Health Monitoring of RAM-Coated Structures by Active Microwave Thermography

In this article, the application of active microwave thermography (AMT) is investigated as an efficient nondestructive technique (NDT) for health monitoring of structures coated with radio frequency absorbing materials (RAMs). The uniqueness of AMT for this particular application is the use of microwave excitation, which acts as a highly efficient thermal source due to the inherent absorptive electromagnetic properties of RAM-coated structures. In addition, the microwave-induced heating takes place subsurface within the absorbing material of an RAM-coated structure, as opposed to the sole surface heating of conventional thermography. Specifically, this work focuses on detection of delamination, a common defect in carbon fiber reinforced polymer (CFRP) structures, via AMT. To this end, a formulation and analysis method based on electromagnetic/thermal modeling is presented, which enables estimation of the output of an AMT inspection of an RAM-coated CFRP structure suffering from delamination. The effect of the structure's constitutive properties, the microwave excitation parameters, and the delamination size and depth on its detectability is investigated. In addition, several simulated and experimental results are provided, which verifies the accuracy of the proposed approach and shows the high efficiency of AMT for detection of delamination in such structures.