Electrochemical fatigue sensor response to Ti–6 wt% Al–4 wt% V and 4130 steel

Abstract

Some of the finer details concerning electrochemical–mechanical interactions under cyclic stressing, observed by the electrochemical fatigue sensor (EFS), are reported, with emphasis on a series of ramp-and-hold cyclic tests on samples of hardened Ti–6 wt% Al–4 wt% V and 4130 steel. These tests were designed to further our understanding of the mechanisms underlying EFS response by revealing its timing and morphology, as well as any dependence on stress range, strain rate or mean stress. The EFS is an instrument that monitors an electrochemical current (‘the EFS current’) as a means of investigating fatigue damage in metals; specimens are fatigued in a benign electrolyte, under controlled conditions, so that their fatigue lives do not differ from those obtained in air. The results show that cyclic loading produces a cyclic EFS current with extrema located at points in the loading cycle where the strain rate is highest. Localized fatigue straining and crack-associated plasticity enhance the EFS current, producing peaks that reveal information about the timing and magnitude of plastic response on a cycle-to-cycle basis. The response depends on the cyclic variables studied in a manner that is convenient for experimentation. The EFS current reflects the superposition of two basic components: one is correlated with elastic strain and is becoming better understood; the other is directly related to plasticity.