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In active infrared thermographic inspection of materials, heat wave is stimulated by activation of heat sources, e.g., through optical heat radiation or vibration-induced heat dissipation. Therefore, the monopolar (i.e., heating only) nature of excitation introduces an inevitable ascending trend in the measured thermal response. To obtain an improved thermal wave imaging quality, it is crucial to remove this ascending trend and to analyze the decoupled bipolar (i.e., AC) component of the thermal response. This study introduces the concept of phase inversion in thermographic inspection, as a deterministic method for (i) decoupling the AC component and (ii) filtering the prominent second-order nonlinearities from the thermal response. First, this "phase inversion thermography (PIT)" is theoretically substantiated by analysis of heat diffusion through the thickness of a solid material subjected to dissipative boundary conditions. Then, the performance of PIT in accurately decoupling the AC response from various excitation waveforms is verified by finite element simulation of optical infrared thermography on an anisotropic composite coupon. It is shown that by proper selection of the signal's initial phase and waveform duration, PIT yields the AC response with a zero-mean amplitude. At last, the experimental applicability of PIT is evaluated for two different test cases: (1) optical thermography on a backside-stiffened carbon fiber-reinforced polymer (CFRP) aircraft panel with a complex cluster of production defects and (2) low-power vibrothermography on an impacted CFRP coupon. It is shown that PIT, as a physics-based signal processing technique, robustly resolves the strongly transient onset of the excitation and systematically decouples an AC response which is equivalent to the thermal response to an ideally linear and bipolar excitation. The recorded thermal responses are post-processed through Fourier transform, and the enhanced thermal imaging quality and improved defect detectability of the decoupled AC component are demonstrated.

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 polymers (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.

International audience; This paper deals with the dynamic characterisation of plate structures with elliptical orthotropic stiffness properties, using an equivalent thin plate theory using a wave fitting approach. The method consists in projecting the experimentally determined transverse displacement field of a plate on an analytical Green’s function of an elliptical orthotropic plate based on Hankel’s functions. The error between the projected and measured fields is then minimized, varying the characteristics of the function until an optimal fit is reached. The thus obtained characteristics are the two flexural rigidities defining the elliptical orthotropy of the plate, and the orthotropy angle. This fitting procedure is applied at each frequency, enabling the determination of frequency dependent dynamic material properties. The method is applied to a honeycomb sandwich panel to validate the proposed fitting approach. The identified flexural rigidities are compared to the estimations obtained by means of an analytical model and the IWC (Inhomogeneous Wave Correlation) method assuming three different type of plate characteristics (anisotropic, orthotropic and elliptical orthotropic). For the elliptical orthotropic assumption, consistent results are observed between the methods and the model over a large frequency range (from 1 to 50 kHz).

In this study, a novel wave processing algorithm: “local wave-direction estimation (LWDE)”, is proposed to localize sources of guided waves using full-field scanning laser Doppler (SLDV) measurements. The LWDE algorithm uses angular bandpass filters in the wavenumber domain to determine the local propagation direction of the guided wave. The obtained local wave-direction map is converted into an error map by the use of a virtual local wave-direction map. The resulting error map reveals the sources of guided waves as local minima. The exploitation of local directional wave information provides the opportunity to scan only a small part of the component for localization of out-of-sight sources of guided waves. The source localization performance of LWDE is illustrated for localization of multiple piezoelectric sources in a quasi-isotropic CFRP plate. LWDE is further coupled to nonlinear vibrations in view of damage localization (NL-LWDE). The second higher harmonic component is extracted from the SLDV measurement and the sources of this higher harmonic component are localized. Damages behave as sources of higher harmonics and are thus localized by this NL-LWDE approach. This is demonstrated for a cross-ply CFRP plate with low velocity impact damage. The proposed combination of (i) nonlinear vibration filtering, (ii) direction estimation using angular bandpass filtering in the wavenumber domain and (iii) error map construction using the virtual wave-direction map results in a robust and baseline-free NDT technique. It can be used for accurate localization of multiple in-sight or out-of-sight damages in composite plates with unknown material properties and layup.

Thermal Wave Radar (TWR) imaging employs the concept of pulse compression in order to obtain an increased probing depth and depth resolution in infrared thermographic testing of materials. The efficiency of the TWR imaging is highly dependent on the nature of the employed excitation signal. Most studies exploit the use of an excitation signal with an analogue frequency modulation (e.g. sweep signal) or a discrete phase modulation (e.g. Barker coded signal). Recently, a novel frequency-phase modulated (FPM) waveform was introduced, and computationally verified by the current authors, which couples the concept of frequency- and phase modulation to each other in view of obtaining an optimized excitation signal for improved TWR imaging. This paper experimentally investigates the performance of the novel optimized FPM waveform for the inspection of glass and carbon fiber reinforced polymer (GFRP and CFRP) composites, using an optical infrared thermography set-up in reflection mode. The response of the halogen lamps to the FPM waveform is measured, and further the influence of the electro-thermal latency of excitation lamps on the applicability of the novel FPM excitation signal is analytically investigated. Then, the performance of the FPM waveform is experimentally investigated for both glass- and carbon fiber reinforced polymers with defects of different depths and sizes. A comparative analysis is performed with amplitude modulated (classical lock-in), frequency modulated (sweep) and phase modulated (Barker coded) excitation, each with the same time duration as the FPM waveform. The novel FPM waveform outperforms these existing waveforms in terms of defect detectability and contrast-to-noise ratio, especially for the deeper defects. Different central frequencies are examined and the improved performance of the FPM waveform in TWR imaging is demonstrated in all cases.

A defect&rsquo, s detectability in flash thermography is highly dependent on the applied post-processing methodology. The majority of the existing analysis techniques operate either on the time-temperature data or on the frequency-phase data. In this paper, we compare the efficiency of time- and frequency-domain analysis techniques in flash thermography for obtaining good defect detectability. Both single-bin and integrated-bin evaluation procedures are considered: dynamic thermal tomography and thermal signal area for the time-domain approach, and frequency domain tomography and adaptive spectral band integration for the frequency-domain approach. The techniques are applied on various carbon fiber reinforced polymer samples having a range of defect sizes and defect types. The advantages and drawbacks of the different post-processing techniques are evaluated and discussed. The best defect detectability is achieved using the integrated procedure in frequency domain.

Vibrothermography using sinusoidal vibration excitation at the resonance frequencies of a defected area (so-called local defect resonance, or LDR) is a promising technique to boost the defect's deformation and its interfacial interactions and as such enhance resultant vibration-induced heating. Contrary to the classical high-power vibrothermography, low power excitation at an LDR frequency results in a reproducible thermal response and adequate quantification of the corresponding damage features. However, the technique is mainly limited by the fact that it requires a priori knowledge of the LDR frequencies (e.g. obtained from prior vibrational measurements). To overcome this limitation, a stand-alone vibrothermographic spectroscopy procedure is introduced in this paper. The proposed technique applies two consecutive broadband sweep vibrational excitations with ascending and descending frequency modulation rates to the sample. The surface of the excited sample is monitored with an IR camera. Both time derivative analysis and superposition of the recorded thermal responses are performed in order to compensate for the thermal latency of the defect-induced heating. This compensation approach enables proper identification of the actual LDR frequencies based on the apparent LDR frequencies of the thermal response. The method is applied on a carbon fiber reinforced polymer (CFRP) with barely visible impact damage (BVID), and multiple LDR frequencies are readily identified. The identified LDR frequencies are also individually evaluated by both lock-in vibrothermography and 3D scanning laser Doppler vibrometry, confirming the competence of the proposed technique for extracting LDR frequencies in a proper and fast way.

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.

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.

Shallow damages can be detected by searching for local defect resonance behavior in the fundamental vibrational response. However, this is not possible for defects which are deeper than half the thickness of the coupon due to the limited bending stiffness difference between the defect and the sound material. In this study, it is shown that delaminations under harmonic excitation at the local defect resonance frequency behave as sources of higher harmonic vibration components. These higher harmonic components are detectable when examining the delaminated component from the side where the delamination is close to the surface, but more importantly, also from the other side where the delamination is at the backside. As such, monitoring of these harmonics allows for the detection of backside delaminations which cannot be found using linear LDR techniques. First, a finite element simulation with nonlinear contact conditions is performed as a proof of concept. Experimental validation is performed for a delaminated CFRP coupon using low-power piezoelectric actuation and scanning laser Doppler vibrometer response measurements.

This paper investigates the performance of the thermal wave radar imaging technique in low-power vibrothermography, so-called vibro-thermal wave radar (VTWR), for non-destructive inspection of composites. VTWR is applied by binary phase modulation of the vibrational excitation using a 5 bit Barker coded waveform, followed by matched filtering of the thermal response. The depth resolvability of VTWR is analyzed by a 1D analytical formulation in which defects are modeled as subsurface heating sources. The obtained results reveal the outperformance of VTWR compared to the classical lock-in vibrothermography (LVT), i.e. a sinusoidal amplitude modulation. Furthermore, the VTWR technique is experimentally demonstrated on a 5.5 mm thick carbon fiber reinforced polymer coupon with barely visible impact damage. A local defect resonance frequency of a backside delamination is selected as the vibrational carrier frequency. This allows for implementing VTWR in the low-power regime (input power < 1 Watt). It is shown that the Barker coded amplitude modulation and the resultant pulse compression efficiency lead to an increased probing depth, and can fully resolve the deep backside delamination. It is also shown that VTWR approach allows depth-selective imaging of defects through the lag of the compressed pulse., 16 pages, 8 figures

Local wavenumber estimation (LWE) applied to a full wavefield response is a powerful approach for detecting and characterizing defects in a composite structure. However, the narrowband nature of the traditional LWE techniques brings several challenges for application on actual test cases. This study proposes a self-reference broadband version of the LWE technique. The broadband vibrations are injected using low-power piezoelectric actuators (sine sweep signal) or using pulsed laser excitation in the thermoelastic regime. The out-of-plane velocity response of the surface is recorded using an infrared scanning laser Doppler vibrometer. The dispersive Lamb wave behavior, corresponding to the damage-free base material, is identified from the broadband vibrational response. Using the identified dispersion curves (i.e. self-reference approach), a Lamb mode passband filter bank in the wavenumber-frequency domain is constructed. Searching for the maximum bandpower density in function of the assumed material thickness provides a robust estimate of the effective local thickness of the tested component, and as such yields a detection and evaluation of damage. The performance of the self-reference broadband LWE algorithm is demonstrated on aluminum plates with various flat bottom holes, as well as on cross-ply CFRP aircraft components with a stiffener disbond and barely visible impact damage. Compared to the traditional narrowband LWE approaches, the proposed self-reference broadband LWE method allows a higher level of automation, removes the need for a priori knowledge on the material and/or defect properties, and results in an improved characterization of defects.

Local defect resonance (LDR) makes use of high frequency vibrations to get a localized resonant activation of the defect. In this study, it is shown for various samples and damage features that the classical out-of-plane local defect resonance can be equally extended towards in-plane local defect resonance. It is found that the in-plane LDR typically occurs at higher frequencies, which is linked to the specific geometry of common defects. This increased frequency allows for a reduction in measurement time. More importantly, the results indicate that the in-plane LDR has an increased sensitivity for defects with dominant out-of-plane defect interfaces (e.g. surface breaking crack). Knowledge of both the out-of-plane and in-plane LDR provides further insight on the internal structure of a defective area. This is explicitly demonstrated on a carbon composite which was subjected to low velocity impact, inducing barely visible impact damage.

Topics Abstract 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 broad-band sweep excitation, measured by a 3D scanning infrared laser Doppler vibrometer. The sample is then inspected through lock-in vibrothermography at the extracted LDR frequencies. The measured thermal contrast induced by LDRXY is significantly higher than that of LDRZ.

Vibrothermography at local defect resonance (LDR) frequencies is a promising technique to localize the vibrational energy at defected area and enhance its detectability under a low excitation power. However, the technique is mainly limited by the fact that it makes use of the LDR frequencies already known from e.g. vibrational measurements. To overcome this limitation, thermal response derivative spectroscopy is suggested as a post-processing strategy for reference-free identification of LDRs in sweep vibrothermography. An impacted carbon fiber reinforced polymer (CFRP) coupon is tested under broadband frequency sweep excitation and its surface temperature is recorded by an infrared camera. The LDR frequencies associated with different regions of the defected area are then extracted by time derivative analysis of the resultant thermal response. Selected LDRs obtained through the proposed technique are individually validated by lock-in vibrothermography at corresponding frequencies.

Non-destructive testing of composites using full wave field analysis of guided waves is illustrated for a CFRP aircraft panel with production defects. The full wave field is measured using 3D scanning laser Doppler vibrometry for broadband chirp excitation through one piezoelectric actuator. First, the A0 mode is extracted through mode filtering in the frequency-wavenumber domain. Next, the measured chirp response is converted to a narrowband burst response. At last, the local wavenumber map is constructed and evaluated. A debonding defect between stiffener and base plate is detected and confirmed by the ultrasonic C-scan time-of-flight map.

Thermal wave radar (TWR) is a state-of-the-art non-destructive testing method, inspired by radio wave radar systems, in order to increase depth resolution and signal to noise ratio of optical infrared thermography through pulse compression. Analogue frequency modulation (i.e. frequency sweep) and Barker binary phase modulation are the two popular and widely researched pulse compression techniques in TWR among which Barker coding has shown the highest performance. This paper introduces a novel modulated waveform with variable discrete frequency-phase modulation (FPM) which distinctively enhances the depth resolvability of TWR compared to the existing techniques. The pulse compression quality and depth resolvability of the novel FPM waveform is initially evaluated through a 1D analytical solution. The analogue frequency modulated and discrete phase modulated waveforms as well as mono-frequency excitation (i.e. lock-in thermography) are also evaluated at the same central frequency as the reference. Objective functions are defined and a large search space is explored for optimal modulation codes. Two FPM waveforms are selected based on their maximized depth resolvability through resultant lag and phase in the output channel of TWR. Furthermore, the excellent performance of the selected FPM waveforms is validated by 3D finite element simulation. A delaminated glass fiber reinforced polymer (GFRP) laminate is simulated in order to evaluate the impact of a dominant lateral heat diffusion on the performance of the novel FPM waveforms. The superior depth resolvability of the introduced FPM waveforms is confirmed and their robustness at various noise levels is demonstrated.

For decades, modal analysis has been a tool to gain insights in the vibrational and acoustical behavior of music instruments. This study provides a critical comparison of various experimental modal analysis approaches with the violin as a case study. Both contact and non-contact excitation and different acquisition approaches are considered. The influence of different boundary conditions (clamped, supported, free/free) on the vibrational response of the violin is investigated. The response is analyzed to extract relevant modal parameters (frequency, damping and mode shape) within the frequency range of 0–1000 Hz. The performance of the different approaches is evaluated in terms of (i) signature vibration modes, (ii) experimental reproducibility, (iii) contact requirement and (iv) cost of required equipment. Two optimal approaches are proposed for performing modal analysis on violins: a contact method for use in the violinmaker’s practice, and a non-contact method for use with fragile instruments.

In this study, a non-destructive testing approach is investigated for finding damages in fiber reinforced polymers using broadband piezoelectric excitation and scanning laser Doppler vibrometer measurements. An efficient broadband filter in wavenumber-frequency domain is considered, in order to calculate a mode-removed broadband weighted root mean square (WRMS) energy map of the guided waves. Compensation of the frequency-dependent wave attenuation is implemented. Because this energy map relates exclusively to abnormalities in the wave field, it shows high sensitivity to all kinds of internal damages. The proposed damage map construction method is automated, and does not require any a priori information on the inspected part or measurement conditions. Further, it is not limited to application on flat coupon samples, but can be equally applied on complex composite parts. The methods are tested on an academic case of a CFRP plate with flat bottom holes and on an industrial case of a large stiffened CFRP aircraft panel with distributed manufacturing defects. It is shown that the broadband mode-removed WRMS energy map is highly sensitive to various kinds of damages, even if they are small and deep inside the material.

This article presents a fatigue model that can robustly capture the effect of high surface roughness on the fatigue crack initiation of additively manufactured Ti6Al4V (AM Ti64) alloys in the moderate cycle fatigue regime based on nonlinear finite element (FE) analyses and the theory of critical distances. We propose two methods to use surface measurements of AM components to create FE models, viz., surface topography of the component is explicitly represented in the FE models or a simplified geometrical model, which has only a single sinusoidal-shaped notch, is proposed to replace the surface topography in the FE models. The geometrical parameters of the simplified model are derived based on the MOTIF method (ISO 12085) and its parameters are determined by using the leave-one-out cross-validation method. In the moderate cycle fatigue regime, the stress applied to components is below the yield stress, but the high surface roughness acts as a stress raiser and causes local plasticity at surface valleys. Thus, a nonlinear hardening elasto-plasticity model is employed in the FE models to capture the cyclic mechanical behavior of AM Ti64 alloys. Afterward, the fatigue analyses are performed by employing our in-house C++ software within which fatigue indicator parameters (FIPs) are computed by post-processing the finite element results. In the present work, the Smith-Watson-Topper (SWT) parameter is used as an FIP and is computed at every element in the FE mesh, i.e., the local FIP. Subsequently, to account for the stress gradient effect in the proposed fatigue model, the SWT parameter is averaged over a so-called critical area to compute a nonlocal FIP. The physical fidelity of the proposed fatigue model is shown by a good agreement between the predicted fatigue life cycles based on the nonlocal FIP and the corresponding experimental data. Moreover, the predicted fatigue cycles obtained from the simplified geometrical model also yield good accordance to the experimental data, thus we can model the effect of the surface roughness on the fatigue properties of AM Ti64 in an efficient way.

In flash thermography, the maximum inspectable defect depth is limited when only the raw thermographic sequence is analyzed. The introduction of pulsed phase thermography (PPT), in which phase (contrast) images at different thermal wave frequencies are obtained, significantly improved the maximum inspectable depth while reducing the effects of non-uniform heating and non-uniform surface properties. However, in a practical environment, the evaluation of many phase images per inspection is a cumbersome procedure. In this paper, a novel Adaptive Spectral Band Integration (ASBI) procedure is introduced for the post-processing of flash thermographic datasets, which yields a unique damage index map. ASBI integrates the most useful spectral information for each pixel individually, obtaining a maximized defect detectability and an almost zero-reference level. The performance of ASBI with respect to defect detectability as well as defect sizing and depth inversion is evaluated thoroughly with both experimentally and numerically generated datasets. The ASBI procedure is successfully applied on various composite coupons with flat bottom holes and barely visible impact damage, as well as on a stiffened aircraft composite panel with a complex cluster of production defects. The ASBI procedure is compared with existing data-processing techniques in literature, illustrating an enhanced performance.

Local Defect Resonance (LDR) is exploited for non-destructive testing (NDT) by using ultrasonic vibrations to get a localized resonant activation of defected zones. The LDR technique relies on the local stiffness difference between the defect and the sound material. Analyzing the structure’s displacement field at this localized resonance frequency reveals the defect’s location and provides information about the defect’s characteristics, i.e. geometry, size and depth. In this study, the opportunities and limitations of linear LDR for NDT of materials are investigated in a parametric way. Both finite element simulations and experiments (using scanning laser Doppler vibrometry) are performed for aluminum alloy and carbon fiber reinforced polymer coupons with flat bottom holes and delaminations ranging in both depth and diameter. The resonance frequencies as well as the associated defect-to-background ratios are parametrically evaluated. For shallow defects, a clear LDR is observed caused by the strong local stiffness reduction at the defect. On the contrary, deep defects are associated with a limited stiffness decrease that results in the absence of LDR behavior. The local stiffness reduction at damages is further exploited using a weighted band power calculation. It is shown that using this technique, deep defects can be detected for which no LDR behavior was observed.

Local defect resonance (LDR) makes use of high frequency vibrations to get a localized resonant activation of a defective region. In this study, the LDR behavior of carbon fiber reinforced polymer (CFRP) coupons with three different types of damages is investigated using broadband measurements obtained with a scanning laser Doppler vibrometer (SLDV). First, the LDR response of flat bottom holes of different depths and sizes is evaluated using a signal-to-noise ratio. Next, results are obtained for ETFE inserts where the difference between (artificial) delaminations and inserts is outlined. At last, the vibrational response of a CFRP coupon with barely visible impact damage is investigated. This type of damage has a more complex structure, and it is shown that frequency band data (an alternative to the single frequency LDR) performs well in identifying such complex damage.

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.

This paper treats an experimental study that focusses on optical infrared thermography for non-destructive testing of composites through lock-in and flash excitation. Different fiber reinforced plastics with various artificial defects have been investigated. Three different postprocessing techniques are applied, namely fast Fourier transform (FFT), principal component analysis (PCA) and thermographic signal reconstruction (TSR). A comparison between the different excitation and post-processing methods is performed, and their strengths and weaknesses in detecting artificial defects in composites are evaluated and discussed.

Local defect resonance (LDR) employs a specific high frequency, the LDR frequency, to get a localized strong resonant activation of the defect. However, one of the major difficulties for applying LDR as a non-destructive testing technique, is the proper identification of the required LDR frequency, and the subsequent LDR localization. In this study, post-processing methods in both time and frequency domain are applied to low-power broadband vibration data in view of automated extraction of LDR parameters, i.e. LDR frequency and LDR location. In order to reduce the computational effort for large datasets (>1 GB), various data compression methods have been considered: power spectral density (PSD), principal component analysis (PCA) and operational modal analysis (OMA). The actual LDR parameter extraction from the (compressed) data is based on an iterative procedure to threshold the vibrational amplitudes. The LDR parameter extraction procedure is demonstrated on different carbon fiber reinforced polymers with various defect types: flat bottom holes, inserts and low velocity impact damage. It is further demonstrated that the procedure can equally handle multiple defects. A comparison of the performance of the various data compression methods is provided.

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 non-destructive testing procedure is proposed for damage detection in composites using full wavefield measurement obtained using a scanning laser Doppler vibrometer. Vibrations are excited using two low-power piezoelectric actuators leading to nonlinear elastic wave modulation at the defect. One actuator is supplied with a broadband chirp signal and the other actuator is supplied with a single-frequency sine signal. First, a time–frequency filtering method is proposed to extract specific nonlinear components of interest (e.g. second higher harmonic and first modulation sideband) without the need for multiple excitation sequences. Next, damage maps are constructed using broadband bandpower calculation of the filtered nonlinear components. It is demonstrated that the modulation sidebands provide an exclusive imaging of defect nonlinearity, and are not affected by potential source nonlinearity. The proposed damage map construction procedure is applied for various carbon fiber reinforced polymer test specimens with different damage features: (1) coupon with quasi-static indentation damage, (2) coupon with artificial delaminations, (3) bicycle frame with impact damage, and (4) stiffened aircraft panel with partially debonded stiffener. The obtained results indicate the high performance of the developed procedure for detection of various defect types in curved and/or stiffened carbon fiber reinforced polymer components.