عملکرد فرسودگی چرخه بالا
Abstract
1-Introduction
2-Material & Experimental Metods
3-Shot Peening
4-Results
5-Conclusions
References
Abstract
Shot peening of titanium alloys is known to enhance the HCF strength 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 initiation 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 initiation 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. Fatigue life of shot peened rotating bending hour-glass shaped and flat bending fatigue specimens was studied and compared to an electrolytically polished reference. 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 initiation 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 strength 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 orientated 90 degrees to the cracking direction seems to be responsible for the increased fatigue strength. 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.
Introduction
The α+β Ti-6Al-4V is widely used in aerospace industry, especially aero engine components like blades, disks, drums or state of the art blisks. Shot peening is a common and broadly used post-surface treatment that increases the overall fatigue performance and/or eliminates the detrimental effects or scattering caused by the manufacturing processes. Shot peening induces compressive residual stresses in near-surface regions that can drastically retard the growth rates of fatigue cracks [1]. In addition, the induced high dislocation densities may increase the resistance to fatigue crack initiation while the typically accompanied high surface roughness can have the opposite effect. Fatigue performance of metallic parts can be markedly influenced by coverage degree in shot peening (SP) [2-3], [4]. Low coverage degrees (<< 100 %) led to fatigue lives even lower than in the untreated electropolished reference condition [5]. These results were explained by residual compressive stresses being not fully developed at low coverage degrees and, therefore, were not able to compensate the early fatigue crack initiation caused by the highly induced surface roughness from the shot peening process. The fully developed residual compressive stresses in minimum 100% coverage were then seen to drastically reduce micro-crack growth by which the early fatigue crack initiation was much overcompensated leading to marked enhancements in the overall fatigue life.