Microcrack propagation under non-proportional multiaxial alternating loading

Non-proportional multiaxial fatigue tests of tubular specimens were performed under purely alternating strain-controlled loading. Both the load paths and the phase shifts were varied. With increasing phase shift at the same equivalent load, the lifetime was found to increase. Based on the Manson-Coffin law, a model for lifetime prediction was developed, which takes the hydrostatic loading part and, thus, the phase shift into account. Consequently it was possible to compare the results of non-proportional multiaxial fatigue tests at different phase shifts with the results of uniaxial tests. To obtain more information about the microcrack behaviour under multiaxial non-proportional loading, sonic-emission studies and fractographic analyses were performed. The results of the sonic emission studies and fractographic analyses suggest a discontinuous microcrack growth with the propagation rate correlating with the event rate. Further more, good agreement has been found between these observations and some microcrack propagation models known from the literature. On the base of both a simplified model for micro and short crack propagation was proposed in terms of J-Integral range DeltaJ and microstructural measures. When applying the crack propagation model to our multiaxial experiments for lifetime prediction fairly well results were obtained.