Fatigue Degradation Models

Although the selection of an appropriate cohesive zone modelling (CZM) shape and the identification of the cohesive parameters are important parts of the fatigue life analysis of bonded joints, it is still necessary to understand how cohesive properties vary (degrade) with the load cycles. This chapter addresses this point, where different CZM based fatigue life prediction models are reviewed and discussed. In this chapter, cohesive based fatigue methods are classified into two main groups, including the loading envelope (cycle jumping) strategy approaches and the cycle-by-cycle analysis models. The first one is mainly used for high cycle fatigue regimes and the second one is more suitable for low cycle fatigue where the fatigue stress level is close or above the yielding point of the adhesive. According to the cycle jumping model, a fatigue loading can be simulated by a static load. Two techniques called LDFA and EXFIT discussed in this chapter follow the load envelope strategy. In LDFA a link is created between the damage mechanics and fracture mechanics. Several models based on the LDFA have been developed by proposing new damage mechanics or by modifying the fracture mechanics part. Another load envelope technique discussed in this chapter is EXFIT where the fatigue damage parameter is defined as a function of an effective stress or strain parameter. In terms of the numerical implementation, the models based on the LDFA need further work than the EXFIT approaches. In addition to these techniques, a cycle-by-cycle method was also considered in some studies. Simulating actual fatigue cycles rather than using a constant static load is the main difference between load envelope strategies and the cycle-by-cycle approach. Cycle-by-cycle fatigue analysis is based on the concepts of irreversible CZMs, where cohesive stiffness in reloading is not the same as in unloading. All of these techniques and their relationships are discussed in this chapter.