Fatigue Damage Evalution Based on Surface Micro Cracks in Base and Weld Metals for Structural Steels

为了寻找对焊接结构构件因疲劳加载而积累的疲劳损伤进行无损检测的可能性，对应变循环疲劳加载过程中表面微裂纹的萌生和扩展行为进行了一系列试验研究。以低碳钢、600 MPa级和800 MPa级高强钢、SUS316NG不锈钢四种钢材的基本金属和横向对焊接头为材料，加工出截面直径为8 mm、规长为12.5 mm的光滑实心圆柱体试样。在最长可达10&7&次的完全反向应变循环疲劳过程中，通过停止疲劳试验机几分钟，将试样表面的微裂纹重新定位在易溶的0.08 mm厚醋酸纤维素薄膜上，干燥后的薄膜用光学显微镜观察，该显微镜连接电视摄像机和图像分析系统。单位面积裂纹密度和单位面积总裂纹长度（即该区域内所有裂纹长度之和）均随疲劳循环比的增加而单调增加，且两者的曲线分别按应变幅值的数量级排列。归一化裂纹密度，即任意循环比下的裂纹密度除以可见裂纹起裂寿命时的裂纹密度，随循环比呈指数增长。在归一化总裂纹长度与循环比之间的关系中也观察到类似的趋势。结果表明，裂纹密度和所有裂纹长度之和是估计钢结构累积疲劳损伤的良好指标，也就是说，这两个参数可用于估计钢结构的疲劳存活寿命。

For the purpose of finding a possivility of non-destructive detection of fatigue damage, that is accumulated in a member of a welded structure due to fatigue loading, a series of experimental investigation was made on initiation and extesion behaviors of surface micro cracks during strain cycling fatigue loading. The smmoth and solid cylinder specimens, 8 mm in test section diameter and 12.5 mm in gage length, were machined out from base metals and transverse butt-welded joints of four kinds of steel : mild steel, 600 MPa class and 800 MPa class high tensile steels and SUS316NG stainless steel. During completely reversed strain cycling fatigue up to 10&7& cycles at the longest, micro cracks on the specimen surface were replocated on readily soluble 0.08 mm thick cellulose acetate films by stopping the fatigue testing machine for a few minutes, the dried films were wxamined by an optical microscope, that was connected to a TV camera and an image analyzing system.Both the crack density in a unit area and the total crack length in a unit area, that is the summation of lengths of all cracks in the area, showed monotonic increase as a function of increasing fatigue cycle ratio, and the curves for both parameters were arranged in an order of the magnitude of strain amplotude, respectively. the normalized crack density, that is the crack density at any cycle ratio devided by that at the visible crack initiation life, increased exponentially against the cycle ratio. A similar tendency was obserbed in the relation between the normalozed total crack length and the cycle ratio. It was concluded that both the crack density and the summation of all crack lengths are promising indications for the estimation of an accumulated fatigue damage in a stractural steel, and, in other words, these two parameters may be usufull for estimation of survival fatigue life.