Fatigue and Stress Concentration

Static or quasi-static loading is rarely observed in modern engineering practice, making it essential for the designer to address themselves to the implications of repeated loads, fluctuating loads, and/or rapidly applied loads. By far, the majority of engineering design projects involves products subjected to fluctuating or cyclic loads. Such loading induces fluctuating or cyclic stresses that often result in failure by fatigue. Fatigue failure investigations over the years have led to the observation that the fatigue process actually embraces two aspects of cyclic stressing or straining that are significantly different in character, and in each of which failure is probably produced by different physical mechanisms.

One aspect of cyclic loading is that for which significant plastic strain occurs during each cycle. This aspect is associated with high loads and short lives, or low numbers of cycles to produce fatigue failure, and is commonly referred to as low-cyclc fatigue. The other aspect of cyclic loading is that for which the strain cycles arc largely confined to the elastic range. This aspect is associated with lower loads and long lives, or high numbers of cycles to produce fatigue failure, and is commonly referred to as high-cycle fatigue.

Low-cycle fatigue is typically associated with cycle lives from 1 up to about 104 or 105 cycles. Fatigue may be characterized as a progressive failure phenomenon that proceeds by the initiation and propagation of cracks to an unstable size. Although there is not complete agreement on the microscopic details of the initiation and propagation of the cracks, processes of reversed slip and dislocation interaction appear to producc fatigue nuclei from which cracks may grow. The crack length reaches a critical dimension and one additional cycle then causes complete failure. The final failure region will typically show evidence of deformation produced just prior to final separation. For ductile materials the final fracture area often appears as a shear lip produced by crack propagation along the planes of maximum shear.

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