Nondestructive Testing of FRP-Bonded Structures Using a Nonlinear Baseline-Free Approach

Abstract Concrete elements are fundamental to civil engineering structures such as buildings, bridges, and pavements. Fiber-Reinforced Polymer (FRP) retrofitting is widely used to enhance their strength and durability; however, debonding the weakening of the bond between concrete and FRP remains a critical challenge. Traditional inspection methods often require baseline data and may fail to detect early-stage debonding. This study presents a baseline-free nonlinear guided wave technique for detecting and localizing debonding in FRP bounded concrete slabs and cylinders. Using a network of three piezoelectric sensors, the method uses frequency mixing to amplify nonlinear wave amplitudes, overcoming the common issue of small nonlinear signal strength. Damage localization functions are proposed for both slab and cylindrical elements to accurately identify debonding locations. Experimental studies, complemented by finite element simulations modeling guided wave propagation, investigate various debonding depths and sizes to assess localization robustness. Results show that the proposed technique is accurate and reliable in detecting and localizing debonding in FRP bounded concrete slabs and cylinders. It achieves high accuracy for deeper defects and maintains acceptable accuracy for shallow ones, with localization errors remaining within practical limits. The results also indicate that defect depth and size are two influential factors affecting the technique’s performance, with depth having a more significant impact. Deeper defects generate stronger nonlinear responses, resulting in more precise localization compared to shallower defects. Considering the simple equipment setup and the use of a minimal number of sensors, the proposed technique can serve as a practical and efficient tool for early defect detection and Structural Health Monitoring (SHM) of FRP bonded structures.