Active Microwave Thermography (AMT)

Active Microwave Thermography (AMT) is a relatively new nondestructive testing and evaluation (NDT&E) technique that utilizes a microwave-based thermal excitation and subsequent thermographic imaging. In AMT, medium power (50 – 200 W) microwave energy is radiated towards the inspection surface of a structure/material under test. This energy is used to minimally heat the structure, and the resulting surface thermal profile is monitored via a thermal camera. A general schematic of an AMT inspection is shown in Fig. 1. Here, a structure under test (SUT) containing a defect is shown under microwave illumination from a radiating antenna. The subsequent surface thermal profile resulting from the incident microwave energy interacting with the SUT is viewed by a thermal camera. The system is controlled via a desktop computer which interfaces with the camera and the data acquisition (DAQ) system. Compared to traditional (flash lamp) thermography, AMT does not require substantial amounts of power, and several electromagnetic parameters (frequency, polarization, etc.) can be optimized to tailor the inspection to a specific material or structure. In this way, AMT offers the advantage of selective heating, as the heat source may be exerted only upon the (lossy) defect(s), with less interaction with the background material(s). In this case, unlike conventional thermography, the defect acts as a heat source, rather than solely affecting thermal diffusion.

Figure 1: AMT system illustration.

As AMT has the capability to selectively heat materials of interest for subsequent detection, one such example of this is detection of moisture ingress. Fig. 2 contains the temporal measured differential surface thermal profile (with respect to ambient) of a SUT containing a few drops of water (~0.05 – 0.1 mL) located between two layers of (low loss) rubber, with the location of the moisture indicated by the white dashed square. This sample was designed to emulate moisture ingress in rubber water pipes, with the measurement setup as illustrated in Fig. 1. As water is very electromagnetically lossy, the 2.4 GHz incident microwave energy (total of 50 W) is absorbed by the water, resulting in the hot spot evident in the video. Similar results are achievable for many applications including crack detection, defects in RAM-coated structures, and defect detection within conductive composites such as carbon fiber-based polymer materials.

Figure 2: Temporal measured surface temperature difference (with respect to ambient) of moisture ingress in rubber.