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A flaw in the surface (or just below the surface) of a material. Cracks at the microscopic level can have a large effect on the mechanical properties of a material because they concentrate stress. In crack growth there is an increase in energy due to new surfaces being generated but this is more than compensated for by a reduction in energy due to stress relief. This means that if there is sufficient energy stored round the crack it is energetically favourable for the crack to grow, and hence for the material to be ruptured.

Subjects: Physics.

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