Effects of Mechanical Surface Treatment on Fatigue Failure in Ti-6Al-4V : Role of Residual Stresses and Foreign-object Damage

Mechanical surface treatments can play a major role in the enhancement of fatigue life in metallic materials. Deep rolling and laser shock peening processes, for example, can induce several favorable effects, including deep cases of compressive residual stresses, work hardening and low surface roughness. Since the fatigue life of smooth specimens is invariably dominated by crack-initiation processes, the presence of high near-surface dislocation densities can improve fatigue lifetimes substantially, even if residual (macro) stresses are not stable during fatigue. Conversely, in engineering applications such as turbine engine airfoils, the surfaces are very often notched or damaged, for instance by foreign-object impacts, which severely reduces the crack-initiation phase. In such cases, the fatigue life is dominated by crack propagation and a stable compressive residual stress state is crucial for enhancing fatigue life. In the present work the effect of mechanical surface treatments on the fatigue behavior and residual stress state of smooth and on foreign-object damaged specimens of the Ti-alloy Ti-6Al-4V is investigated. The results show that deep rolling can substantially improve the fatigue life of smooth specimens at elevated temperature (450°C) due to a higher resistance against fatigue crack initiation. For laser shock peened and foreign-object damaged specimens, information on the residual stress state in and around the foreign-object impacts yields a deeper insight into the nature of local fatigue damage and microcrack propagation. The results indicate that laser shock peening process parameters have to be very carefully chosen in order to diminish the effects of foreign object damage.