Fatigue

Fatigue is a form of failure that occurs in structures subjected to dynamic stresses over an extended period.

Under these conditions it is possible to fail at stress levels considerably lower than tensile or yield strength for a static load.

Single largest cause of failure in metals; also affects polymers and ceramics.

Common failure in bridges, aircraft and machine components

¨Variation of stress with time that accounts for fatigue failures.

¨The stress may be axial (tension-compression), flexural (bending) or torsional (twisting) in nature.

¨There are 3 fluctuating stress-time modes (a) reversed stress cycle - symmetrical amplitude about a mean zero stress level; (b) repeated stress cycle ?- asymmetrical maxima and minima relative to the zero stress level; (c) variable (random) stress level

Fracture surface with crack initiation at top. Surface shows predominantly dull fibrous texture where rapid failure occurred after crack achieved critical size.

Fatigue failure

1.Crack initiation

2.Crack propagation

3.Final failure