Effects of stress-relieving slots and friction on fretting fatigue behavior of titanium alloys

Fretting alters the stress/strain fields in the contact region between two bodies, due to micro-slip, which decreases the fatigue resistance of materials. This study investigates the effect of stress-relieving slots (U-shaped, V-shaped, and circular holes), machined in flat pads, on the fretting fatigue behavior of Ti-6Al-4V alloy. Conventional fretting fatigue model, involving a contact between a flat pad with rounded edges and a fatigue specimen loaded by a cyclic axial load, was used. The finite element analysis software ABAQUS were used to calculate the stress/strain and displacement distribution in the contact region of fatigue specimen. The fretting fatigue parameters were further analyzed to evaluate the effectiveness of the slots in improving the fretting fatigue resistance. All the parameters decreased due to the presence of the slots and that the U-slot was found to be most effective in improving the fretting fatigue resistance. The location and size of the U-slot was further optimized by the response surface method. The slot length has the dominant effect in reducing the fretting parameters followed by slot height. The effect of slot width can be neglected. With the optimum shape design, the fatigue parameter can be reduced as much as one-third of initial design. The analytical results were verified by the experiments carried out at AFIT.; In fretting, the coefficient of friction (CoF) varies during the fretting process due to the cyclic micro-slip. A variable CoF model which characterizes the distribution of local CoF (q/p) in the contact zone was developed and investigated. A close-form equation was derived to determine the average CoF for the case when no axial load is applied on the specimen and was verified by FEM. For the case when an axial load is applied, the average CoF was determined by FEM. The stress distribution and slip amplitude for the variable CoF models were investigated and compared with the constant CoF models.