Grad-CAM-Assisted Deep Learning for Mode Hop Localization in Shearographic Tire Inspection

Abstract

In shearography-based tire testing, so-called “Mode Hops”, abrupt phase changes caused by laser mode changes, can lead to significant disturbances in the interference image analysis. These artifacts distort defect assessment, lead to retesting or false-positive decisions, and, thus, represent a significant hurdle for the automation of the shearography-based tire inspection process. This work proposes a deep learning workflow that combines a pretrained, optimized ResNet-50 classifier with Grad-CAM, providing a practical and explainable solution for the reliable detection and localization of Mode Hops in shearographic tire inspection images. We trained the algorithm on an extensive, cross-machine dataset comprising more than 6.5 million test images. The final deep learning model achieves a classification accuracy of 99.67%, a false-negative rate of 0.48%, and a false-positive rate of 0.24%. Applying a probability-based quadrant-repeat decision rule within the inspection process effectively reduces process-level false positives to zero, with an estimated probability of repetition of ≤0.084%. This statistically validated approach increases the overall inspection accuracy to 99.83%. The method allows the robust detection and localization of relevant Mode Hops and represents a significant contribution to explainable, AI-supported tire testing. It fulfills central requirements for the automation of shearography-based tire testing and contributes to the possible certification process of non-destructive testing methods in safety-critical industries.