High cycle fatigue behavior of Ti–6Al–4V with simulated foreign object damage

Abstract

This study investigated the high cycle fatigue (HCF) behavior of a titanium alloy, Ti–6Al–4V, after being subjected to simulated foreign object damage (FOD). For this purpose, rectangular specimens were damaged to various depths by steel indenters of different diameters under quasi-static loading to simulate damage found in turbine engine airfoils during service. The fatigue strength of these specimens for 10 7 cycles was measured. Finite element analysis was used to compute the deformation and stress state created by the simulated FOD and to explain the relationship between damage and fatigue strength. The examination of damage mechanisms on the fracture surface near FOD showed a region of either macro bands (in the X-shape) of intense plasticity (i.e. intensely deformed material) having strain levels of 15–20%, shear bands, or shear stress-induced cracks depending upon the indentation depth and indenter diameter. The depth from the specimen edge up to the center of these X-shaped macro bands on the fracture surface had a simple empirical relationship with the reduction in fatigue strength. The residual stresses, introduced from the FOD, caused the effective stress ratio or mean stress in the vicinity of FOD to be different than that applied to the specimen. These modified stress states provided the explanation for reduction in the fatigue strength of the material with FOD.