Rolling-direction-dependent fatigue crack growth in thin Cu-Sn-P alloy strips: Role of crack morphology and local J-integral redistribution

The fatigue crack growth behavior of thin rolled Cu-Sn-P alloy strips depends on specimen orientation relative to the rolling direction, but the mechanism responsible for this difference remains unclear. In this study, the difference between rolling-direction (RD) and transverse-direction (TD) specimens of a 0.15-mm-thick Cu-Sn-P alloy strip was investigated from the viewpoint of crack-driving force. The J -integral was evaluated experimentally by digital image correlation (DIC) and compared with values obtained by plane-stress two-dimensional finite element analysis. Fracture surfaces and cross-sections were examined, and three-dimensional finite element models based on the experimentally observed crack geometries were constructed. The results showed a much larger discrepancy between DIC and 2D FEM in the TD specimen than in the RD specimen. The observed crack morphology also differed between RD and TD, consistent with the anisotropic grain morphology introduced by rolling. Three-dimensional analysis showed that this morphology redistributed the local J -integral through the thickness, whereas surface-based evaluation was relatively insensitive to the detailed crack geometry. These results indicate that the RD/TD difference cannot be understood only from the apparent surface crack-driving force, but must be interpreted in terms of morphology-dependent redistribution of the local crack-driving force.