Process of Cracking and Damage Due to Plastic Fatigue of Steel Elements and Members under Very Low Cycle Loading

对钢构件和角钢在重复载荷作用下的低周疲劳破坏进行了试验研究。这项研究的目的是提取在几到二十个循环的加载重复过程中导致裂纹和断裂的决定性因素。试件在经历非弹性屈曲后反复承受轴向荷载。实验结果表明，弯曲变形的凹面在拉伸时产生了明显的裂纹。随着宽厚比的减小和长细比的增大，破坏时的荷载循环次数减少。耗能能力在很大程度上取决于整个加载历史、破坏模式、长细比和宽厚比。所有试件的残余净应变(不包括裂纹张开的贡献)的最大值在25%-40%之间，它们与加载方式、破坏模式、长细比和宽厚比无关。此外，为了模拟钢角钢的试验行为，并弄清构件关键部位的局部应力-应变历史，利用有限元程序MSC/NASTRAN对极低周荷载作用下的钢角钢进行了非线性数值分析。从数值研究的结果来看，有限元方法在描述钢构件在后屈曲范围内严重循环变形下的局部行为和逐步跟踪变形方面具有重要意义。在此基础上，提出了以承受较大重复变形的钢构件关键部位的累积塑性应变作为破坏指标的概念。

An experimental investigation has been performed on very low-cycle fatigue failure of steel elements and angles under repeated loading. The objective of this study is to extract decisive factors causing cracks and rupture in the course of loading repetitions of the order of a few to twenty cycles. The test specimen was subjected to axial load repeatedly after undergoing inelastic buckling. The experimental results show that visible cracks were initiated on the concave side of the bending deformation when it was stretching. The number of load cycles at rupture decreases as width-to-thickness ratio becomes smaller and slenderness ratio becomes larger. Energy dissipation capacity depends heavily on the entire history of loading, failure mode, slenderness ratio and width-to-thickness ratio. The maximum values of residual "net" strains, excluding contributions from the crack opening, of all the specimens are of the order of 25-40% and they do not depend on loading pattern, failure mode, slenderness ratio and width-to-thickness ratio. Furthermore, to simulate the experimental behavior of steel angles, previously done by the authors, and to make clear the local stress-strain history at the member's critical part, the nonlinear numerical analysis of steel angles under very low-cycle loading was performed by using the FEM program, MSC/NASTRAN.From the results of the numerical study, the significance of finite element method has been established in the description of local behavior and the step-by-atep tracing of deformation for steel members under severe cyclic deformations in the post-buckling range. On this basis, a concept is proposed that cumulative plastic strain of critical parts of the steel members subjected to large repeated deformations can be a key index of failure.