An entropy based low-cycle fatigue life prediction model for solder materials

Fatigue damage is an irreversible progression which can be represented by the entropy increase, and it is well known that the second law of thermodynamics can describe an irreversible process. Based on the concept of entropy, the second law of thermodynamics can provide the changing direction of system. In the current study, a new entropy increment model is developed based on the frame work of continuum damage mechanics. The proposed model is applied to determine the entropy increment during the fatigue damage process. Based on the relationship between entropy and fatigue life, a new fatigue life prediction model is proposed with clear physical meaning. To verify the proposed model, eight groups of experiments were performed with different aging and experimental conditions. The theoretical predictions show good agreement with the experimental data. It is noted that with higher aging temperatures, the value of ε_th/ε_cr becomes larger and the residual fatigue life reduces. The value of ε_th/ε_cr goes larger and the residual fatigue life becomes shorter with higher strain amplitude.