Numerical investigation on the effect of thickness and stress level on fatigue crack growth in notched specimens

The effect of specimen thickness on the behavior of fatigue crack growth rate (FCGR) requires rigorous investigation because the thickness effect is not independent and could be a function of the specimen geometry, material properties, loading conditions, and environmental conditions. The purpose of this study is to numerically investigate the effect of specimen thickness and stress level on the behavior of FCGR at the specimen-free surface and mid-thickness in notched mild steel specimens. Elastic-plastic numerical calculations were performed based on FEM using the domain integral method to calculate the cyclic J-integral parameter, Delta J, which was used for calculating FCGR. Different specimen thicknesses and various constant amplitude stress levels were used in the numerical calculations. Crack growth calculations were carried out using the node release technique in which the history of the former crack was considered. The calculated fatigue lives were firstly verified against the experiments taken from the literature. The results of numerical calculations showed that no significant thickness effect was noticed at the mid-thickness for calculations carried out below general yield while when the stress level is close to general yield the thinner specimens showed faster crack growth rates. Further, thickness has a remarkable effect on the crack growth rate at the free surface when the applied stress level is below and close to general yield where the thinner specimens gave faster crack growth rates. The crack-tip driving force Delta J and the local strains and stresses induced ahead of the crack front could explain the mechanics and behavior of FCGR for the applied thicknesses and stress levels. The distributions of the local strains and stresses could also reveal the effect of the local material ahead of the crack front on the behavior of crack growth.