Multiaxial Fatigue Evaluation of Ti-6Al-4V under Simulated Mission Histories

The authors have previously evaluated over twenty fatigue damage parameters for application to titanium alloys (Ti-6Al-4V) subjected to constant amplitude loadings. Equivalent stress based parameters and critical plane parameters were considered for both proportional and non-proportional multiaxial load paths. The evaluations were based on the ability of the models to correlate tension/torsion fatigue data to a uniaxial baseline. Only a few models displayed good agreement for the range of experimental data. In the present study, the extension of the multiaxial fatigue parameters to simple variable load histories is considered. Several simulated mission histories were applied to solid round bars of Ti-6Al-4V under tension/torsion loading. The mission histories consisted of a single highly damaging LCF cycle (often non-proportional), coupled with 5 to 50 small, low-damage HCF cycles. Each specimen was subjected to an individual mission history applied repeatedly until failure. The differences between the HCF and LCF cycles incorporated both cycle “shape ” (or path) and amplitude of loading. Comparisons between experimental and predicted fatigue lives will be presented. Specific attention will be given to the interaction effects between the HCF and LCF cycles. Also, the degree to which the load path influences the multiaxial fatigue damage will be discussed