Modes of Failure

A mechanical component is called failed if it is unable to perform its desired functions satisfactorily. In static loading, the failure can be in any one of the following modes:

1. Elastic Deflection

A member of structure, such as shafts, beam, column, can have elastic deflection beyond the allowable limits. In such cases, the material does not fail but it is unable to function. Elastic deflection vanishes after removal of loads from the member. This mode of failure is concerned with stability of the machine elements, where modulus of elasticity and rigidity of the material play a key role.

2. Yielding

A component made of ductile material loses its engineering usefulness due to significant amount of plastic deformation after exceeding the yield stress. Only ductile materials can yield significantly. Presence of cracks in a ductile material can cause sudden fracture at stress levels below the yield strength, even under static loads.

3. Fracture

Components made of brittle materials fail due to sudden fracture, without any plastic deformation or significant change in shape. In such cases, ultimate strength of the material is of primary importance.