Improvement of experimental device for high cycle fatigue tests of copper alloy strips and characterizing fatigue properties

<p>In this research, we improved the fatigue test method for copper alloy strips which was standardized by Japan Copper and Brass Association. In order to increase test frequency and obtain repeatable test results for 108 cycles, the design of fixed block jigs was improved and the tolerance of the knife edge shaper was restricted. Also, lubricant was used for reducing wear on the specimen. From results from the fatigue tests, it shows that the fatigue life of specimens created by a transverse rolling direction was longer than that of specimens created by a rolling direction. To investigate this reason, fracture surfaces of specimen were observed. The areas where the cracks began were found inside the copper alloy strip, not on the edge, and they were consistent with the stress concentration areas estimated with finite element analysis. In addition, we found that in specimens created in the rolling direction transgranular fractures occurred, while in specimens created in the transverse rolling direction intergranular fractures occurred. The difference in fracture mode between the rolling directions is thought to be due to the fact that the grain size is slightly larger than that in the rolling direction due to rolling during strip fabrication, and it is assumed that the degree of plastic strain mismatch at the grain boundary is larger in the orthogonal direction than in the rolling direction, creating resistance to crack propagation. Moreover, the effect of specimen thickness on the fatigue life was investigated. Regardless of the rolling directions, it was confirmed that fatigue strength decreased with increasing thickness. The relationship between thickness and fatigue strength of the strip in the rolling direction tended to decrease at an almost constant rate with increasing cycles, but in the transverse rolling direction, the amount of decrease in fatigue strength decreased with increasing thickness with increasing cycles.</p>