On the effect of coatings on the tensile and fatigue properties of 7075-T6 aluminum alloy monitored with acoustic Emission (AE): Towards lifetime estimation

Acoustic Emission (AE) is used to monitor the fatigue behaviour of specimens made of aluminium alloy 7075-T6 and covered with different coating combinations: anodic oxidation (alumina) -primer layer (organic coating) -topcoat layer (organic coating)). AE monitoring allows to understand and differentiate the processes and damage mechanisms in action during fatigue for all types of specimens. The type of coating applied has a significant influence on the damage process and the fatigue lifetime of the material. AE-based indicators are defined to assess the overall damage state of the specimens during fatigue testing. Some indicators are based on the distinction of the acoustic activity acquired during the loading and unloading phases of the cycles. As an example, the joint analysis of these indicators on anodized samples allows to highlight characteristic times ranging from 15% to 70% of the specimen's lifetime. The early time at 15% is related to the saturation of cracking of the oxide layer during the first cycles of fatigue tests. By 70% of failure life, the AE activity correlates with the propagation of the main crack in the substrate, assisted by brittle fracture of the surrounding oxide layer. These specific times provide information on the Remaining Useful Lifetime (RUL) of the material and could be used to anticipate future failure.