疲労亀裂開閉口に及ぼす応力比と応力拡大係数範囲の影響について

Abstract

The effective stress intensity factor range based on the measurements of crack opening loads gives the same crack propagation behavior for various stress ratios. Many authors have reported that the crack opening ratio U depends only on the stress ratio R. It is observed in experiment, however, that U depends also on stress intensity factor range ΔK.In this study, fatigue crack propagation tests were carried out on a high strength steel with yield strength of 450 MPa class under various stress ratios. And a crack opening model, that contains the both effects of R and ΔK was investigated based on the cyclic elasto-plastic finite element analysis (FEA) and the fracture mechanics approach with the small scale yielding concept. The FEA can simulate well the plasticity-induced closure even under the plane strain condition. Generally, it is difficult to apply the FEA for oxide- and roughness- induced closures that play an important role in the near threshold region. On the other hand, in the small scale yielding model with the assumption of the existence of the residual deformation on the wake of a crack, the crack opening stress intensity factor can be calculated as the value at which the fatigue crack opening displacement becomes zero.The resultant formula of U contains two parameters, one is for the plastic deformation at a crack tip and the other is for the oxide and the roughness in a fatigue crack. This model can explain successfully the experimental results. As a result, the fatigue crack propagation behavior under various stress ratios is described by the equation with five parameters which can be obtained with only one specimen. The predicted crack propagation rates showed good agreement with empirical results. Therefore, the formula derived in this study may be applied well to the evaluation of fatigue crack extension.