PREDICTIONS OF LIFE OF METALS SUBJECTED TO MULTIAXIAL FATIGUE LOADING (CRACK PROPAGATION, J-INTEGRAL, MIXED MODE, EXPERIMENTAL DATA)

A review of the literature on multiaxial fatigue is presented; it summarizes empirical trends and discusses available life prediction approaches. An evaluation of three of the more promising approaches for predicting multiaxial fatigue life is then presented. The first approach is that used in Code Case N-47-12 of Section III of the ASME Boiler and Pressure Vessel Code. This approach is based on a strain range parameter similar to the octahedral shear strain range. The other two approaches are recently proposed ones. The first uses plastic work done during multiaxial cyclic straining as a fatigue damage parameter. The other is a criterion based on maximum range of shear strain, modified by a fraction of normal strain range action on the plane experiencing maximum shear strain range. Evaluation of the approaches is based on data available in the literature as well as data from the author's tests of specimens of A533B pressure vessel steel. In these deflection-controlled tests, specimens were subjected to combined bending and torsion, applied both in-phase and 90(DEGREES) out-of-phase, at two different fixed ratios of applied torsional shear strain range to bending strain range, and at several strain ranges to produce lives between approximately 10('3) and 10('6) cycles. In general, it was found that the plastic work approach was superior to the others. Differences in crack formation and growth behavior with differences in the above test parameters are presented, and their significance discussed in terms of development of improved methods for predicting multiaxial fatigue life. A new theory for predicting crack propagation rates for complex multiaxial loading is subsequently developed. This theory is based on physical mechanisms of crack propagation and uses the J-intergral as the crack growth parameter for both high-cycle and low-cycle fatigue. A procedure for estimating the J-integral for mixed mode cracking is developed. Subsequent evaluation of this theory, using data from tests done on A533B steel, proves promising.