摘 要 在实际的工程机械结构服役过程中，偶尔会出现负荷在很短时间内突然增大，之后又回落到正常载荷的一次超载现象。相关研究和实验表明，一次超载会带来裂纹尖端闭合现象，进而影响裂纹的扩展历程。为了了解一次超载对含缺陷结构裂纹尖端力学状态的影响，本文以压水堆核电站一回路安全端含缺陷焊接接头作为研究对象，借助有限元模拟方法，对一次超载情况下异种金属焊接接头裂纹尖端应力应变场进行数值模拟计算与分析。主要研究工作如下： (1) 以核电结构材料600镍基合金为研究对象，建立了单一材料的紧凑拉伸试样力学有限元模型，对裂纹尖端力学场进行了数值模拟计算，得到了一次超载过程中，裂纹长度和一次超载系数对镍基合金材料裂纹尖端应力应变场的影响规律。 (2) 以182合金和A508低合金钢异种金属焊接接头为研究对象，对材料界面及附近区域裂纹力学场进行了理论分析及模拟计算，获得了一次超载情况下，不同位置裂纹尖端力学场和塑性区的变化规律。 (3) 基于焊接接头非均质材料力学性能和三维实体模型，模拟分析了正常载荷和一次超载情况下安全端异种金属焊接接头径向内表面裂纹前沿应力应变场，得到了一次超载情况下，不同的裂纹位置、裂纹深度及一次超载系数对焊接接头裂纹尖端力学状态的影响规律。

ABSTRACT It is known as a single overload that the load increases suddenly and returned to normal level in a very short period, which is occasionally occurred in mechanical structures during service. Corresponding researches and experiments indicate that a single overload would induce the crack closed phenomenon at the crack tip, and which would affect the crack growing course. To understand the effect of a single overload on the mechanical state at the crack tip in the structure with flaws, the stress-strain fields at crack tip were simulated and analyzed under a single overload by using the finite element simulation method, in which the dissimilar metal joint in safety end of the primary loop in nuclear power plant was as a research object. Main works were performed as follows: (1) The finite element model of a single material compact tensile specimen was established, and the mechanical field at the crack tip in nickel-based alloy 600 was simulated and analyzed. The effects of different crack length and a single overload ratio on the stress-strain field at the crack tip in nickel-based alloy were obtained. (2) The local stress and strain field at the crack tip nearby the interface of the two material specimen in the 182 alloy-A508 low alloy dissimilar metal welded joint were analyzed and simulated in detail, and the effects of the different location of the crack on the stress and strain field and the plastic zone at the crack tip under a single overload condition were obtained. (3) The stress and strain field in the inner surface radial crack fronts of the dissimilar metal joint in safety end under the normal load service and a single overload condition were simulated respectively, in which the mechanical heterogeneous properties of welded joint was considered and three dimensional model was constructed, and the effects of the crack location, crack depth and a single overload ratio on the mechanical state around crack fronts were obtained.