疲劳裂纹闭合的数值模拟方法

用弹塑性有限元法模拟含中心裂纹试件在等幅循环拉伸应力作用下的疲劳裂纹扩展过程,对塑性变形导致的裂纹闭合效应进行了分析。试件材料为2024-T351铝合金板,平面应力状态,线性随动强化假设。分析中考虑裂纹面的弹塑性接触和几何非线性变形,通过逐步释放裂尖节点来模拟裂纹扩展。考察了裂尖局部网格单元尺度、裂纹长度和平均应力对裂纹张开/闭合应力水平的影响。裂纹闭合效应会随疲劳裂纹长度增加而逐渐增强,呈先爬升再缓慢下降的趋势,应力比的增大会导致裂纹张开/闭合应力水平提高。此外,网格细度也对张开/闭合应力的大小有一定影响。

A Study of the Numerical Simulation Technique for Fatigue Crack Closure

Using the elastic-plastic finite element method,the paper simulated the fatigue crack propagation and the plasticity-induced crack closure under constant-amplitude cyclic stress and strain loading for a middle-crack coupon(MT) tension made of 2024-T351 aluminum alloy plates which are under plane strain conditions and assumed to be linear,kinematic and hardened.The crack closure effects brought about by plastic deformation were analyzed.The nonlinear geometrical deformation and the crack surface contact in elastic and plastic ways were considered.The crack propagates by releasing the nodes of crack tips gradually.The influence of mesh size,length of crack and average stress of crack tips on crack opening and closure stress levels was examined.The crack closure effect gets stronger as the fatigue crack becomes longer,following the trend that the crack ascends abruptly and then descends slowly.The opening and closure stress levels,which were also influenced to a certain degree by mesh refinement,increase as the stress ratio increases.