An incremental multiaxial life prediction law (IMLP) is proposed which consists of the three-dimensional thermoviscoplasticity theory based on overstress (TVBO) combined with a multiaxial damage accumulation law (MDA) to compute the life-time or cycles-to-crack initiation. Crack growth is not considered in this paper but is needed to ascertain the useful life of a component. The method is intended for application to high-temperature low-cycle fatigue with and without hold times and for triangular and trapezoidal waveforms when creep-fatigue interaction takes place.
The deformation behavior is determined by solving the coupled differential equations of TVBO for the strain variation of interest, i.e., continuous cycling, hold times, or fast/slow or slow/fast loading. Only the cyclic neutral version of TVBO is used here, although cyclic hardening and recovery of state formulations are available.
The incremental damage accumulation law consists of a fatigue and a creep damage rate equation. When the sum of creep and fatigue damage reaches 1, crack initiation is said to occur. The damage accumulation equations assume that the combined actions of stress and inelastic strain rate contribute to damage and that damage evolution does not influence the constitutive equation. Fatigue damage always accumulates, but a negative creep damage rate is possible to allow for healing (creep damage is, however, always positive). In accordance with scarce experimental evidence, the maximum inelastic shear strain rate, a hydrostatic pressure-modified effective stress, as well as a parameter which depends on the multiaxiality of loading are used in each damage rate equation. The multiaxiality loading parameter depends on maximum inelastic shear strain rate for fatigue damage, while it is a function of maximum principal stress for creep damage.
All material constants for TVBO and MDA are determined from isothermal tests on Type 304 Stainless Steel (SS) at 538°C using data of Zamrik , Blass and Zamrik , and Blass . The damage accumulation law correlates fatigue life under biaxial (tension-torsion) cycling with and without hold times. Only the results of one biaxial test series were available to compare the generally favorable predictions (correlations) with experiments.