Time- and Phase-Domain Thermal Tomography of Composites

Abstract

Active infrared (IR) thermographic nondestructive testing (NDT) has become a valuable inspection method for composite materials due to its high sensitivity to particular types of defect and high inspection rate. The computer-implemented thermal tomography, based on the analysis of heat diffusion in solids, involves a specialized treatment of the data obtained by means of active IR thermographic NDT, thus allowing for the “slicing” of materials under testing for a few layers where discontinuity-like defects can be underlined on the noise-free background (binary thermal tomograms). The time-domain thermal tomography is based on the fact that, in a one-sided test, temperature “footprints” of deeper defects appear later in regard to shallower defects. The phase-domain tomography can be applied to collected IR data in a direct way, for instance, by using the Fourier transform, but quantification of results is more difficult because the relationships between phase and defect depth depend on experimental parameters, and the corresponding “phase vs. defect depth” calibration functions are ambiguous. In this study, the time- and phase-domain thermal tomography techniques have been compared on simulated IR thermograms and experimentally applied to the evaluation of carbon fiber reinforced plastic composite containing impact damage defects characterized by impact energy 10, 18, and 63 J. Both tomographic techniques have demonstrated similar results in the reconstruction of thermal tomograms and, in some cases, supplied complementary information about the distribution of single defect zones within impacted areas.

An Experimental Study on the Defect Detectability of Time- and Frequency-Domain Analyses for Flash Thermography

“…The effectiveness of this prior standardization is demonstrated in Figure 3 Right Column. Dynamic thermal tomography (DTT) is an often-used time-domain analysis technique, which operates on the thermal contrast Δ [21,22,35]. The thermal contrast is calculated by subtracting the thermal response of a manually chosen reference (sound) pixel's thermal response from all other pixel responses (see Figure 4a).…”

“…In Ref. [21], defect detection is performed by evaluating the phase at arbitrary frequencies. In contrast, the present approach focuses on a systematic search of the maximum phase contrast and its corresponding frequency, analogous to DTT.…”

“…The defect detectability is significantly improved by post-processing of the recorded thermal data sequence using image processing [14][15][16] and data processing [2,3,[17][18][19][20] techniques. Some of the data processing algorithms operate directly on the time-domain data (i.e., the recorded thermal sequence), such as dynamic thermal tomography (DTT) [21,22], differential absolute contrast (DAC) [23], and thermal signal area (TSA) [24]. On the other hand, performing the analysis in the frequency domain (i.e., after performing a fast Fourier transform) exhibits the advantageous property that undesirable effects of non-uniform heating and non-uniform surface emissivity are strongly reduced.…”

“…On the other hand, performing the analysis in the frequency domain (i.e., after performing a fast Fourier transform) exhibits the advantageous property that undesirable effects of non-uniform heating and non-uniform surface emissivity are strongly reduced. Some of the analysis techniques operating in frequency domain are pulsed phase thermography (PPT) [18,25,26], modified pulse-phase thermography (PPT1/PPT2) [27], phase-domain thermal tomography (PDTT) [21], and adaptive spectral band integration (ASBI) [19]. PDTT may be considered as a phase-domain equivalent of DTT, where a pixel's phase angle instead of an indicative time provides information on the defect's depth [21].…”

“…Some of the analysis techniques operating in frequency domain are pulsed phase thermography (PPT) [18,25,26], modified pulse-phase thermography (PPT1/PPT2) [27], phase-domain thermal tomography (PDTT) [21], and adaptive spectral band integration (ASBI) [19]. PDTT may be considered as a phase-domain equivalent of DTT, where a pixel's phase angle instead of an indicative time provides information on the defect's depth [21]. PDTT, however, requires the operator to select both the evaluation frequency and the phase range of interest, and can scarcely be used for quantitative defect evaluation.…”