Factors influences fatigue crack propagation life

The modeling of interaction effects requires consideration of crack-tip plasticity and its subsequent influence. In metals of all types, cracks will remain closed or partially closed for a portion of the applied cyclic load as a consequence of plastically deformed material left in the wake of the growing crack. Under cyclic loading, crack growth will occur during the loading portion of the cycle. Given that a plastic zone exists at the crack tip prior to crack extension, as the material at the crack tip separates, the newly formed crack surfaces will exhibit a layer of plastically deformed material along the newly formed crack faces. Subsequent unloading will compress this plastically deformed material, closing the crack while the applied stress remains tensile. This phenomenon is known as plasticity-induced fatigue crack closure and was first discussed by Ether.” Discussion to this point has been limited to the growth of thmugb-thickness cracks under mode I loading. While mode I loading is often dominant, under the most general circum-stances the applied cyclic loads will generate stress intensity factor ranges AK  AK, , and OK , at the crack tip, and mixed-mode fatigue crack growth must be considered. Modeling methodologies for mixed-mode fatigue c rack growth are discussed in Refs. 34 and 35. In addition, fatigue cracks in machine elements and structures are often not through-thickness cracks but rather surface cracks that extend partially through the thickness. Such surface cracks are often semielliptical in shape and the analysis of these cracks is considerably more complicated. Information regarding surface cracks may be found in Refs. 4 and 29
Research has suggested that when fatigue cracks are small, crack growth rates are larger than would be predicted using Eq. (15) for a given 4K36 Small-crack behavior is often important, as a significant portion of the fatigue life may be spent in the small-crack regime
The fatigue crack propagation life will also be influenced by the presence of residual stresses such as might exist as a consequence of welding, heat treatment, carburizing, grinding, or shotpeening. Compressive residual stresses are beneficial, decreasing the rate of fatigue crack growth and increasing propagation life. While approximate methodologies exist for incorporating the effects of residual stress within fatigue crack growth predictions,” residual stress distributions are often difficult to characterize. Reasonable design estimates for the fatigue crack propagation life may be obtained using Eq. (17). However, the many uncertainties typically associated with fatigue life predictions emphasize the essential requirement to conduct full-scale fatigue tests to provide acceptable reliabtily.

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