A PLASTIC FRACTURE-MECHANICS MODEL FOR CHARACTERIZATION OF MULTIAXIALLOW-CYCLE FATIGUE

The near-tip stress and strain fields of small cracks in power-law har dening materials are investigated under plane-stress, general yielding , and mixed mode I and II conditions by finite element analyses. The c haracteristics of the near-tip strain fields suggest that the experime ntal observations of shallow surface cracks (Case A cracks) propagatin g in the maximum shear strain direction can be explained by a fracture mechanics crack growth criterion based on the maximum effective strai n of the near-tip fields for small cracks under general yielding condi tions. The constant effective stress contours representing the intense straining zones near the tip are also presented. The results of the J integral from finite element analyses are used to correlate to a fati gue crack growth criterion for Case A cracks. Based on the concept of the characterization of fatigue crack growth by the cyclic J integral, the trend of constant J contours on the Gamma-plane for Case A cracks compares well with those of constant fatigue life and constant crack growth rate obtained from experiments. (C) 1998 Elsevier Science Ltd. All rights reserved.