A Study Of Testing Method And Fitting Method For P-da/dN-△k Curve Under Constant Amplitude Loading

The essence of the process of fatigue fracture is the tiny defect extending continuously and finally causing the part broken when reaching a critical condition. Fatigue crack propagation (FCP) is one of the theories to reflect and predict the fatigue fracture process. Crack growth rate curve (da/dN-ΔK curve) is one of the most important curves in FCP, and a crack growth rate curve with reliability parameter, namely p-da/dN-ΔK curve, is needed when the reliability theory is introduced into fatigue fracture field. In order to obtain p-da/dN-ΔK curves, a study of the testing method and data fitting method is necessary.This thesis firstly refers to standards on test methods for fatigue crack growth rates and other related references, sums up and summarizes methods now available, and mainly makes suggestions on data recording according to followed data processing methods.The theory on normal distribution, least square method and fitting goodness is studied and summarized combining with this thesis. For the reason of the lack of real test data, some simple simulation test methods are studied. Considering the importance of the calculation of da/dNand ΔK, four different usual methods are discussed, and the conclusion is drawn that there are some advantages, even not obvious, in seven points incremental polynomial method.Five different methods on data processing have been discussed. They are the method based on (a, N) data set, the method based on (da, dN) data set, the method based on randomization, the method based on new randomization and the method based on the assumption of the independence of parameters. The theoretical base and the final fitting result of each method are discussed separately, and the fitting goodness, security and the reflection of the dispersion of crack growth rate. The comparison of five methods is discussed at last to draw conclusion about the advantages and disadvantages of these methods. The conclusion can be a reference for the later research on fitting p-da/dN-ΔK curve.