考虑超载迟滞效应的一个新模型

 根据有效残余压应力的概念,并考虑基本循环中残余压应力的松弛.导出了计算超载后裂纹扩展速率的公式其中为未受超载影响的裂纹扩展速率;应力松弛系数 用本文的方法对铝合金LYl2,2024,7075,钛合金Ti-6Al-4V以及铆接加劲板等试件在不同载荷情况下超载迟滞期进行了计算。如令系数n=4,发现计算结果与实验结果符合得相当好。

A NEW MODEL FOR PREDICTING OVERLOAD RETARDATION EFFECT IN FATIGUE CRACK PROPAGATION

The effect of the residual compressive stress in the overload plastic zone is equivalent to an effective stress σr, and as proposed by WillenborgwhereIn the present paper it is further assumed that σr is not constant during one baseline cycle, but it varies from σr at σb max to ασr at σb min , α is a relaxation factor. Thenso the effective stress range （△σb）eff during the baseline cycle isOr alternatively the above equation may be written in the following formIt is easy to show that the maximum value of Kap during the transient period following the overload will be where the overload ratioMany investigators have found that when the overload rate increases to some critical value ra, the crack will cease to propagate at all after the overload. It is reasonable to assume that when r reaches ra, （△Kb）eff min will decrease to a value （△Ab）eff min = △Kth, so the crack will cease to propagate. Using this condition, we getorIf the crack propagation rate is expressed by Paris formulathen the reduced rate of crack propagation following an overload will bethe unretarded rate of crack propagation correspondingto （△Kb）.The retarded number of cycle ND can be calculated bywhere △aa is the length of the retarded zone.The retarded number of cycle of some specimens of LY12, 2024,7075 aluminium alloys, Ti-6Al-4V titanium alloy and a stiffened plate under different loading conditions are predicted by the present method. By putting the coefficient n = 4 , we find that the agreement between the predicted values and the tested values is good, as shown in fig. 2 of this paper.