Active Microwave Thermography for Defect Detection of CFRP-Strengthened Cement-Based Materials

Nondestructive testing (NDT) of rehabilitated cement-based materials (RCMs) with carbon-fiber-reinforced polymer (CFRP) composites is quite important in the transportation and infrastructure industries. Among various NDT methods, active microwave thermography (AMT) has shown good potential. This method uses microwave energy to heat a structure of interest, and subsequently the surface thermal profile is measured using a thermal camera. In this paper, the application of AMT for defect detection (unbond, delamination, and crack) in CFRP composites used in RCMs is presented. More specifically, the effect of defect size and depth and polarization on the resultant surface thermal profile with defects is first studied through simulation. The effect of polarization on detection of defects with regard to the orientation of CFRP fibers is also experimentally investigated. Finally, a quantitative analysis of the measured results based on the thermal contrast and signalto- noise ratio (SNR) for all the three aforementioned defect types is presented. The results show that the SNR is improved when utilizing perpendicular (compared with parallel) polarization, and that the maximum effective heating time is similar to 60 s, even for small defects.