Material | Young's modulus [\(GPa\)] |
---|---|
Mild Steel | 210 |
Copper | 120 |
Bone | 18 |
Plastic | 2 |
Rubber | 0.02 |
Yield Strength
Perfect plastic or ideal plastic: well-defined \( \sigma_Y \), stress plateau up to failure. Some materials (e.g. mild steel) have two yield points (stress plateau at \( \sigma_{YL} \)). Most ductile metals do not have a stress plateau; yield strength \( \sigma_{YS} \) is then defined by the 0.002 (0.2\%) offset method.
Strain HardeningAtoms rearrange in plastic region of ductile materials when a higher stress is sustained. Plastic strain remains after unloading as permanent set, resulting in permanent deformation. Reloading is linear elastic up to the new, higher yield stress (at A') and a reduced ductility.
Ultimate StrengthThe ultimate strength (\( \sigma_u \)) is the maximum stress the material can withstand.
NeckingAfter ultimate stress (\( \sigma_u < \sigma \)), the middle of the material elongates before failure.
FailureAlso called fracture or rupture stress (\( \sigma_f \)) is the stress at the point of failure for the material. Brittle and Ductile materials fail differently.
Ductile materials generally fail in shear. There is a large region of plastic deformation before failure (fracture) at higher strain and necking.
Brittle materials are weaker in tension than shear. There is a small plastic region between yield and failure (fracture) and no necking.
Note the difference between engineering and true stress/strain diagrams: ultimate stress is a consequence of necking, and the true maximum is the true fracture stress.
Example: Concrete is a brittle material.Heads up!
Strain Energy builds on this content in Engineering Materals.
Deformation does work on the material: equal to internal strain energy (by energy conservation).
Heads up!
Fatigue builds on this content in Engineering Materals and Mechanical Design.