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Science Materials Science & Engineering

Crack theory

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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.

Introduction

The cost of crack failures may result in loss of human life, injury and increased costs. It is critically important to design against failure.

Ductile Fracture

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

Brittle fracture

Brittle fractures are characterised as having little or no plastic deformation prior to failure.

Fatigue failure

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

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.

Crack elimination

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.