Summary

This TLP covers the following points:

1. In the linear elastic region, where the shape change is recoverable and stress is proportional to strain (the material obeys Hooke’s law), the Young’s modulus, or elastic modulus, or modulus of elasticity, or stiffness E is defined as the ratio between stress and strain. The value of E measures the resistance to elastic deformation.
2. Yield stress \( \sigma_\rm{y} \) is the maximum stress that can be applied before the material begins to change shape permanently. This measures the resistance to plastic deformation.
3. Ultimate tensile stress, or UTS \( \sigma_\rm{u} \) is the maximum stress that a material can withstand while being stretched or pulled before breaking. This measures resistance to failure and also indicates the onset of necking.
4. Ductility (in %EL), or failure strain εf is the nominal strain at failure. This measures the ability of a material to be drawn or plastically deformed without fracture.
5. Hardness \( H_\rm{v} \) is a measure of the resistance to localised plastic deformation induced by either mechanical indentation or abrasion. This comes from a combination of high \( \sigma_\rm{y} \) and high \( \sigma_\rm{u} \).
6. Toughness is the amount of energy per unit volume that a material can absorb before fracture. This can be measured by the area under stress–strain curve. This indicates resistance to fracture by undergoing plastic deformation.
7. These mechanical properties can be investigated usingr a tensile test. A load displacement curve is obtained, which can be transformed into geometrically independent stress strain curve for a more valid comparison between different materials.
8. Viscoelasticity is time dependent elastic deformation.
9. Creep is the time dependent plastic deformation under stress lower than the yield stress.