In this study, evaluation of fatigue crack properties in the Sn5.0Sb/Cu joint was performed by using the inelastic strain energy density Win around the crack tip which was calculated by FEM. Win-c given by the multiplication of Win obtained from an arbitrary-sized square region surrounding the crack tip by the side length of the square region did not depend on the size of the square region and the element size. The fatigue crack propagated in the solder layer and the power law of Paris law type between its fatigue crack propagation rate and ¦Win-c held. However, the power exponent in the fatigue crack propagation law differed depending on the regions of ¦Win-c. The power exponent became about 1 in the low ¦Win-c region and very large in the high ¦Win-c region. In the low ¦Win-c region, fatigue fracture propagated along the high angle grain boundaries formed ahead of the crack by the continuous dynamic recrystallization. On the other hand, the fracture transformed to static fracture mode in the high ¦Win-c region and the cleavage fracture was observed. The large power exponent in the high ¦Win-c region was attributed to the cleavage fracture.
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