ELASTIC-PLASTIC FRACTURE MECHANICS FOR INITIATION AND PROPAGATION OF NOTCH FATIGUE CRACKS

Initiation and propagation are considered to be controlled by the extent of total plastic shear deformation φ. Crack initiation and crack propagation occur when φ, exceeds a critical threshold value which can be equated to threshold conditions determined from linear elastic fracture mechanics analyses. When a crack is in a plastically deformed zone φt=φp+φe. where φp is the component of φt due to notch bulk plasticity and φe, is the component of φt due to a linear elastic fracture mechanics (LEFM) analysis of the crack tip plasticity field. When cracks initiate at notch roots φt > φth. As the crack propagates in the notch plastic zone the rate of decrease of vp will be different from the rate of increase of φe and it is possible for φt to decrease to a level below φth thereby creating a non-propagating crack.

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