Failure Mechanisms

· The lifetime before failure can be divided into periods of crack initiation, crack propagation and final fast fracture.
· Crack initiation occurs at some stress concentration site such as a surface flaw. At low stress levels most of the lifetime is associated with crack initiation.

· Crack propagation is initially slow but its rate increases as the crack size increases.
· Final fracture corresponds to the crack having grown to the critical size for rapid fracture, the material failing in the next stress cycle.
· N _f = N _i+ N _p At high stress levels, N_p > N_i , and as much as 90% of the fatigue life may be due to crack propagation.