We don’t want engineering materials to fail because they can cause loss of life, economic loss or a loss of products or services. The usual causes of material failure are incorrect materials selection, incorrect processing, incorrect manufacturing procedures, inadequate design or incorrect use. Fracture is the separation of a body into two or more pieces as a result of an imposed stress.

We don’t want engineering materials to fail because they can cause loss of life, economic loss or a loss of products or services.

In a tensile test (where the load is in tension) a material can either, behave in a highly ductile manner:

Large plastic deformation

Polymers and very soft metals deform this way

Many applications exist where a material is subjected to a repeated cyclic stress. These materials are found to fail after a large number of applications of the stress even though the cyclic stress that is applied is below their yield strength. This mode of failure is termed fatigue.

Fracture mechanics is the discipline concerned with analysing the failure of materials containing cracks and flaws. With fracture mechanics we can determine the stress level at which cracks of known size can propagate through the material to give failure.

When engineers design parts for service, they do so with the mechanical properties of the material in mind. In other words, materials are not used in applications where they will be subject to stresses above their yield strength. So why then do materials often fail? The answer lies in stress concentrations.