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

为了探索无损检测焊接结构构件疲劳损伤的可能性，对焊接构件在应变循环疲劳载荷作用下表面微裂纹的萌生和扩展行为进行了一系列实验研究。采用低碳钢、600 MPa级和800 MPa级高强度钢、SUS316NG不锈钢4种钢的母材和横向对接接头，加工出截面直径为8 mm、标距为12.5 mm的小型实心圆柱试样。在最长达10.75个循环的完全反向应变循环疲劳期间，通过停止疲劳试验机几分钟，将试样表面上的微裂纹重新定位在易溶的0.08mm厚的乙酸纤维素膜上，通过光学显微镜检查干燥的膜。该系统与电视摄像机和图像分析系统相连，通过测量单位面积上的裂纹密度和单位面积上的裂纹总长度，即该面积上所有裂纹长度的总和，随着疲劳循环比的增加，应变幅值呈单调增加的趋势，两个参数的曲线均按应变幅值的大小排列。归一化裂纹密度，即任何循环比下的裂纹密度除以可见裂纹萌生寿命下的裂纹密度，随循环比呈指数增加。归一化裂纹总长度与循环比的关系也呈现出类似的趋势。结果表明，裂纹密度和裂纹长度之和是估算结构钢疲劳累积损伤的有效指标，也就是说，这两个参数可用于估算结构钢的疲劳寿命。

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.