Influence Of Shot Peening Induced Surface Roughness, Residual Macrostresses and Dislocation Density on the Elevated Temperatu

Author:  Gray, Wagner, Lutjering
Source:  Conf Proc: ICSP-3, (p.447-458)
Doc ID:  1987049
Year of Publication:  1987
Abstract:  
The fatigue behavior of engineering components is known to be strongly dependent on the properties of the surface and near surface regions. Mechanical surface treatments such as shot peening induce a high dislocation density in near surface regions, residual stresses are developed and the surface topography is changed. Without a change in the bulk properties of the material, the fatigue life can significantly increase due to an improved resistance to fatigue crack nucleation and propagation of small surface cracks. Since the shot peening procedure is a fatigue process to the surface layer it was argued that microcracks may already nucleate during shot peening. Even if cracks are not formed during shot peening the fatigue life of shot peened parts will nevertheless be crack propagation controlled because of the remaining defects, dents and overlaps in the surface layer which lead to early crack nucleation. Therefore, the fatigue strength of shot peened specimens should correlate with the threshold value Delta K(th) for small crack propagation. Previous work has shown that the beneficial effect of shot peening on the fatigue behavior of high-strength titanium alloys at room temperature mainly derives from a marked retardation of the growth rate of small surface cracks propagating within the regions of induced residual compressive stresses. Unlike the compressive residual stresses, the high dislocation densities were found to even accelerate the growth rate of small surface cracks. Obviously, the effect of the last parameter the surface roughness can be very pronounced since the fatigue behavior can change from crack propagation controlled (rough surface condition) to crack nucleation controlled (smooth surface condition) if additional polishing is used after shot peening. In that case the fatigue strength is more determined by surface layer yield stress and slip length. By separating the individual effects of residual stresses, dislocation density, and surface roughness on fatigue crack nucleation and propagation of small surface cracks at various temperatures the present study was aimed to contribute to the understanding of the mechanisms by which shot peening can improve or worsen the elevated temperature fatigue behavior of titanium alloys.