Crack identification and quantification methods under the condition of lift-off effect using alternating current field measurement technique

Alternating current field measurement (ACFM) technique is an emerging non-destructive testing method for detecting cracks in conductive material. However, the existence of lift-off effect brings great challenges to the identification and accurate quantification of cracks using ACFM technique. In this paper, crack identification and quantization methods are studied under the condition of lift-off effect using ACFM technique. A novel array image feature method based on a longitudinal array probe is proposed to identify the crack with lift-off variations. The influence of constant lift-off height on characteristic signal is studied. The Peak-Trough distance of the Bz signal is not affected by the lift-off height, which can be used to measure the length of cracks. A newfound attenuation coefficient is proposed to develop a lift-off compensation method for evaluation of the depth of crack at any constant lift-off height. The crack identification and quantification methods are verified by crack testing experiments in an aluminum workpiece. The results show that the crack can be identified effectively under the condition of lift-off variations using the array image feature method. The length and depth of cracks with different constant lift-off heights can be measured by the Bz Peak-Trough distance and the lift-off compensation method respectively. The measured errors of the crack depth and length are limited within 10%