Elasticity

Deformation is temporary. Returns to its original shape when load is released.

Plasticity

Opposite to elasticity. Deformation is permanent.

Linear elastic materials

When elastic deformation portion in stress-strain diagram is a straight line.

Young’s modulus

Aka. alastic modulus. Can be thought of as stiffness.

$$\text{Young's modulus }E=\frac{\text{stress}}{\text{strain}}=\frac{\sigma}{\epsilon}$$

Nonlinear elastic materials

When elastic deformation portion in stress-strain diagram is not a straight line.

Secant modulus

Equal to the tangent of the line connecting a point in the stress-strain diagram and the origin.

Tangent modulus

Equal to the instantaneous tangent on a point in the stress-strain diagram.