Numerical Simulation Of Crack Initiation Under Creep-fatigue Interaction

During its service life, the plant structure will bear steady loads at elevated temperatures as well as variable loads due to repeated start and stop processes; together these can lead to the interaction of creep and fatigue. For initially defect-free structures, crack initiation occupies a key role in their whole failure life. So, it is of great significance to research crack initiation and the factors influencing it under the interaction of creep and fatigue.By combining the theory of continuum damage mechanics with initial stress and strain fields and criteria for element failure and crack initiation, a predictive model of crack initiation under creep-fatigue interaction was constructed. Based on the model, a user defined material subroutine(UMAT) was compiled and creep-fatigue damage accumulation was investigated by the finite element method using ABAQUS codes. Crack initiation was simulated in a structure without initial defects under a creep-fatigue interaction. Factors affecting the time of crack initiation were investigated.Analysis shows that stress and plastic strain level are the key factors affecting crack initiation; cracks initiate where stress and plastic strain are concentrated. In contrast to linear cumulative damage(LCD) theory, mutually promoting effects exist between creep damage and fatigue damage at crack initiation locations. Interaction between creep and fatigue shortens the time to crack initiation. Three kinds of sequence of creep and fatigue loading are simulated. It shows that the sequence of creep and fatigue loading greatly influences accumulation of damage at the crack initiation location. Cyclic loads cause larger damage after long periods of creep load compared to the other two kinds of loading sequence, hence time to crack initiation is shorter. Approximate linear relationship is found between stress concentration factor and the logarithm of cycle life. The percentage of creep damage is larger when crack initiates in case of greater stress concentration factor.