A comparison of the pulsed, lock-in and frequency modulated thermography nondestructive evaluation techniques

TLDR: Pulsed, lock-in and frequency modulated thermography are three alternative nondestructive evaluation techniques. as discussed by the authors compared the defect imaging performance of these techniques using: matched excitation energy; the same carbon fiber composite test piece and infrared camera system.

Abstract: Pulsed, lock-in and frequency modulated thermography are three alternative nondestructive evaluation techniques. The defect imaging performance of these techniques are compared using: matched excitation energy; the same carbon fiber composite test piece and infrared camera system. The lock-in technique suffers from “blind frequencies” at which phase images for some defects disappear. It is shown that this problem can be overcome by using frequency modulated (chirp) excitation and an image fusion algorithm is presented that enhance phase imaging of defects. The signal-to-noise ratios (SNRs) of defect images obtained by the three techniques are presented. For the shallowest defects (depths 0.25 and 0.5 mm, 6 mm diameter), the pulsed technique exhibits the highest SNRs. For deeper defects the SNRs of the three techniques are similar in magnitude under matched excitation energy condition.

Figures

Figure 4: Experimental setup

Table 3: Estimated amplitude of surface temperature oscillations for lock-in tests at various frequencies as imposed by the energy matching criterion

Figure 2: Plot of logarithmic temperature rise (digital level) against logarithmic time over sound region in pulsed thermography experiment. The inset plot zooms around t = 1 second.

Figure 8: Experimental phase images at 16.7 mHz, 33.3 mHz, 50.0 mHz, 66.7 mHz and 83.3 mHz for lock-in thermography (8a-8e), and FMTWI (8f-8j), respectively.

Figure 10: Plot of maximum SNR as a function of defect depth, as in Table 6

Figure 13: Fused image obtained from the extracted FMTWI phase images at the five analysis frequencies.