Y-Y Ma et aL/Engineering Failure Analysis 47(2015)162-177 ection rigin of the crack body Fig. 7. Macro morphologies of sample A: (a)the crack, (b) two corresponding cross sections, (c)magnified morphology of LoP2 and(d)origin site in Section show, fatigue cracks could be clearly observed, propagating along the arrows marked in the figures. According to the surface morphologies, the crack seemed to be torn by external force, exhibiting a kind of secondary fracture rather than the first fracture So it can be sure that the fracture appeared in this location happened after the fracture in the weld joint of sample A. The fracture in the weld joint of the flange led to an unbalanced of the CwP and large force was applied suddenly on the of sample B, which cause it to be finally fractured. As a result, further studies are not needed to conduct for the 4. Failure analysis 4.1. Lack of penetration Based on the analysis results presented above, it is clear that lack of penetrations(LOPs)existing in the weld joints duced by improper welding procedures should be blamed for the serious fracture of the circulating water pump(CWP). Just as Fig. 7(b)illustrates, LOP not only lies in the weld joint that connects the flanges, but also in the weld joint that connects the flange and the cylindrical body decreasing the strength of the weld joint LOPI locating in the weld joint of the flange joint. These LOPs strength and fatigue life of the joints significantly It has been proved by many researchers that LOP, as a kind of common defect in the weld joint, would markedly decrease the strength of the weld joint and reduced greatly the fatigue life, leading to a severe fracture. Kim [21 studied the effect of LOP on the fatigue strength of butt weld by a number of fatigue tests of high steel containing partial and full penetration butt elds, revealing that fatigue strength of partial penetration butt weld was lower remarkably than that with full penetration, and the fatigue cracks initiated at the LOP section. Wahab 22 pointed out that the weld imperfections such as LOP, porosity, lack of fusion, undercut effectively reduced the fatigue crack propagation life and fatigue strength of welded joints bshow, fatigue cracks could be clearly observed, propagating along the arrows marked in the figures. According to the surface morphologies, the crack seemed to be torn by external force, exhibiting a kind of secondary fracture rather than the first fracture. So it can be sure that the fracture appeared in this location happened after the fracture in the weld joint of sample A. The fracture in the weld joint of the flange led to an unbalanced of the CWP and large force was applied suddenly on the location of sample B, which cause it to be finally fractured. As a result, further studies are not needed to conduct for the sample B. 4. Failure analysis 4.1. Lack of penetration Based on the analysis results presented above, it is clear that lack of penetrations (LOPs) existing in the weld joints induced by improper welding procedures should be blamed for the serious fracture of the circulating water pump(CWP). Just as Fig. 7(b) illustrates, LOP not only lies in the weld joint that connects the flanges, but also in the weld joint that connects the flange and the cylindrical body, decreasing the strength of the weld joint. LOP1 locating in the weld joint of the flange even runs through the weld joint. These LOPs would result in high stress concentration on the crack tip, and reduce the strength and fatigue life of the joints significantly. It has been proved by many researchers that LOP, as a kind of common defect in the weld joint, would markedly decrease the strength of the weld joint and reduced greatly the fatigue life, leading to a severe fracture. Kim [21] studied the effect of LOP on the fatigue strength of butt weld by a number of fatigue tests of high steel containing partial and full penetration butt welds, revealing that fatigue strength of partial penetration butt weld was lower remarkably than that with full penetration, and the fatigue cracks initiated at the LOP section. Wahab [22] pointed out that the weld imperfections such as LOP, porosity, lack of fusion, undercut effectively reduced the fatigue crack propagation life and fatigue strength of welded joints by Fig. 7. Macro morphologies of sample A: (a) the crack, (b) two corresponding cross sections, (c) magnified morphology of LOP2 and (d) origin site in Section 2. 170 Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177