Following the developments in pulse compression techniques for increased range resolution and higher signal to noise ratio of radio wave radar systems, the concept of thermal wave radar (TWR) was introduced for enhanced depth resolvability in optical infrared thermography. However, considering the highly dispersive and overly damped behavior of heat wave, it is essential to systematically address both the opportunities and the limitations of the approach. In this regard, this paper is dedicated to a detailed analysis of the performance of TWR in inspection of carbon fiber reinforced polymers (CFRPs) through frequency and/or phase modulation of the excitation waveform. In addition to analogue frequency modulated (sweep) and discrete phase modulated (Barker binary coded) waveforms, a new discrete frequency-phase modulated (FPM) excitation waveform is introduced. All waveforms are formulated based on a central frequency so that their performance can be fairly compared to each other and to lock-in thermography at the same frequency. Depth resolvability of the waveforms, in terms of phase and lag of TWR, is firstly analyzed by an analytical solution to the 1D heat wave problem, and further by 3D finite element analysis which takes into account the anisotropic heat diffusivity of CFRPs, the non-uniform heating induced by the optical source and the measurement noise. The spectrum of the defectinduced phase contrast is calculated and, in view of that, the critical influence of the chosen central frequency and the laminate's thickness on the performance of TWR is discussed. Various central frequencies are examined and the outstanding performance of TWR at relatively high excitation frequencies is highlighted, particularly when approaching the so-called blind frequency of a defect.

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It is well known that the efficiency of the vibrothermographic non-destructive testing (NDT) technique can be enhanced by taking advantage of local defect resonance (LDR) frequencies. Recently, the classical out-of-plane local defect resonance was extended towards in-plane LDR for enhanced efficiency of vibrometric NDT. This paper further couples the concept of this in-plane LDR to vibrothermography, on the basis of the promising potential of in-plane LDRs to enhance the rubbing (tangential) interaction and viscoelastic damping of defects. Carbon fiber-reinforced composites (CFRPs) with barely visible impact damage (BVID) are inspected and the significant contribution of in-plane LDRs in vibrational heating is demonstrated. Moreover, it is shown that the defect thermal contrast induced by in-plane LDRs is so high that it allows for easy detection of BVID by live monitoring of infrared thermal images during a single broadband sweep excitation. Thermal and vibrational spectra of the inspected surface are studied and the dominant contribution of in-plane LDR in vibration-induced heating is demonstrated.

E.V.); Joost.Segers@UGent.be (J.S.); Saeid.Hedayatrasa@UGent.be (S.H.); Wim.VanPaepegem@UGent.be (W.V.P.) 2 SIM Program M3 DETECT-IV, Technologiepark-Zwijnaarde 935, B-9052 Zwijnaarde, BelgiumAbstract: Different non-destructive testing techniques have been evaluated for detecting and assessing damage in carbon fiber reinforced plastics: (i) ultrasonic C-scan, (ii) local defect resonance of front/back surface and (iii) lock-in infrared thermography in reflection. Both artificial defects (flat bottom holes and inserts) and impact damage (barely visible impact damage) have been considered. The ultrasonic C-scans in reflection shows good performance in detecting the defects and in assessing actual defect parameters (e.g., size and depth), but it requires long scanning procedures and water coupling. The local defect resonance technique shows acceptable defect detectability, but has difficulty in extracting actual defect parameters without a priori knowledge. The thermographic inspection is by far the fastest technique, and shows good detectability of shallow defects (depth < 2 mm). Lateral sizing of shallow damage is also possible. The inspection of deeper defects (depth > 3-4 mm) in reflection is problematic and requires advanced post-processing approaches in order to improve the defect contrast to detectable limits.