Cyclic Event Identification and Fatigue Damage Assessment for Multiaxial Mission Loadings

Several multiaxial (tension/torsion) mission-history tests were conducted on smooth bars of Ti-6Al-4V. The mission histories were constructed of a single out-of-phase LCF cycle (in the form of a “box ” or “check ” path) coupled with 5 to 50 small HCF cycles. The stress paths and magnitudes for the LCF and HCF cycles were chosen to produce significantly different damage levels on different planes, as predicted by critical plane models that have been found to provide good agreement with constant amplitude fatigue data. Using Miner’s algorithm for damage summation and traditional event identification, the HCF cycles should have had a negligible effect on the LCF mission lives. However, the experimental results illustrated widely differing effects. In this paper, alternative techniques are discussed which relate to the identification of cyclic events, the calculation of fatigue damage resulting from an event, and the accumulation of damage which takes into account the nonlinear behavior resulting from LCF/HCF interactions. A nonlinear damage summation method based on the Damage Curve Approach was found to provide excellent agreement with the experimental results when applied in conjunction with the Findley critical plane model