According to the change characteristics in the toughness of the metal material during the fatigue damage process, the fatigue tests were carried out with the standard 18CrNiMo7-6 material. Scanning the fracture with an electron microscope explains the lack of linear cumulative damage in the mechanism. According to the obtained results, a nonlinear damage accumulation model which considered the loading sequence state under the toughness dissipation model was established. The recursive formula was devised under two-level. The fatigue test data verification of three metal materials showed that using this model to predict fatigue life is satisfactory and suitable for engineering applications.

Residual strength is one of the important properties of materials and is the basis for material fatigue life prediction. In this article, the residual strength of 18CrNiMo7-6, a commonly used material for wind power gears, is degraded with the load of fatigue load. According to the law of fatigue damage generation and development, a residual strength degradation model is proposed. According to the comparison between the test results and the model, the accuracy of the proposed model is high. When the number of loading/total number of cycles is close to 0.7, the residual intensity will change significantly. It is helpful to further study the reliability change of wind power gears under high cycle fatigue.