(f) elastic and plastic deformations of materials.

The idea

Elastic deformation is reversible: when the deforming force is removed, the material returns to its original shape and size. Atoms are displaced slightly from equilibrium positions but return, and the material obeys Hooke's law up to the elastic limit, where stress is proportional to strain. Plastic deformation is permanent: the force exceeds the elastic limit, atomic planes slip past one another, and the material does not return to its original shape when unloaded. A wire stretched beyond its elastic limit remains longer; a ductile metal may neck and eventually fracture. The elastic limit, yield point and breaking point mark transitions on a stress–strain graph. Energy stored elastically is recoverable; energy used in plastic deformation is dissipated, often as heat.

What earns the marks

• Defines elastic deformation as reversible, with the material returning to its original dimensions when the load is removed.
• Defines plastic deformation as permanent, with the material retaining a changed shape after unloading.
• Links elastic behaviour to Hooke's law and the elastic limit, and plastic behaviour to atomic plane slip.
• Interprets the elastic limit, yield point and breaking point on a stress–strain graph.
• Distinguishes recoverable elastic strain energy from energy dissipated during plastic deformation.

Common mistakes and how to put them right

• Believing elastic deformation means the material does not change shape at all.

  Put it right: It changes shape but returns to the original shape when unloaded.

• Thinking plastic deformation occurs before the elastic limit.

  Put it right: Plastic deformation begins once the elastic limit is exceeded.

• Assuming all energy put into deforming a material is recovered.

  Put it right: Energy used in plastic deformation is dissipated, often as heat.

• Confusing the elastic limit with the breaking point.

  Put it right: The elastic limit is where permanent deformation starts, while the breaking point is where the material fractures.

Exam tips

• Use the words reversible and permanent explicitly when defining elastic and plastic deformation.
• Refer to a stress–strain graph and mark the elastic limit, yield point and breaking point to support your explanation.
• When discussing energy, state whether it is stored elastically and recovered or dissipated during plastic flow.