FATIGUE-LIFE PREDICTIONS INCLUDING THE EFFECTS OF HOLD TIME AND MULTIAXIAL LOADS ON CRACK-COALESCENCE BEHAVIOR

摘要

Two aspects of crack-coalescence behavior are reported. The first concerns a regime frequently referred to in the literature as creep fatigue interactions but which in this paper is essentially a time-dependent, fatigue-failure process. The second relates to crack coalescence under a wide range of different multiaxial stress-strain states. In the framework of the first approach, a fatigue-crack growth model is derived based on experimental observations during high-temperature, high-strain, reversed-bend, hold-time tests on AISI 316 stainless steel. Essential features of these tests are the compressive and the tensile 60-min hold periods on different surfaces, which induce, respectively, transgranular-short and intergranular-long cracks. The latter, more damaging cracks involve the coalescence of numerous short cracks to form a dominant Stage II crack that leads to failure. Then, in the framework of the second approach, the crack-coalescence model is advanced to predict the fatigue lifetimes for multiaxial, variable amplitude, proportional loading of a medium carbon steel commonly used to manufacture engineering components. It is shown that under high strain fatigue conditions the models used for the calculations of lifetime must necessarily involve crack-coalescence behavior if unsafe lifetime predictions are to be avoided.