Fatigue cumulative damage models based on strength degradation

Abstract: 80%-90% of failures in structures of construction machinery result from fatigue and fracture. Fatigue process is usually considered as degradation process of material strength under cycle loading. Strength degradation mainly relates to the number of cycles and stress level of loading. This paper discussed cumulative fatigue damage of metal structure based on strength degradation. A correlation model between strength degradation and cumulative fatigue damage was established based on characteristics of strength degradation under metal fatigue and further verified that the strength degradation process met the irreversible thermodynamics condition. A strength degradation coefficient was obtained by non-linear fitting method based on the data from the material fatigue test. The biggest relative error of residual strength, 4.6%, was obtained from the fatigue and strength test data, further verifying the accuracy of this correlation model. Two new fatigue cumulative damage models under two-level and multilevel loading were established. First formula was established based on equivalence rule of relative strength degradation. The other was established by introducing slope value of material S-N characteristics as the power law of strength. Through calculation, it was concluded that both models met common fatigue rule that cumulative damage is above 1 for increasing loading amplitude, and cumulative damage is below 1 for decreasing loading amplitude under any two level loading. Coefficients of strength degradation models were respectively calculated through nonlinear fitting method based on the data from two-level and three-level loading fatigue test and two new fatigue cumulative damage models were established. The last level fatigue damage and the level last fatigue life of fatigue cumulative damage models and Miner model were calculated and relative error of each model was obtained by considering the relative error of the last level fatigue damage as evaluation index. Moreover, the cumulative fatigue damage and the fatigue life of each were obtained. The results showed that whether in two-level or three-level loading fatigue test, the accuracy ranking of the above three models was as follows: the second model> the first model> Miner model. The equivalence damage model based on strength degradation may have some relevance to the "power" phenomenon, which corresponds to the second model. So, the results of this study will provide certain reference in the accurate calculation of the metal fatigue life.