Development of Active Microwave Thermography (AMT) for Nondestructive Testing and Evaluation (NDT&E) Applications
Nondestructive testing and evaluation (NDT&E) of composite infrastructure and aerospace structures is a challenging issue to fully address with traditional NDT&E techniques. It can often be quite difficult to use a single modality for a complete inspection of structures or materials due to the complexity and variation in material type. Various NDT&E methods including microwave ultrasonic, eddy current, and thermography have been applied for inspection of such materials and structures, each with their own benefits and limitations. Integrated NDT methods have become more popular in recent years, including those utilizing thermographic measurements. Thermography conventionally uses a flash/heat lamp to generate thermal contrast and facilitate detection of surface or subsurface defects. However, the main limitation of traditional thermography is that the thermal energy cannot be targeted to an area of interest, but rather is applied over a large area. Thus, integrated thermography is another approach that has gained in popularity in research years, including ultrasonic thermography, eddy current thermography, and most recently, active microwave thermography (AMT). AMT uses a microwave excitation to generate heat and the surface thermal profile of the material or structure under test is subsequently measured using a thermal camera. Utilizing a microwave heat excitation provides advantages over other thermal excitations including the potential for non-contact, selective and focused heating. Additionally, the unique properties of microwave signals including frequency, power, and polarization can be utilized to customize an AMT inspection to a specific application. During an AMT inspection, two heating mechanisms are possible, referred to as dielectric and induction heating. Dielectric heating occurs as a result of the interaction of microwave energy with lossy dielectric materials, and induction heating is a result of induced surface current on conductive materials. Thus, AMT has strong potential for application in various industries including infrastructure, transportation, aerospace, etc. In this presentation, the application of AMT for inspection of carbon-fiber reinforced polymer (CFRP)-strengthened and steel-fiber reinforced cement-based (SFRC) structures will be discussed in detail. In order to quantitatively study the effect of various parameters on the measurement results, the thermal contrast (TC) between healthy and defective areas and the signal-to-noise ratio (SNR) will be explored. From this analysis, important measurement-related conclusions will be made about numerous measurement parameters (e.g., minimum required heating time) and their relationship with specific measurement system parameters (thermal camera sensitivity, power level, etc.). Finally, other applications that have successfully employed AMT including evaluation of corrosion on metal structures and detection of covered surface cracks will be briefly highlighted.