Ultrasonic evaluation for the detection of contact defects of the timber and fiber-reinforced polymer

Fiber-reinforced polymer (FRP) composites have been used tremendously to repair and rehabilitate timber structures due to their formability, ease of use, and high specific strength. The bond quality between FRP and timber substrate is critical for complete composite action. In this paper, a comprehensive set of linear and nonlinear ultrasonic methods was performed to investigate the bond between carbon-FRP (CFRP) and timber. For this purpose, 126 specimens of reinforced timber were prepared. Two techniques were considered to bond CFRP and timber: (1) externally bonded reinforcement (EBR), and (2) externally bonded reinforcement on the groove (EBROG). The effect of the number of CFRP layers, adhesive types, the extent of artificial surface defects, and the depth of the groove were contemplated. By analyzing linear methods, differences in stiffness and modulus of elasticity of CFRP and timber led to velocity variation and attenuation phenomena. Higher-order harmonics were also used to investigate the nonlinear behavior of ultrasonic waves at the bond. The second harmonic generation detected surface damage at CFRP and timber bond. Single-lap shear tests were employed to determine the bond strength of CFRP and timber and evaluate the accuracy of the linear and nonlinear methods. Finally, using the linear and nonlinear features extracted from the ultrasonic response signals and available physical parameters, three artificial neural networks (ANNs) were developed to predict three important CFRP-to-timber bond characteristics quantitively: (1) to predict the groove depth, (2) to predict the volume of voids, and (3) to predict the CFRP-to-timber bond strength. The three proposed models achieved an average [Formula: see text] of 0.95, 0.96, and 0.92 in predicting the groove depth, voids volume, and bond strength, respectively.