Effect of Signal Modulation on Active Microwave Thermography

Active microwave thermography (AMT) is a coupled electromagnetic and thermographic nondestructive testing and evaluation (NDT&E) technique. AMT utilizes an active electromagnetic source to induce heat on or within a specimen under test (SUT) with subsequential infrared measurements via an infrared camera. AMT has been successfully employed in multiple fields, including aerospace, space, and infrastructure, among others. Generally, for AMT, the excitation (electromagnetic source) is applied over a specified period of time (referred to as the heating time). If inspection continues for additional time after the excitation is removed (turned off), this period is referred to as the cooling time. This general inspection scheme differs from traditional thermography, where the excitation is applied once over a very short time period or pulsed (i.e., modulation), as is the case for pulsed thermography (PT). This work applies the pulsed excitation approach to AMT, herein referred to as P-AMT, and compares the thermal contrast (TC) and signal-to-noise ratio (SNR) of traditional and pulsed AMT inspections. P-AMT was also expanded to other modulation waveforms (referred to as M-AMT) and duty cycles. The results indicate that P-AMT and M-AMT can provide equivalent detection with a reduction in the induced thermal energy. This is important as it relates to concerns of thermal damage to a given SUT as well as an overall reduction in required inspection time.