Correlation of fatigue life with potential NDE corrosion metrics

Abstract

Attempts are being made to move away from the 'Find-and-Fix' approach and to manage corrosion damage in aircraft so that it can be rectified during scheduled maintenance periods. This 'Identify-and-Manage' approach will only be possible if effective non-destructive evaluation (NDE) methods are available to detect, characterise and monitor the corrosion that is present within the air frame and suitable life-prediction models exist to predict the interaction of corrosion and fatigue from the characterised corrosion damage. In order to investigate potential NDE corrosion metrics, a set of dog-bone fatigue specimens was fabricated from unclad 2024-T3 aluminium alloy. The specimens were corroded in a salt-fog chamber for a range of time intervals to induce both pitting and crevice corrosion, which was then characterised using ultrasonic and eddy current inspection techniques. Ultrasonic inspection was carried out at three frequencies (10, 22 and 50 MHz) using focused immersion transducers. Amplitude and depth scans at each frequency were obtained. Eddy current inspection was carried out at a single frequency of 10 kHz. A number of parameters were calculated from the scans and correlated with the fatigue life.