On the engineering approach to estimating the parameters in an improved crack growth rate model for fatigue life prediction

摘要

Metal structures in ships and offshore platforms are subjected to complex loading histories and one of the most significant failure modes is fatigue. Accurate prediction of the fatigue life of marine structures is very important for both safe and economic design and operation. It has been realised that fatigue crack propagation theory can provide a more rational basis for fatigue strength assessment than traditional cumulative fatigue damage theory. At the same time, more and more fatigue crack growth models are proposed along with a good understanding of metal fatigue mechanisms. In our previous work, a new crack growth rate model extended from the McEvily model was proposed. This model shows promising capability to explain various fatigue phenomena. However, there are still some shortcomings in this model such as the insufficient description of unstable fracture condition and the effect of stress/strain state. At the same time, it is difficult to determine a large number of model parameters without amounts of fatigue test data, which restricts its use in practical engineering problems. In this paper, the model will be further improved first by adopting a continuous empirical formula for estimating the value of variable fracture toughness during crack propagation and introducing a modified continuous equation for the crack tip stress/strain constraint factor used for calculating the stress intensity factor at the opening level. In order to quickly obtain the fatigue crack growth characteristic of a structural element before carrying out enough fatigue tests, the engineering approach to estimating the model parameters by recurring to some research findings on evaluation of fatigue properties was investigated. Based on the engineering approach, costly fatigue tests may be saved and limited tensile or fatigue data will be sufficient for the evaluation of the crack growth rate curve. The prediction results will be proved to agree well with the observed phenomena and results in test.