Introduction

In response to an applied force, a material might behave in different ways.

For example, a spring extends when there is a load suspended from it and after the load is removed, it returns to the original shape; this behaviour is known as elastic deformation.

Bending a paper clip and crimping cause permanent shape changes; this behaviour is known as plastic deformation.

Breaking a glass and snapping a piece of dry spaghetti are examples of fracture.

Let us define these behaviours:

• Elastic deformation: a shape change that is recoverable once the load is removed.
• Plastic deformation: a shape change that is permanent; materials do not return to the original shape after the load is removed.
• Fracture: the failure of the material by crack propagation.

These are the most common types of mechanical behaviour observed in materials. However, some exceptions do exist. For example, when placed on a flat surface, silly putty balls will flatten out over time. This response to the load (its own weight) is not instantaneous and therefore time-dependent. Materials like silly putty have a mechanical property called viscoelasticity, which means that they exhibit both viscous and elastic characteristics. Other types of time-dependent behaviour include creep for gradual plastic deformation and fatigue for gradual failure.