An Examination of Fatigue Initiation Mechanisms in Thin 35Co-35Ni-20Cr-10Mo Medical Grade Wires

Abstract

Knowledge of the intrinsic defect size distribution, surface grain size distribution, and prior deformation history are important factors in determining fatigue crack initiation mechanisms and total life variability in thin, metallic, medical grade wires. The ASTM F562 alloy system is used extensively as a fine wire coil or cable in the production of cardiac rhythm management leads which require excellent fatigue life, and a good understanding of life variability. In the present investigation, samples of 0.0070 in. diameter ASTM F562, 35Co-35Ni-20Cr-10Mo wires were produced with a variety of grain sizes and strain hardening conditions. Samples were then cyclically loaded to failure in rotary beam testing and preserved for post mortem fractography using high resolution scanning electron microscopy (HR-SEM). Fatigue cracks were found to initiate from three sources: intrinsic microstructural inhomogeneities, persistent slip bands (PSBs), and extrinsic surface defects. The dominance of the various initiation mechanisms was shown to be a function of the constituent particle and grain size as well as the fatigue loading conditions and prior deformation history. In samples exhibiting a surface grain size significantly larger than the constitutive particle distribution, cracks were observed to preferentially nucleate from surface intersecting PSBs rather than near-surface-particles. Understanding of these phenomena is important in the design of robust cardiac lead systems that will outlive the patient.