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Fatigue damage mechanisms and damage evolution near cyclically loaded edges

Ebner R., Gruber P., Ecker W., Kolednik O., Krobath O., Jesner G.

Bulletin of the Polish Academy of Sciences. Technical Sciences

2010

Vol. 58, nr 2

267-279

A combined experimental and numerical approach was applied to develop a basic understanding of the fatigue damage process taking place at edges exposed to cyclic mechanical loads. A recently developed cyclic edge-loading test was used in order to simulate the fatigue loading of the edges of manufacturing tools and to study the microscopic damage mechanisms. Accompanying finite element calculations were performed to provide a better understanding of the loading conditions at edges subjected to cyclic mechanical loads. A comparison of the numerical simulation with the experimental results revealed good accordance. Main results of the investigations are the distribution of plastic strains and their evolution with increasing number of cycles, the distribution of the residual stresses, the localisation and the evolution of damage at the microscale (microcracks and voids), and the localisation and growth of fatigue cracks. Micro-damage develops in the entire plastically deformed region. Fatigue crack nucleation was mainly found in deformation bands and fatigue crack growth was only observed near the transition region between the extensively and the slightly plastically deformed zone not at the loaded area but at the side area. The reason for that phenomenon is the formation of tensile residual stresses in this region which is favouring fatigue crack growth.

Mechanical surface treatments for improving fatique behavior in titanium alloys

Ludian T., Wagner L.

Advances in Materials Science

2008

Vol. 8, nr 2(16)

44-52

Mechanical surface treatments such as shot peening or ball-burnishing induce high dislocation densities and residual compressive stresses in near-surface regions. In addition, the surface roughness is changed. Micro-hardness and residual stress-depth profiles are evaluated in Ti-6Al-4V as a function of the Almes intensity being the main process parameter in shot peening. In the finite life regime, residual compressive stresses are shown to drastically increase the fatigue life by retarding crack growth from thesurface into the interior. However, in the HCF regime a shift in crack nucleation site occurs from the surface to subsurface regions where residual tensile stresses balance the outer compressive stress field. Therefore, the tensile mean stress sensitivity of the fatigue strength which can largely vary in Ti-6Al-4V needs to be taken into account in order to understand the observed differences in HCF responds to shot peening.