Graphs

Elasticity graphs

Normal force-extension graph

Stress-strain graph

Limit of proportionality = Hooke's Law is no longer obeyed beyond this point
Elastic limit = Material is permanently stretched beyond this point

Gradient = Spring constant
Area under graph = Elastic potential energy (strain energy)

Gradient = Young modulus
Area under graph = Elastic potential energy per unit volume

Load/unload graphs

Load-unload graph of rubber