On some basic problems of fatigue research in engineering

Research on the basic fatigue formula, the criterion for omitting small loads, the rule for cumulative fatigue damage, and models for predicting the fatigue crack initiation (FCI) life and probability distribution under variable-amplitude loading are comprehensively studied and summarized in the present paper. First, the formula for FCI life is given as a function of the equivalent stress amplitude, and the procedures for investigating overload effects are introduced. Using the above formula to analyze test results of the overload effect on FCI life yields the formula for FCI life containing the overload effect factor, z, which can be adopted to characterize the load interaction effect when Minerʹs rule is used to compute the cumulative fatigue damage. Fatigue test results and analysis of the underload effect show that the FCI threshold in the FCI life formula can be taken as the criterion for omitting small loads in life prediction and in compilation of the load spectrum. Then, the fatigue damage function is defined, the cumulative fatigue damage can be computed and the FCI life of structure members under variable-amplitude loading can be obtained by using Minerʹs rule. Procedures for predicting the probability distribution of FCI life or fatigue life are described and examples are introduced. Furthermore, factors affecting the value of z and the applicability of Minerʹs rule are briefly discussed. Finally, further areas for fatigue research are tentatively proposed. Most interest should be focused on research into generally applicable life prediction models and the approach for fatigue reliability assessment of structure details made of the metals with continuous strain-hardening characteristics, which have been widely adopted in aeronautical engineering, under service load and environments.