Fatigue testing of thin CoNiCr wire up to 1010 cycles

Abstract

The CoNiCr alloy MP35N in the form of thin wires is commonly used as conductor in cardiac leads. The application requires high fatigue strength, in particular in the very high cycle fatigue (VHCF) regime. High frequency fatigue testing techniques are of great interest for material characterization and development since existing techniques require very long testing times.In this study, adapting the ultrasonic fatigue testing method to sub-millimeter wires has been accomplished for the first time. Fatigue tests of as-drawn low-Ti MP35N wire with 100μm diameter have been performed with a novel technique at 20kHz cycling frequency at load ratio R=0.3. Measured lifetimes show good agreement with conventional servo-hydraulic tension-tension fatigue data of the same material at 30Hz. Ultrasonic tests were run up to 2×1010cycles, and no fatigue limit was observed.Crack initiation occurs at Al2O3-inclusions at the surface, where the interfaces between particles and matrix fail. Sizes of the crack initiating inclusions, areainc. are between 1.9μm and 3.5μm. Fatigue lifetimes can be approximated with a single S-N curve between 104 and 1010 cycles. No change of crack-initiation mode into the interior is observed when lifetimes extend into the VHCF regime. Inclusions can be considered as initial cracks, and a fracture mechanics concept is used to describe fatigue lifetimes. On average a stress intensity range as low as 1.0MPam can propagate the crack to fracture at lifetimes above 108cycles.