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• A conditional PDF model is presented to characterize fatigue life distribution under CAC loading. • A fatigue damage accumulation model is developed to account for the loading sequence effects. • The fatigue damage accumulation rule of material under the multi-level cyclic loading is elaborated. • A general framework is proposed to predict fatigue life under the multi-level cyclic loading. Fatigue life prediction of materials under the cyclic loading is still a particularly challenging task remains to be resolved. This study aims to develop a generic framework for fatigue life prediction under the multi-level cyclic (MLC) loading with consideration of the loading sequence effects. Firstly, a conditional probability density function (PDF) model is presented to quantify the fatigue life distributions of materials under any constant amplitude cyclic (CAC) loading. Subsequently, a fatigue damage accumulation model is proposed from the perspective of cumulative failure probability, which is capable of accounting for the nonlinear characteristics of damage accumulation and the loading sequence effects. Finally, a generic framework for fatigue life prediction of materials under the MLC loading is developed based on the proposed models. The fatigue life data of fiber reinforced composites under the two-level and three-level cyclic loading are utilized to verify the proposed framework. The results show that the predicted fatigue lives agree well with the fatigue test data of fiber reinforced composites. Moreover, the proposed framework can be easily extended to calculate fatigue life of materials under any MLC loading. [ABSTRACT FROM AUTHOR]

• The inherent correlation between stiffness degradation and strength degradation of FRP is explored. • A probability model of stiffness degradation is proposed based on the same damage state principle. • The proposed model is flexible enough to fit the stiffness degradation data of FRP laminates with good agreement. The stiffness degradation of fiber reinforced polymer (FRP) has significant effects on its fatigue life and reliability. In this study, the inherent correlation between stiffness degradation and strength degradation is explored, and a probability model that describing stiffness degradation of FRP is presented. Firstly, the evolution of fatigue damage in FRP is analyzed, and then it is characterized by stiffness degradation and strength degradation, respectively. Secondly, the randomness of initial stiffness and critical stiffness of FRP is considered, and a probability model of stiffness degradation is proposed based on the same damage state principle. Finally, the test data of FRP laminates are used to verify the proposed model, and the fitting accuracy is compared with several existing models. The results show that the stiffness degradation of FRP presents a fast-slow-fast trend during fatigue failure process, and the proposed model is capable of fitting stiffness degradation data of FRP with good agreement. [ABSTRACT FROM AUTHOR]

• • A unified fatigue damage model is proposed based on the performance degradation of composites. • • A pair of residual strength and stiffness models is presented to characterize the degradation rules. • • Correlation between strength degradation and stiffness degradation of composites is elaborated. • • Fatigue life prediction methods are developed from the viewpoint of performance degradation. The performance degradation of composite materials under fatigue loading is so complex that the present knowledge is far from complete. This study aims to present a pair of residual strength and residual stiffness models based on a unified fatigue damage formula, and predict fatigue life under variable amplitude cyclic loading. Firstly, a unified fatigue damage model for composite materials is proposed, which is capable of characterizing fatigue damage development caused by strength degradation and stiffness degradation, respectively. Subsequently, a pair of residual strength and residual stiffness models is presented to estimate the performance degradation of composite materials. Finally, the corresponding life prediction methods are developed from the viewpoints of strength degradation and stiffness degradation, respectively. A set of experimental data for composite laminates are utilized to validate the proposed models and methods. The results show that the proposed residual strength and stiffness models are flexible enough to fit strength and stiffness degradation data with good agreement. Moreover, the developed life prediction methods are capable of calculating fatigue life of composite laminates with high accuracy. [ABSTRACT FROM AUTHOR]