Effects of shot peening on the HCF performance of Ti-6Al-4V by variation of coverage and impact angleThursday (09.11.2017) 09:55 - 10:15 Part of:
Shot peening of titanium alloys is known to enhance the HCF performance by inducing in near-surface regions residual compressive stresses which can drastically retard the growth rates of surface cracks. In addition, the induced high dislocation densities may increase the resistance to fatigue crack nucleation while the typically accompanied high surface roughness has the opposite effect.
The present work aimed at studying the effect of coverage and peening angle effects in shot peening on fatigue crack nucleation and micro-crack growth on Ti-6Al-4V.
The coverage was varied to a wide extent ranging from 20% to 1200% and the peening angle from 90 to 30 degrees. Residual stresses were evaluated by the incremental hole drilling method. Fatigue performance of shot peened rotating bending hour-glass shaped and flat bending fatigue specimens was studied and compared to an electrolytically polished.
The results indicate that low (20%) coverage peening leads to a loss in HCF strength presumably, caused by insufficient residual compressive stress fields which cannot compensate the early crack nucleation caused by the high roughness. In contrast, full (100%) up to a high (1200%) coverage was found to result in a marked increase in HCF strength.
The variation of peening angles resulted in a significant beneficial effect on the fatigue performance of the flat bending fatigue samples in direction to increasingly flat angles and high coverages (1200%) at constant Almen intensities of 0,20 mmA. Beside the deeper compressive stress layer at flat impact angles especially the increased dislocation density and the creation of a material texture with an orientation 90 degrees to the cracking direction seems to be responsible for the increased fatigue performance.
These results are even more interesting as those peened surfaces resulted in the highest roughness values and a topography with a starting waviness on the surface.
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