Frequency-phase modulated thermal wave radar : stepping beyond state-of-the-art infrared thermography

Thermal wave radar is a state-of-the-art non-destructive testing method inspired by radio wave radar systems. The underlying principle of the technique is the application of a modulated excitation waveform by which the total energy of the response signal can be compressed in time-domain through cross-correlation. This leads to an enhanced depth resolution and increased signal to noise ratio in optical infrared thermography. Frequency sweep and Barker binary phase modulation are the two popular and widely researched excitation waveforms of the technique. In this research, a novel frequency-phase modulated waveform is introduced, which is designed for optimized performance of thermal wave radar

Multi-scale gapped smoothing algorithm for robust baseline-free damage detection in optical infrared thermography

Flash thermography is a promising technique to perform rapid non-destructive testing of composite materials. However, it is well known that several difficulties are inherently paired with this approach, such as non-uniform heating, measurement noise and lateral heat diffusion effects. Hence, advanced signal-processing techniques are indispensable in order to analyze the recorded dataset. One such processing technique is Gapped Smoothing Algorithm, which predicts a gapped pixel’s value in its sound state from a measurement in the defected state by evaluating only its neighboring pixels. However, the standard Gapped Smoothing Algorithm uses a fixed spatial gap size, which induces issues to detect variable defect sizes in a noisy dataset. In this paper, a Multi-Scale Gapped Smoothing Algorithm (MSGSA) is introduced as a baseline-free image processing technique and an extension to the standard Gapped Smoothing Algorithm. The MSGSA makes use of the evaluation of a wide range of spatial gap sizes so that defects of highly different dimensions are identified. Moreover, it is shown that a weighted combination of all assessed spatial gap sizes significantly improves the detectability of defects and results in an (almost) zero-reference background. The technique thus effectively suppresses the measurement noise and excitation non-uniformity. The efficiency of the MSGSA technique is evaluated and confirmed through numerical simulation and an experimental procedure of flash thermography on carbon fiber reinforced polymers with various defect sizes

Backside delamination detection in composites through local defect resonance induced nonlinear source behavior

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Flash thermography of composites : evaluation of advanced post-processing approaches

Carbon fiber reinforced polymers (CFRP) are composite materials that offer a high stiffness-to-weight ratio in comparison to traditional metals, which explains their increasing use in many high-end applications (e.g. aerospace). However, composites are prone to internal damage that may deteriorate the structural integrity, and thus require reliable and non-destructive testing (NDT) approaches. Infrared thermography (IRT) is a promising NDT technique which provides fast, full-field measurements, and in which hidden defects are detectable based on their thermal signatures. In flash thermography (FT), which is the thermographic technique of interest for this contribution, the component’s surface temperature is rapidly elevated through the application of an intense optical flash. Subsequent recording of the cooling down of the stimulated surface, by means of a high-end infrared camera, allows to detect defects by searching for anomalies in the surface temperature (due to heat build-up above the defect). Considering the anisotropic diffusivity and high damping of thermal waves in CFRP, advanced post-processing techniques are indispensable to detect deep defects (> 2 mm in CFRP). In this paper, FT is performed on several CFRPs with various defects (flat bottom holes, Teflon inserts and barely visible impact damage). This thermographic dataset is then analyzed using various post-processing techniques, including pulsed phase thermography (PPT), principal component thermography (PCT), thermographic signal reconstruction (TSR) and dynamic thermal tomography (DTT), in order to improve the defect detectability and assessment. The performance of the employed processing techniques is critically evaluated

Enhanced detectability of barely visible impact damage in CFRPs : vibrothermography of in-plane local defect resonances

This paper demonstrates the enhanced detectability of barely visible impact damage in CFRPs through low power vibrothermography of in-plane local defect resonances (LDR). In-plane LDR (LDRxy), with a higher cut-off frequency than out-of-plane LDR (LDRz), generally enhances the rubbing interaction and viscoelastic damping of defects and leads to higher vibration-induced heating. The most prominent LDRz and LDRxy frequencies of an impacted CFRP are extracted from its vibrational spectra under a broadband sweep excitation, measured by a 3D infrared laser Doppler vibrometer. The sample is then inspected through lock-in vibrothermography at the extracted LDR frequencies and the distintively higher detectability of LDRxy compared to LDRz is evidenced. Moreover, it is observed that the thermal contrast induced by LDRxy is so high, that it allows for easy detection of impact damage by live monitoring of infrared thermal images during a single broadband sweep vibration excitation

Nonlinear elastic wave energy imaging for the detection and localization of in-sight and out-of-sight defects in composites

In this study, both linear and nonlinear vibrational defect imaging is performed for a cross-ply carbon fiber-reinforced polymer (CFRP) plate with artificial delaminations and for a quasi-isotropic CFRP with delaminations at the edge. The measured broadband chirp vibrational response is decomposed into different components: the linear response and the nonlinear response in terms of the higher harmonics. This decomposition is performed using the short-time Fourier transformation combined with bandpass filtering in the time-frequency domain. The linear and nonlinear vibrational response of the defect is analyzed by calculation of the defect-to-background ratio. Damage maps are created using band power calculation, which does not require any user-input nor prior information about the inspected sample. It is shown that the damage map resulting from the linear band power shows high sensitivity to shallow defects, while the damage map associated to the nonlinear band power shows a high sensitivity to both shallow and deep defects. Finally, a baseline-free framework is proposed for the detection and localization of out-of-sight damage. The damage is localized by source localization of the observed nonlinear wave components in the wavenumber domain

Performance of frequency and/or phase modulated excitation waveforms for optical infrared thermography of CFRPs through thermal wave radar : a simulation study

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 defect-induced 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

A robust multi-scale gapped smoothing algorithm for baseline-free damage mapping from raw thermal images in flash thermography

Flash thermography is a promising non-destructive testing technique for the inspection of composite components. However, non-uniform heating, measurement noise and lateral heat diffusion complicate the interpretation of thermographic measurements. In order to overcome these difficulties, a novel baseline-free processing technique called ‘Multi-Scale Gapped Smoothing Algorithm’ is presented. This algorithm constructs a damage map directly from the measured data, in which an (almost) zero-reference background is obtained, and where measurement noise and excitation non-uniformity are effectively suppressed. The efficiency of the proposed technique is evaluated and confirmed through synthetic data and experimental results of a carbon fiber reinforced polymer with various artificial defects