Correlation of Fracture Surface Fractography to Fatigue Test Results

Silver-cored MP35N wire is commonly used in medical devices, utilizing the MP35N shell for strength and the silver core for improved conductivity. Fractures were observed in silver-cored MP35N multi-filar coiled wire during tension-tension fatigue testing. Fracture analysis showed surface discontinuities, inclusions, laps, and cracking in the wire surface at the crack origins. Supplier process improvements were implemented to improve the wire surface. In order to evaluate the process improvements, a rapid, straight wire stepped-stress, tension-tension fatigue test was developed to correlate straight wire fatigue life to coiled wire fatigue life of assembled devices. The fracture surfaces of the straight wires were examined and compared to the coils. The overall straight wire showed surface discontinuities, inclusions, laps, and cracking in the wire surface, however; only inclusions, laps, and cracking were observed at crack origins of the straight wire. The relationship between wire surface features at the crack origin and tension-tension cycle count to failure was also documented. The trends observed support the hypothesis that the larger the flaw at the fracture origin, the lower the number of cycles to failure. It would also be expected that the larger the flaw size, the lower the maximum strain at failure, but this was not the case observed. However, the stepped strain may be convoluting the data between flaw size, strain level, and cycles at strain.