Numerical Simulation Of Plastic Induced Fatigue Crack Closure Behavior Under Overloading

The load state of metal components is often not constant during their service life.In constant amplitude cyclic load,periodic overload occurs from time to time,which affects the fatigue life of metal components.Therefore,it is very important to accurately evaluate the change mechanism of crack growth rate after overload.At present,the research on the action mechanism of overload mainly focuses on the theory of plastic induced crack closure.However,the research on crack closure mostly focuses on the change of closure level.The crack tip opening displacement（CTOD）and the crack surface displacement curve are also important parameters to characterize the closed state of the crack tip.Therefore,this paper studies the change mechanism of crack closure after overload based on the curve of crack closure level,crack tip opening displacement and crack surface displacement.The main research work is as follows:（1）Analysis of the change of crack closure level under single peak overload.The changes of crack closure level,CTOD and crack surface displacement curve under different overload conditions are studied by a two-dimensional finite element model.It is considered that the residual plastic deformation CTOD_-0)forms overload"Hump"deformation,and the appearance of"Hump"leads to the change of closure level and the discontinuity of crack surface closure.（2）The changes of crack closure level under different multi overload modes are discussed.The results show that when tension compression composite overload is applied,the application of compression overload has little effect on crack closure,while the application of continuous multi peak overload and over guaranteed load is equivalent to the single peak overload with higher overload ratio,and the increase of overload interval in interval multi peak overload will slightly reduce the peak R_op,peak of overload closure level,And the previous overload has a certain inhibitory effect on the plastic deformation caused by the subsequent overload.（3）Study on three-dimensional crack closure level under constant amplitude and overload.The von Mises stress and Y-direction plastic strain on the crack surface during propagation are studied and discussed.It is considered that the overload"Hump"on the crack surface is intrinsically related to the residual plastic strain.Finally,the change of closure level in the thickness direction under different loading conditions is quantitatively analyzed.It is found that the crack closure level in the area near the free surface in the thickness direction is always higher than that in the interior of the sample,and the closure level is more sensitive to the occurrence of overload and stress ratio in the plane strain state of the interior area of the sample.