A Study On Intrinsic Mechanism Of Crack Propagation Behavior Subjected Overload

Engineering components often fail due to fatigue when subjected to cyclic loading. For most engineering components/structures, overload condition is more relevant than constant amplitude loading. The intrinsic mechanism of crack propagation behavior subjected overload remains open to discussion. There still are not a unify method to evaluate the crack growth rate and fatigue life. Therefore, the study of the intrinsic mechanism of crack propagation behavior subjected overload has both values of theory and engineering applicantion. The crack propagation experiments and an approach of fatigue crack growth modeling is used to study the fatigue behavior of a pressure vessel steel Q345R.A series of fatigue crack growth experiments were conducted using compact tension specimens of Q345R steel. The influences of single tensile overload, underload and sequence effect between overload and underload were observed and studied. The influence of stress ratio on overload effect, sequence effect and underload effect were firstly studied with the Q345R steel. The influence of overload ratio, stress ratio and thickness were analyzed and discussed with the experimental results. The resuls show that1) the larger overload ratio induce the more obvious retardation after single tensile overload.2) as the increased stress ratio, the retardation effect decreased;3) as the increased thichness of specimen, the retardation effect decreased.The approach for fatigue crack initiation and crack growth developed by Jiang and co-workers is used to predict the crack growth rate of Q345steel. To obtain accurate numerical results of stress-strain field of components, a robust Armstrong-Frederick type cyclic plasticity model is implemented into ABAQUS as UMAT. The predicted results were compared with the results of the crack growth experiments. The influence of mesh size, crack propagation method and the loading cycles are discussed with the predicted results.Based on the crack growth experiment and the finite element results, the intrinsic mechanism were discussed by looking at the detailed cracked surface contact, compressive stress field distribution and the crack tip deformation. The cracked surface contact behavior were analysed by different period. The relationship was creted between cracked surface contact and crack grack rate. The residual stress field which is relate to laoding history change the behavior of cracked surface contact, and the behavior of cracked surface contact determine the crack growth behavior after overload. The proposed intrinsic mechanism can perfectly explaine the crack propagation behavior with different overload condition.