Design - BEng2 - DSGN 215

Failure Mechanisms

The following types of failure are of interest to Mechanical Engineers:

Plastic Deformation / Fracture – Due to excessive load.

Excessive elastic deformation – Due to the structure being insufficiently stiff.

Plastic collapse, buckling – Due to instability.

Fracture – In a brittle material, or a normally ductile material that is behaving in a brittle manner (eg. because it is operating below its ductile / brittle transition temperature) - Due to load causing a stress intensity exceeding the critical stress intensity factor, or fracture toughness, of the component that contains a defect.

Fracture (fatigue failure) - typically in low strength metals - Due to cyclic load causing a crack to grow, usually from a defect at or close to the surface, reducing the area of material available to support the load, so the stress generated by the maximum load exceeds the UTS of the material.

Fracture (fatigue failure) - typically in high strength metals - Due to cyclic load causing a crack to grow, usually from a defect at or close to the surface, so the maximum stress intensity at the crack tip exceeds the fracture toughness of the material.

Further reading - 'Case Studies in Engineering Design', by C Matthews.

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David J Grieve, updated: 27th October 2004, original: July 1998.