高载作用下的疲劳裂纹闭合与残余应力作用

Fatigue Crack Closure and Residual Stress Effect of Overload

工程结构经常受到变幅载荷的作用,施加的高载对结构中的疲劳裂纹扩展有明显影响,了解高载作用机理对于随机载荷谱下的裂纹扩展预测十分重要。基于塑性诱导裂纹闭合原理,运用弹塑性有限元法模拟疲劳裂纹扩展。阐述了所采用的裂纹扩展模拟方法及确定裂纹张开和闭合应力的原理,计算获得等K基本载荷循环下的裂纹闭合特性和残余应力分布规律。重点分析在基本循环中插入单个拉伸超载、单个压缩超载和单个拉伸超载后紧跟单个压缩超载等情况下裂纹的张开、闭合应力及残余应力分布随裂纹扩展的变化规律。结果表明,超载在裂尖前方和裂纹尾迹区引起的压缩残余应力是导致裂纹闭合应力水平升高和裂纹扩展迟滞的重要原因。裂纹闭合效应在拉伸超载后瞬时减弱,但会随着裂纹扩展快速上升至超过正常水平;单纯的压缩超载对裂纹闭合的削弱可以忽略不计,但紧跟在拉伸超载之后的压缩超载将导致裂纹闭合效应减弱,削弱拉伸超载下的裂纹扩展迟滞效应。

Engineering structures are frequently subjected to the variable amplitude loads, which have a significant impact on the fatigue crack growth. In order to understand the relationship between the overload effect and the plas-ticity induced crack closure, we simulated fatigue crack propagation using elastic-plastic finite element method. The techniques of the simulation and the determination of crack opening/closing stresses are introduced. Crack growth induced residual stress distribution and the crack closure behavior are firstly obtained for the baseline loading which is the constant K （ keeping the maximum stress intensity factor as constant and the stress ratio R as 0 during crack growth） cyclic loading. The values of the crack opening/closing stress levels relative to the maximum applied stress keep constant during the crack extension. Then similar results are obtained for different overload cases inserted into baseline cycles, such as a single tensile overload cycle, a single compressive overload cycle, or a single tensile o-verload followed by a compressive overload. The results demonstrate that the compressive residual stress induced by the tensile overload in front of crack tip and later in the wake of the crack causes the increase of crack closure level and the retardation of crack growth. The crack closure effect is weakened instantaneously after the tensile overload and enhanced rapidly along with the crack growth until a level much higher than that of the baseline cycles. The effect of a single compression overload itself on crack closure should be overlooked, but the beneficial effect of a tensile overload can be attenuated by a compressive overload immediately after it by reducing the crack opening/closing levels.