Crack characterisation using invariable feature extraction in stainless steel specimen used for absorber tubes of CSP applications via EMAT

Absorber tubes are one of the most critical components of parabolic trough Concentrated Solar Plants (CSPs). Due to the high temperatures where these tubes perform at with concentration of sunlight, it is very likely for them to get damaged such as crack and mass loss etc., and lose functionality of power generation. Therefore, the monitoring of their structural health via Non-Destructive Testing (NDT) techniques is regarded as essential for preventing them from being significantly defective and thereby reducing maintenance cost. Non-contact method is one of the best inspection candidates, which is more reliable to the tubes at high temperature through a review and the access to the absorber tubes is limited. In this paper, the crack detection and quantification for stainless steel specimen used for absorber tube using Electromagnetic Acoustic Transducers (EMATs) is presented. Through numerical and experimental studies, features are extracted to quantify the crack. Among these features, the ratio between the first edge echo and the second crack echo (Ac2/Ae1) is investigated as invariable feature to factors such as electromagnetic coupling, lift-off distance between EMATs and the specimen etc. In addition, the feature Ac2/Ae1 has linear relationship with the depth of the crack when the depth is more than 0.75 mm, which proves the feature Ac2/Ae1 is invariable for crack quantification via both numerical modelling and experimental studies.