Variation of Fatigue Threshold Limit of Ti-6Al-4Vat Different Stress Ratios

Numerous factors affect the propensity of a metal to fatigue, especially the surface heterogeneities and the surface stress (residual stress) states. S-N curves are generally developed using smooth-polished specimens which give the fatigue limit of the microstructural short crack state. However, the threshold curves (Kitagawa-Takahashi type diagrams) provide a clear insight of the fatigue threshold conditions. These threshold curves correlate the S-N properties and fracture mechanics-based damage tolerant approach to understand the safe fatigue envelop of the materials. Specifically, according to the stress amplitude and surface damage (roughness), the fatigue threshold limits can be investigated. Rotary-bending tests were conducted using samples with four known surface roughness values (0.2 μm, 6 μm, 21 μm and 52 μm) with two subsets, annealed and without-annealed conditions. Residual stress states were examined, and they vary between -313 MPa and +175 MPa. Threshold curves were developed for four different stress ratios, and a clear variation of the threshold stress intensity factor (ΔKth) was identified. The results can be used to understand the vulnerability of fatigue threshold limits and propagation rates based on the surface roughness and residual stress states, thereby to predict and minimise the in-vivo failures of components made of Ti-6Al-4V implant grade material.