DEVELOPMENT OF A NOVEL SPECIMEN GEOMETRY FOR FATIGUE TESTING OF FINE WIRE

Despite a critical need to predict component life based on wire fatigue data, a survey of the literature indicates that no standard specimen exists for fatigue testing of fine wires. This article describes a plastically formed, precurved loop specimen developed to meet this need. The loop geometry localizes damage, permitting the use of insitu, automated crack detection systems. The loop specimen is appropriate for wire diameters between a few hundred micrometres to a few millimetres, and for materials with a high elastic modulus. Material properties are presented for a cobalt-chromium-based alloy (Elgiloy) wire used as a practical example. Analytical and finite element stress analyses are presented which quantify the maximum stress at the loop apex. Fatigue life results using the present specimen are given for Elgiloy wire. Post-test SEM examinations are presented to show that fracture initiation takes place at the maximum stress location as predicted by the analyses. A fracture path and morphology characterization is also shown to be consistent with the anticipated stress regimes.