Imaging of debonds in a FRP strengthened concrete beam using linear and nonlinear features of surface guided waves generated by a wedge transducer

In fiber-reinforced polymer (FRP) systems, debonding is the most common failure mode that emphasizes importance of timely detection and assessment of debonded areas to ensure structural integrity. In this study, a novel debond imaging algorithm is proposed that utilizes nonlinear features of surface guided waves generated by a movable contact type wedge transducer. The technique employs a newly developed damage index (DI) based on higher and combined harmonic peaks (HCHP) along with Convex Hull algorithm for localization and sizing of debonds. Three defect cases – single, multiple and irregular are studied using an experimentally validated three-dimensional numerical model. In addition to the proposed HCHP damage index, the side-band peak count (SPC) based non-linear DI —is also used for debond imaging. All three cases are also studied using an algorithm based on the conventional linear ultrasonics feature i.e., signal amplitude. To assess and compare imaging accuracy across different scenarios, Intersection over Union (IoU) value is utilized. The findings reveal that while all three techniques demonstrate satisfactory debond localization and sizing, the nonlinear ultrasonics-based DI algorithms exhibit significantly superior performance in terms of IoU values. In addition, the technique allows rapid scanning of the area with a movable contact transducer pair that enhances its feasibility for real-world applications. The improved performance of the nonlinear ultrasonics-based DI algorithms (HCHP and SPC) demonstrates their potential to enhance the full imaging of debonds of different sizes and shapes in FRP-strengthened structures, thereby advancing nondestructive evaluation techniques.