On the effect of coatings and pre-corrosion during fatigue tests of 7075-T6 aluminum alloy monitored with acoustic emission (AE)

Abstract

Strength hardened aluminum alloys as 7075-T6 al are widely used in aeronautical industry due to their low density and high mechanical properties. Their specific microstructural composition makes them sensitive to corrosion damage at intermetallic particles. Aeronautical parts are often submitted to corrosive environments and fatigue loadings. For safety and economic matters, it is important to be able to monitor the development of damages during application. In this context, Acoustic Emission (AE) is a useful tool to monitor the state of material damage and predict its remaining useful lifetime. This work, which is part of a larger European project (Early detection and progress monitoring and prediction of corrosion in aeronautic Al alloys through calibrated Ultrasonic-CorROSion Sensor application), has for main objective to understand, identify and quantify, via AE, how corrosion defects impact the fatigue behaviour of aluminum alloy 7075-T6 specimens and covered with different types of coatings (top coat, primer and a conversion coating obtained by anodizing process). Therefore, tensile-tensile fatigue tests (R? = 0.1) monitored with AE are performed at room temperature on non-corroded samples and on corroded samples. Precorrosion defects are generated by the complete immersion of the samples in an NaCl bath (3.5% wt). Pre-corrosion defects tend to decrease the fatigue lifetime of the material tested and create damage in the substrate and coatings generating new AE sources. For all types of coated specimens, damage indicators based on the AE activity are studied in order to find characteristic damage times giving information on the remaining useful lifetime of the material during fatigue tests. Characteristic times linked to damage initiation in the substrate and propagation the main fatigue crack in the material are defined.