In-Phase and Out-of-Phase Multiaxial Fatigue

In this investigation, thin-walled circular cylindrical specimens fabricated from a low alloy pressure vessel steel (ASTM A-516 Gr. 70) were subjected to various multiaxial loading conditions. The tests were conducted under strain-controlled condition, and loading was provided through an axial actuator and internal and external pressure across the specimen wall. Four in-plane strain ratios (ρ = Δε_t/Δε_a) were tested, and the most damaging case was the equi-biaxial in-plane straining, ρ = 1. For the latter condition, 90 deg out-of-phase loading was also investigated. These tests indicated a dramatic decrease in the number of cycles to failure, N_f, as a result of out-of-phase loading. The influence of the plastic strain path on life is thus clearly demonstrated. It is shown that the total strain energy density, ΔW^t = ΔW^e+ + ΔW^p, correlates with both the in-phase and out-of-phase cyclic tests, and therefore is a proper damage parameter to be used for life predictions. A brief description of how ΔW^t can be calculated is given for the case of proportional loading. The predicted results are compared with the experimental data, and the agreement is found to be very good indeed.