Electrochemical detection of fatigue damage

Abstract

Electrochemical reoxidation currents are shown to provide a new method of detecting fatigue damage in metals. The basic principle is demonstrated by measurements on anodized samples of aluminum. The initial anodic oxide film is ruptured at the site of plastic deformation and the flow of charge is measured during subsequent reanodization, or healing of the oxide. In a preliminary series of experiments on tensile specimens, the total flow of charge is shown to be the sum of two components: i) that associated with charging the capacitor created by the metal specimen, the oxide film, and the electrolyte ii) that associated with reoxidation of the microcracks in the oxide film. The reoxidation component is in agreement with a simple model calculation of oxide rupture, and provides a quantitative measure of the total area of the microcracks in the oxide film. In the fatigue experiments these two components of charge flow are separated by a simple immersion scanning procedure, which also reveals the localized distribution of the damage. The flow of charge to these localized regions correlates with observations of the severity of the damage by scanning electron microscopy. Fatigue cracks ∼50 μm long or longer can be detected. In our specimens, these occurred after 10 percent of the fatigue life. The magnitude of the associated reanodization current transient is an accurate measure of the crack length and can predict the fatigue life within ±50 percent.