gpa). Material Technology Course Code: MET-403 Credit Hour: 3
- 11. Stress To Strain Ratio A stress on a rubber band produces larger strain (deformation) than the same stress on a steel band of the same dimensions. Due to the fact that the elastic modulus of steel is two times greater than the elastic modulus of rubber. a ductile material will therefore have a higher modulus of toughness than a brittle material with the same yield strength. A ductile material can withstand much more plastic strain than a brittle material. The highest known Young's modulus value is that of diamond, which is both the hardest material known and has the highest elastic modulus known of ~ 1210 Gpa. Material Technology Course Code: MET-403 Credit Hour: 3
- 12. Stress To Strain Ratio Material Technology Course Code: MET-403 Credit Hour: 3 Young Modulus: This is the ratio of normal stress to normal strain Shear Modulus: This is the ratio of shear stress to shear strain bulk Modulus: This is the ratio of volumetric stress to volumetric strain
- 13. Stress-Strain Curve A stress-strain curve is a graphical way to show the reaction of a material when a load is applied. It shows a comparison between stress and strain. The stress-strain diagram provides a graphical measurement of the strength and elasticity of the material The behaviour of the materials can be studied with the help of the stress-strain diagram, which makes it easy to understand the application of these materials Material Technology Course Code: MET-403 Credit Hour: 3
- 14. Stress-Strain Curve Material Technology Course Code: MET-403 Credit Hour: 3 Stress Strain Curve for Ductile Material
- 15. Stress-Strain Curve Proportional Limit (From o to A) According to hook’s law proportional limit is the limit where stress is directly proportion to strain. The stress strain curve is a straight line within the proportional limit. A material young modulus of elasticity is constant within the proportional limit. Elastic Limit (From o to B) The elastic limit for a material is the limit beyond which the material will not come to its original shape when we remove the external force. The material exhibits the elastic properties from A to B (yield point) in the stress-strain curve. If the external load goes beyond the elastic limit, the material will not come to its original shape. Material Technology Course Code: MET-403 Credit Hour: 3
- 16. Stress-Strain Curve Upper Yield Point (Point B) Beyond the elastic material, ductile material exhibits the plastic properties. At the upper yield point, the material require the maximum stress to initiate the plastic deformation inside the material. The strength of the material corresponding to point B is known as yield strength. Lower Yield Point (Point C) After point B, the material length will increase with a small increase in tensile load. In other words, at lower yield point a minimum load is required to exhibits the plastic deformation in the material. Material Technology Course Code: MET-403 Credit Hour: 3
- 17. Stress-Strain Curve Ultimate Tensile Strength (Point D) A material has the ultimate tensile strength at point D in stress-strain diagram. The ultimate tensile strength of a material is maximum stress material can withstand before breaking. After point D, necking start inside the material. Rapture/Fracture/Breaking Strength (Point E) Point E is the point where material fracture of breaks. Stress at point- E is known as breaking strength of material. Material Technology Course Code: MET-403 Credit Hour: 3
- 18. Stress-Strain diagram comparison for Ductile, Brittle and Plastic Materials We can classify most material available in the market into three categories. 1. Ductile Materials 2. Brittle Materials 3. Plastic Materials Material Technology Course Code: MET-403 Credit Hour: 3
- 19. Stress-Strain diagram comparison for Ductile, Brittle and Plastic Materials Ductile Materials A ductile material has different point for ultimate stress and fracture on stress-strain diagram because they have elastic and plastic deformation. Copper, Aluminum and steel are the example of ductile materials. Ductile Material Behavior: Steel sheets regain their initial position up to the elastic limit during the sheet metal bending process. But after the elastic limit, the material start showing plastic behavior and does not come to it’s initial position. If we continue applying force beyond this elastic limit, the material will break at the fracture point. Material Technology Course Code: MET-403 Credit Hour: 3
- 20. Stress-Strain diagram comparison for Ductile, Brittle and Plastic Materials Brittle Materials Brittle material break with the small elastic deformation and without the plastic deformation due to the external forces. In other words, a brittle material’s elastic limit, yield strength, ultimate tensile strength, and breaking strength are equal. Brittle material absorbs relatively little energy before fracture. Ceramic, Wood, Glass, PMMA, Graphite, and cast iron are example of brittle materials. Brittle Material Behavior: Brittle material such as pencil or glass break suddenly with a snapping sound and a small deformation. Material Technology Course Code: MET-403 Credit Hour: 3
- 21. Stress-Strain diagram comparison for Ductile, Brittle and Plastic Materials Plastic Materials Similar to the ductile materials, plastic material also exhibit the elastic properties up to the proportional limit. But plastic material requires very little stress (Compare to the ductile materials) to produce deformation. Plastic materials do not show any work hardening during the plastic deformation. Plastic Material Behavior: If we apply an external force to bend a plastic spoon. After the elastic limit, the spoon will not retain its original shape. Material Technology Course Code: MET-403 Credit Hour: 3
- 22. Stress-Strain diagram comparison for Ductile, Brittle and Plastic Materials Material Technology Course Code: MET-403 Credit Hour: 3
- 23. Stress-Strain diagram comparison for Ductile, Brittle and Plastic Materials Material Technology Course Code: MET-403 Credit Hour: 3
- 24. Mechanical properties Stiffness: Stiffness relates to how a component bends under load while still returning to its original shape once the load is removed. Stiffness is used to indicate whether a material is compliant (soft) or rigid (hard). Objects with a high stiffness will resist changes in shape when being acted on by a physical force. For example, loose, wet clay has low stiffness, changing shape with just a few pounds of pressure. The stiffness of aluminum is considerably stiffer than wet clay. In biology, stiffness has been used to collectively represent mechanical properties of a biological substrate. Substrate stiffness depends on scaffold molecular- constituent-structure interaction. Material Technology Course Code: MET-403 Credit Hour: 3
- 25. Mechanical properties Hardness: Hardness is the resistance of a material to localised plastic deformation. Hardness is defined as the force per unit area of indentation or penetration. Hardness is one of the most important parameters for comparing properties of materials. Hardness is used for finding the suitability of the clinical use of biomaterials Biomaterial hardness is desirable as equal to bone hardness. If higher than the biomaterial, then it penetrates in the bone. Biomaterials sample are very small, therefore micro- and nano-scale hardness tests (Diamond Knoop and Vickers indenters) are used Material Technology Course Code: MET-403 Credit Hour: 3
- 26. Mechanical properties Toughness: Toughness is a fundamental material property measuring the ability of a material to absorb energy and withstand shock up to fracture; that is, the ability to absorb energy in the plastic range. Toughness is helpful to evaluate the serviceability, performance and long term clinical success of biomaterials. High fracture toughness biomaterial improved clinical performance and reliability as compare to low fracture toughness. The enamel (paper-thin) that covers your teeth is much stronger than your bones. In fact, the only substance on earth that is stronger than enamel is diamond. Material Technology Course Code: MET-403 Credit Hour: 3
- 27. Mechanical properties Strength: the strength of a material is its ability to withstand an applied load without failure or plastic deformation. Strength of biomaterials (bioceramics) is an important mechanical property because they are brittle. In brittle materials like bioceramics, cracks easily propagate when the material is subject to tensile loading, unlike compressive loading. Biomaterials with high strength show the resistance against crack propagation. Fatigue: Fatigue is defined as a process of progressive localized plastic deformation occurring in a material subjected to cyclic stresses and strains at high stress concentration locations that may culminate in cracks or complete fracture after a sufficient number of fluctuations. Fatigue refers to a mode of failure that results from repeated stress at magnitudes lower than that required to cause failure in a single application. Material Technology Course Code: MET-403 Credit Hour: 3
- 28. Mechanical properties Corrosion: Corrosion is the deterioration and loss of a material and its critical properties due to chemical, electrochemical and other reactions of the exposed material surface with the surrounding environment. Corrosion of metallic biomaterials causes the loss of their structural integrity and surface function. It accelerates their fatigue, fretting fatigue and wear and, conversly, such damage accelerates the corrosion. Rusting of iron, or the forming of a brown flaky material on iron objects when exposed to moist air, is the most common example of metal corrosion. Corrosion is one of the major processes that cause problems when metals and alloys are used as implants in the body. Corrosion of implants in the aqueous medium of body fluids takes place via electrochemical reactions. Material Technology Course Code: MET-403 Credit Hour: 3
- 29. Mechanical properties Creep: Creep may be defined as a time-dependent deformation at elevated temperature and constant stress. It follows, then, that a failure from such a condition is referred to as a creep failure or, occasionally, a stress rupture. The temperature at which creep begins depends on the alloy composition. Creep in bone is a complex phenomenon and varies with type of loading and local mechanical properties. One manifestation of creep damage is elderly people's decreased stature as a result of skeletal creep damage. Material Technology Course Code: MET-403 Credit Hour: 3
- 30. A stress on a rubber band produces larger strain (deformation) than the same stress on a steel band of the same dimensions Due to the fact that the elastic modulus of steel is two times greater than the elastic modulus of rubber.