Research on ultra-long life duplex S-N curve model

Based on some special metal materials, such as high strength steel, titanium alloy, and so on, which represent the character of duplex S-N (stress-life) curves due to the difference of the fracture mechanism (surface and internal fracture mode), high carbon-chromium bearing steel is used for the rotary bending fatigue test research. Its S-N curve relation is described respectively by Basquin, Langer and three-parameter model. Its probabilistic S-N curve model is researched respectively by conventional least square method and general maximum likelihood method. Test results show that the test data follow the logarithm normal distribution better based on differentiating the fracture mechanism. Taking the fit effect and reliability design into account, three-parameter model is better for describing the duplex S-N curves relation of special metals. Simultaneously, it is discovered that its probabilistic S-N curve model should be described by general maximum likelihood method. It can avoid the possible non-conservative predictions of the conventional least square method, which is resulted from the influence of local statistical characteristics of the testing data.