Research On Three Parameters P-S-N Curve Fitting Method Based On Sample Aggregation Principle

With the rapid development of modern industry in my country,the requirements of accuracy and reliability are higher and higher,whether it is the fatigue performance test of new materials or the fatigue reliability evaluation of large and complex mechanical structure system.The P-S-N curve represents a more objective,complete and comprehensive stress-life relationship,which plays an important role in reliability design,analysis,reliability calculation and life prediction of mechanical parts.However,when performing fatigue tests,it is often faced with problems such as fewer test samples,long test cycles,and high costs,which limits the P-S-N curve fitting method based on the theory of large samples.Therefore,the research on the P-S-N curve fitting method under small samples has important value and practical significance.In this paper,the three-parameter P-S-N curve fitting method is studied on the basis of small sample test data and sample aggregation principle.The main research contents are as follows:(1)A new hypothesis relationship between logarithmic life standard deviation and stress level in medium and long life region is proposed.On this basis,a small sample test scheme for fitting three parameter P-S-N curve is designed,and the fatigue life distribution under any stress level is obtained.Then,the new hypothesis relationship is verified by comparing with the P-SN curve fitted by traditional method.(2)Based on the principle of equal failure rate of fatigue life and failure rate of fatigue strength,the obtained distribution of fatigue life under given stress level is transformed into the distribution of fatigue strength under given life,and then the conditional fatigue limit values under different reliability are obtained by assuming that the variation coefficient of fatigue strength distribution under given life(N=5×105)is constant.(3)According to the distribution results of fatigue life obtained by the principle of sample aggregation and the conditional fatigue limit values under different reliability,the P-S-N curve is fitted by direct and indirect methods under two types of small sample test schemes(X-x-x-x)and(x-x-x-x),and the influence of different test schemes,sample number and different methods on the P-S-N curve fitting is analyzed,The P-S-N curve fitting method which can ensure accuracy and save cost is determined.