中文摘要：

基于英国R6规范选择3的方法构建了核压力容器接管安全端两种异种金属焊接结构（含隔离层的四材料结构和不含隔离层的三材料结构）的失效评定图（Failure assessment diagram, FAD）,并分析了裂纹位置和裂纹深度对FAD的影响。结果表明，两种结构的极限载荷基本相同，裂纹位置对极限载荷基本没有影响。随裂纹位置由管嘴向316L安全端管区域靠近及随裂纹深度的增加，失效评定曲线（Failure assessment curve，FAC）下移，结构的安全性降低。由于两种焊接结构中，相同位置、相同尺寸裂纹的FAD和极限载荷基本相同，因而其抗断裂性能和结构强度基本相同。

英文摘要：

Background： Dissimilar metal welded joints （DMWJ） were indicated to be vulnerable components in the primary system, and an accurate structural integrity assessment for such DMWJ is needed. Three-material DMWJ structures without buttering layer are also applied in recent years. Purpose ＆ Methods： In order to understand the effects of crack locations and crack sizes on failure assessment diagrams （FAD）, FADs of two types of dissimilar metal welded structures （four-material structure with buttering layer and three-material structure without buttering layer） for joining safe end to pipe-nozzle of nuclear pressure vessel were constructed by finite element numerical analysis. Results： Limit loads of the two welded structures are basically the same, and the crack locations almost have no effects on the limit loads. With moving crack location from pipe nozzle to 316L safe end pipe and increasing crack depth, the failure assessment curves （FACs） shift downward and the safety margin of the cracked structure decreases. Conclusions： Because the FADs and limit loads of the cracks with the same location and size in the two welded structures are basically the same, their properties against fracture failure and structural strength are almost the same. It is feasible to connect pipe-nozzle with safe end using simplified manufacture process without buttering layer for the consideration of cost saving.