MECHANISM OF FATIGUE FAILURE MECHANISM OF FATIGUE FAILURE Understanding fatigue mechanism will help improve fatigue resist Understanding fatigue mechanism will help improve fatigue resist ance ance There are three stages in fatigue failure: There are three stages in fatigue failure: 1. 1. Crack initiation ( Crack initiation (fatigue always starts at a crack fatigue always starts at a crack) 2. 2. Crack propagation Crack propagation 3. 3. Fast fracture, which can be ductile or brittle ( Fast fracture, which can be ductile or brittle (failure with no warning failure with no warning) The initiation site is very small, extending only about two to f The initiation site is very small, extending only about two to f ive grains around ive grains around the origin. the origin. The location of the initiation is at a stress concentration and The location of the initiation is at a stress concentration and may be may be difficult to distinguish from the succeeding stage of propagatio difficult to distinguish from the succeeding stage of propagation, or crack n, or crack growth. growth. The The crack initiation site is always parallel to the shear stress dir crack initiation site is always parallel to the shear stress direction ection. Stage I Crack Growth Stage I Crack Growth Cyclic stress causes dislocation movements Cyclic stress causes dislocation movements A slip band develops within a grain and then accumulates permane A slip band develops within a grain and then accumulates permanent damage nt damage This damage results in This damage results in intrusions intrusions and and extrusions, extrusions, and a and a crack is initiated crack is initiated Slow propagation of crack along crystal planes with high resolve Slow propagation of crack along crystal planes with high resolved shear stress d shear stress (the order of angstroms per cycle) (the order of angstroms per cycle) Stage II Crack Growth Stage II Crack Growth The crack that leads to fracture is usually The crack that leads to fracture is usually nucleated in a slip nucleated in a slip band on which the maximum shear stress is acting. band on which the maximum shear stress is acting. This is because slip is a shear process so more plastic This is because slip is a shear process so more plastic deformation will occur in slip bands on which the maximum deformation will occur in slip bands on which the maximum shear stress is acting than in other slip bands shear stress is acting than in other slip bands In stage II, the crack propagates very fast normal to the applied stress (crack propagation rates, microns per cycle) The crack will eventually reach a critical size and failure occurs
MECHANISM OF FATIGUE FAILURE
Apr 28, 2023
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