Transient Thermography for Flaw Detection in Friction Stir Welding: A Machine Learning Approach

A systematic computational method to simulate and detect subsurface flaws, through nondestructive transient thermography, in aluminum (AL) sheets and friction stir (FS) welded sheets is proposed in this article. The proposed method relies on feature extraction methods and a data-driven machine learning modeling structure. Here, we propose the use of a multilayer perceptron feed-forward neural network with feature extraction methods to improve the flaw-probing depth of transient thermography inspection. Furthermore, for the first time, we propose thermographic signal linear modelling (TSLM), a hyper-parameter-free feature extraction technique for transient thermography. The new feature extraction and modeling framework was tested with out-of-sample experimental transient thermography data, and results show effectiveness in subsurface flaw detection of up to 2.3 mm deep in AL sheets [99.8% true positive rate (TPR) and 92.1% true negative rate (TNR)] and up to 2.2 mm deep in FS welds (97.2% TPR and 87.8% TNR).