1994 Volume 43 Issue 495 Pages 1584-1590
X-ray fractography is a method of analysing the causes of accidental fracture of machine components or structures. Almost all of the previous research on this problem has been carried out using constant amplitude fatigue tests. However, the actual loads on components and structures are usually of variable amplitudes.
In this study, X-ray fractography was applied to fatigue fractured surfaces produced by variable amplitude loading. Fatigue tests were carried out on Ni-Cr-Mo steel CT specimens under the conditions of repeated, two-step and multiple-step loading. Residual stresses were measured on the fatigue fractured surface by an X-ray diffraction method. The relationships between residual stress and stress intensity factor or crack propagation rate were studied. They were discussed in terms of the quantitative expressions under constant amplitude loading, proposed by the authors in previous papers.
The main results obtained were as follows:
(1) It was possible to estimate the crack propagation rate of the fatigue fractured surface under variable amplitude loading by using the relationship between residual stress and stress intensity factor under constant amplitude loading.
(2) The compressive residual stress components on the fatigue fractured surface correspond with cyclic softening of the material rather than with compressive plastic deformation at the crack tip.