Pulse-compression Thermography Research Articles

Barker code Pulse-Compression thermography for defects detection in CFRP composites based on 3D-Matched filter

Carbon fibre reinforced polymer (CFRP) composites have been extensively applied in the aerospace industry. However, during the service life, CFRP composites always suffer from delamination and other subsurface defects. In order to improve the safety performance of the CFRP composites, Barker code pulse-compression thermography (PuCT) has been proposed for detecting and identifying subsurface delamination defects. A series of 15 artificial flat-bottomed holes (FBHs) were prepared to simulate the subsurface delamination defects in CFRP specimen. A 13-bit Barker code PuCT test scheme was proposed to heat the surface of specimen, and the infrared image sequence was captured by an infrared camera A655sc. The Barker code PuCT original image sequences was processed by 3D-matched filter (3D-MF) technique, combined with various sub-filtering methods included simple matched filter (SMF), matched subspace filter (MSF), linear discriminant analysis (LDA) and quadratic discriminant analysis (QDA). The signal-to-noise ratio (SNR) was calculated to evaluate the detection capabilities of different processing methods. The results show that the 3D-MF significantly enhances the SNR and reduces the background noise, and obtains better defect recognition than PCA. 3D-MF-QDA combined the advantages of Barker code PuCT, 3D-MF and supervised learning (SL) technique to accurately identify defects with a diameter-to-depth ratio of 4 or more, and the actual defect diameters identification accuracy was 97.5%, 96.7%, 86%, 86% and 71.5%, respectively. The results demonstrate the potential of Barker code PuCT with 3D-MF to effectively detect defects in CFRP composites and improve the accuracy of defect size identification.

LFM-Chirp-Square pulse-compression thermography for debonding defects detection in honeycomb sandwich composites based on THD-processing technique

ABSTRACT Honeycomb Sandwich Composites (HSCs) have been widely used in aerospace and aircraft industries because of their high temperature and corrosion resistance, etc. As defects and damage to HSCs are unavoidable, non-destructive testing (NDT) is essential to ensure the reliability of HSCs. A series of artificial flat-bottomed holes (FBHs) defects of HSCs were prepared for pulse thermography (PT) and LFM-Chirp-Square pulse-compression thermography (PuCT). LFM-Chirp-Square PuCT has the advantage of frequency modulation over PT to detect different depth ranges of FBHs defects. In order to improve the efficiency of defect detection, pulse phase thermography (PPT) and total harmonic distortion (THD) techniques were applied to process the infrared image sequences. The results show that THD processing technique outperforms the PPT processing technique in improving the signal-to-noise ratio (SNR) from feature images. The combination of LFM-Chirp-Square PuCT and THD techniques can be applied to improve the defect detection identification results in HSCs.

Diffusion-compensated correlation analysis of frequency-modulated thermal signal for quantitative infrared thermography

Application of a broadband modulated excitation signal and its cross-correlation with the corresponding thermal response, so-called thermal wave radar or pulse compression thermography, is a well-known technique in active infrared thermography. The technique benefits from both the broadband spectrum and the pulse compression efficiency of excitation signal, providing lag and phase quantities which can be used for characterization of defects. However, the significant distortion of the thermal response due to heat diffusion makes the cross-correlation analysis inefficient, particularly for the detection of deep defects or the inspection of a material with low thermal diffusivity. To tackle this issue, diffusion-compensated correlation analysis (DCCA) of thermal signal is proposed for quantitative infrared thermography, using a frequency-modulated sweep signal as excitation waveform. DCCA is based on the correlation of the measured thermal response with a library of template thermal responses calculated using the 1D analytical solution of the heat diffusion problem. DCCA can then more reliably resolve the thermal response in the presence of measurement noise, and depending on the definition of the library, directly map it to the corresponding depth or diffusivity. Two inspection modes are studied: (i) transmission of heat from defects which act as sub-surface heat sources (e.g. vibrothermography), and (ii) reflection of an externally applied heat flux from sub-surface defects (e.g. optical infrared thermography in reflection mode). Outperformance of DCCA to thermal wave radar is analytically substantiated and verified by finite element simulation on a carbon fibre reinforced polymer plate. Furthermore, the technique is experimentally studied for thermographic inspection of a carbon fiber reinforced polymer (CFRP) coupon including artificial defects, and the advantages as well as limitations of the technique are discussed.

Multi-Technique Approach by Traditional and Innovative Methodologies to Support the Restoration of a Wall Painting from the 16th Century at Palazzo Gallo in Bagnaia, Viterbo, Central Italy

This paper summarizes the main results of the study, diagnostics, and restorations conducted and applied to a 16th century wall painting (a portion of the frieze and the Riario coat of arm) in Palazzo Gallo (Bagnaia, Viterbo district—central Italy) recently concluded, which was also the subject of a master’s degree thesis in Conservation and Restoration of Cultural Heritage at the University of Tuscia, Viterbo. Innovative imaging techniques were used for the first time on a wall painting on-site: hypercolorimetric multispectral imaging (HMI) and pulse-compression thermography (PuCT), combined with more traditional analysis such as X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy, and cross-section investigation. HMI allowed for mapping the conservation status before and after the removal of the scialbo layer that covered the original paintings. It also allowed different areas of the painting to be compared and for verifying the effectiveness of the cleaning. PuCT enabled the detection of cracks and discontinuities in the ground layers and to evaluate the depth of such anomalies, giving valuable support in the consolidation step. Moreover, passive thermography was used to monitor the penetration level of a hydraulic mortar in real time, a technique that was greatly helpful for verifying the successful fill and consolidation of voids beneath the pictorial layer. Overall, the multi-technique approach reported here was of considerable assistance for restoration of the mentioned artwork, the result of which has also been documented.

IR Reflectography, Pulse-Compression Thermography, MA-XRF, and Radiography: A Full-Thickness Study of a 16th-Century Panel Painting Copy of Raphael

The potential of any multi-analytical and non-invasive approach to the study of cultural heritage, both for conservation and scientific investigation purposes, is gaining increasing interest, and it was tested in this paper, focusing on the panel painting Madonna della Tenda (Musei Reali, Turin), identified as a 16th-century copy of the painting by Raffaello Sanzio. As a part of a broader diagnostic campaign carried out at the Centro Conservazione e Restauro, La Venaria Reale in Turin, Italy, the potential of the combination of X-ray radiography, pulse-compression thermography, macro X-ray fluorescence, and IR reflectography was tested to investigate the wooden support and all the preparatory phases for the realization of the painting. The results of the optical microscopy and SEM/EDS analyses on a multi-layered micro-sample were used for a precise comparison, integration, and/or confirmation of what was suggested by the non-invasive techniques. Particularly, the radiographic and thermographic techniques allowed for an in-depth study of a hole, interestingly present on the panel’s back surface, detecting the trajectory of the wood grain and confirming the presence of an old wood knot, as well as of a tau-shaped element—potentially a cracked and unfilled area of the wooden support—near the hollow. The combination of radiography, macro X-ray fluorescence, Near Infrared (NIR), and Short Wave Infrared (SWIR) reflectography allowed for an inspection of the ground layer, imprimitura, engravings, and underdrawing, not only revealing interesting technical-executive aspects of the artwork realization, but also highlighting the advantages of an integrated reading of data obtained from the different analytical techniques.

Surface and Interface Investigation of a 15th Century Wall Painting Using Multispectral Imaging and Pulse-Compression Infrared Thermography

Before starting the restoration of an artwork, good practice involves the evaluation of the item healthiness possibly carried out through non-invasive techniques. In the specific case of painting inspection, hyper- and multi- spectral techniques are commonly used to analyze the outer layers (varnish, pictorial, and drawing), while X-ray, tomography, and many others can be employed to investigate its inner structure. Although highly desirable, a single technique providing all the information about a painting is still not available. Thus, it is of great interest to define the analysis’ protocols that could optimally exploit the complementarities of a minimal set of techniques. To this aim, the present paper shows the combined use of the hypercolorimetric multispectral imaging and pulse-compression thermography on a 15th century wall painting attributed to the Italian artist Antonio del Massaro, also known as Pastura, representing the Madonna with the Child and the Saints Jerome and Francis. The capabilities and the complementarities of the two techniques, whose information can also be fused through post-processing, are illustrated in detail in this paper. In addition, a false-colour imaging approach is proposed to improve the readability and analysis of the thermography results.

Hypercolorimetric multispectral Imaging and Pulse Compression thermography as innovative combined techniques for painting investigation: the case of a detached wall painting by Pastura

This contribution focuses the attention on an innovative approach in diagnostics of paintings, based on the combine use of two imaging techniques named Hypecolorimetric Multispectral Imaging (HMI) and Pulse Compression Thermography (PuCT) applied to a 15th century wall painting, attributed to the Italian artist Antonio del Massaro, also known as Pastura. HMI technique is based on the simultaneous exploitation of the electromagnetic spectrum from the ultraviolet to the near infrared region. The acquisition, made under a standard metric, allows for characterizing the investigated surfaces in a more detailed way than the standard colorimetry. The system transforms any spectra in the range 300-1000nm into sevenfold hypecolorimetric coordinates. HMI guarantees very high radiometric (better than 95%) and colorimetric precision (better than ΔE = 2). PuCT is a thermography technique based on the use of coded modulated heating stimuli in combination with the pulse-compression technique. A PuCT scheme, based on coded LED excitation capable of optimizing the estimation of the impulse responses compared to the state-of-the-art PuCT literature has also been proposed. The combined use of HMI and PuCT recently revealed its potentiality in the investigation of important panel paintings by highlighting hidden details, mapping the conservation status, characterizing painting materials, etc. in a completely non-invasive way. Their combined capabilities are here tested on a wall painting representing the Madonna with the Child and the Saints Jerome and Francis, which was investigated during the restoration in the Laboratory in order to supply information about the materials and techniques.

A Comparison of Feature Extraction Techniques for Delamination of CFRP Using Eddy Current Pulse-Compression Thermography

CFRP (carbon fiber reinforced plastic) has replaced conventional metallic materials in many industrial applications because of its outstanding mechanical performance such as high strength to ratio and resistance of fatigue. During the service life of the aerospace composite component, delamination left without detection can cause a sudden breakdown of the structure. Eddy current pulse-compression thermography (ECPuCT) is an emerging technique that combines traditional ECPT and pulse compression techniques. In this work, feature extraction techniques of impulse response have been exploited in terms of principal component analysis (PCA), kernel principal component analysis (K-PCA) and independent component analysis (ICA). Each technique is evaluated using SNR as the index to compare their performance. The results indicate that Kernel-PCA performs better than PCA and ICA based features when dealing with delamination ranged from defect#1(0.46mm) to defect#9 (2.30mm).

Low Thermal Conductivity Materials and Very Low Heat Power: A Demanding Challenge in the Detection of Flaws in Multi-Layer Wooden Cultural Heritage Objects Solved by Pulse-Compression Thermography Technique

An inlay sample with artificial defects was inspected via the pulse-compression thermography (PuCT) technique. The sample belongs to the cultural heritage field, and it was realized by a professional restorer based on his long-time experience, imitating historical art crafting styles. The tesserae composing the inlay were not treated by any protective paints, so that external thermal stimuli may cause physical/mechanical alterations of the cell walls, with consequent colour changes, cracks, and eventually damage to its surface. To avoid any alteration of the sample, the PuCT technique was used for inspecting the inlay sample as it allows the heating power to be very low, while assuring enough thermal contrast for the defects to be detected after the exploitation of the pulse-compression algorithm. Even if a maximum ΔT slightly exceeding 1 °C was detected during the PuCT test of the inlay sample, it is shown that this is enough for detecting several defects. Further, image processing based on the Hilbert transform increases defect detection and characterization. In addition, a novel normalization technique, i.e., a pixel-by-pixel data normalization with respect to the absorbance estimated by considering a characteristic value of the compression peak, is introduced here for the first time. The proposed normalization enhances the defect detection capability with respect to the standard pixel-by-pixel amplitude visualization. This has been demonstrated for two experimental setups, both exploiting the same LED chips system as heating source but different thermal camera sensors, i.e., one in the mid-infrared spectrum, the other in the far infrared one. Thus, the present work is also the first small-scale test of a future portable system that will include low-power LED chip feed in DC by metal-oxide-semiconductor field-effect transistor (MOSFET) devices, and a handy far-infrared camera.

Quantitative Evaluation of Crack Depths on Thin Aluminum Plate Using Eddy Current Pulse-Compression Thermography

Eddy current stimulated thermography is an emerging technique for non-destructive testing and evaluation of conductive materials. However, quantitative estimation of the depth of subsurface defects in metallic materials by thermography techniques remains challenging due to significant lateral thermal diffusion. This work presents the application of eddy current pulse compression thermography to detect surface and subsurface defects with various depths in an aluminum sample. Kernel Principal Component analysis and Low Rank Sparse modelling were used to enhance the defective area, and cross point feature was exploited to quantitatively evaluate the defects depth. Based on experimental results, it is shown that the crossing point feature has a monotonic relationship with surface and subsurface defects depth, and it can also indicate whether the defect is within or beyond the eddy current skin depth. In addition, the comparison study between aluminum and composites in terms of impulse response and proposed features are also presented.

Looking Through Paintings by Combining Hyper-Spectral Imaging and Pulse-Compression Thermography.

The use of different spectral bands in the inspection of artworks is highly recommended to identify the maximum number of defects/anomalies (i.e., the targets), whose presence ought to be known before any possible restoration action. Although an artwork cannot be considered as a composite material in which the zero-defect theory is usually followed by scientists, it is possible to state that the preservation of a multi-layered structure fabricated by the artist’s hands is based on a methodological analysis, where the use of non-destructive testing methods is highly desirable. In this paper, the infrared thermography and hyperspectral imaging methods were applied to identify both fabricated and non-fabricated targets in a canvas painting mocking up the famous character “Venus” by Botticelli. The pulse-compression thermography technique was used to retrieve info about the inner structure of the sample and low power light-emitting diode (LED) chips, whose emission was modulated via a pseudo-noise sequence, were exploited as the heat source for minimizing the heat radiated on the sample surface. Hyper-spectral imaging was employed to detect surface and subsurface features such as pentimenti and facial contours. The results demonstrate how the application of statistical algorithms (i.e., principal component and independent component analyses) maximized the number of targets retrieved during the post-acquisition steps for both the employed techniques. Finally, the best results obtained by both techniques and post-processing methods were fused together, resulting in a clear targets map, in which both the surface, subsurface and deeper information are all shown at a glance.

Evaluation of debonding in CFRP-epoxy adhesive single-lap joints using eddy current pulse-compression thermography

Contaminations arising during the manufacturing or usage of Carbon Fiber Reinforced Plastic (CFRP)-epoxy adhesive single-lap joints can drastically affect the mechanical performance of the structure and eventually lead to its catastrophic failure. Eddy current stimulated thermography has been recently proposed for detecting and characterizing defects in composite materials. The characterization of debonding in adhesive single-lap joints in composites by eddy current stimulated thermography can be challenging due to the anisotropic thermal diffusion caused by the anisotropic conductivity and multi-layer nature of the composites. This work tackles the challenge of detecting the interface contamination by combining eddy current pulse-compression thermography and image post-processing algorithms. The impulse responses from samples containing either brass, release film or release agent contaminations obtained through pulse-compression combined with Eddy Current Pulsed Thermography (ECPT). Non-uniform heating patterns are removed by partial least square technique. Then, the time instants containing meaningful information about the contaminated interface layer are inferred by comparing each norm of kernel principal component over suitable time windows. The evaluation of contamination depths and properties are conducted in selected time windows by principal component analysis and time-phase analysis. Results proved that release agent contamination plays the minor role in changing the electrical and thermal properties of the single-lap joints compared with brass and release film.

Halogen optical referred pulse-compression thermography for defect detection of CFRP

In this paper, pulse-compression thermography nondestructive testing method is proposed for defect detection in carbon fiber reinforced composite in reflection mode, using halogen lamps as excitation source. It is known that care should be taken when pulse-compression algorithm is applied to data obtained using halogen lamps. This because standard commercial halogen lamps suffer from high on/off switching delay time, thus they are not well-following the time behavior of the coded waveform employed for the heating emission modulation. The hereby-proposed method, named “Referred Pulse-Compression Thermography”, relies on subtracting the step heating contribution from the recorded coded thermal sequence before applying the pulse-compression algorithm. In particular, the step heating contribution is recorded separately by mean of an additional measurement. It is shown that the proposed method is inherently immune to the halogen-lamp switching time delay as both the reference recorded step signal and the coded signal sequence are affected by the same setback value. The proposed method can effectively enhance the defect information and improving the thermal contrast between defect and non-defect areas when halogen lamps are used in combination with pulse-compression in reflection mode. The validity of the proposed method is verified by experimental results and a comparison study with step heating thermography on different composite samples, having complex shapes and embedded artificial defects at different depths.

Development of integrated innovative techniques for paintings examination: The case studies of The Resurrection of Christ attributed to Andrea Mantegna and the Crucifixion of Viterbo attributed to Michelangelo's workshop

Abstract This paper presents the contextual use of Pulse-Compression Thermography and Hypercolorimetric Multispectral Imaging for the diagnostic study of historical heritage paintings. The comparison and the integration of images provided by the two techniques allows the conservation state of both the painting layers and wooden support to be investigated. Relevant information on the painting technique and figurative scene can be obtained as well. The proposed approach was applied to two Italian Renaissance panel paintings. The first object tested was a 16th century panel painting representing a Crucifixion, exposed in the Museum of Colle del Duomo in Viterbo, Italy, and attributed to the workshop of the master Michelangelo Buonarroti. The second artwork was a late 15th century panel painting, representing The Resurrection of Christ, currently preserved at Museo Carrara in Bergamo, Italy, and recently re-attributed to Andrea Mantegna; it was identified as being the upper half of a whole composition together with the Descent into Limbo painting. HMI acquisitions and digital image processing tools allowed to investigate the upper painting layer, while PuCT imaging data gave relevant information on the structure of the wooden support proving to be an innovative stratigraphic investigation method. The combination of HMI and PuCT imaging techniques supplied information on the whole structure of the artworks, identifying surface degradation, different layers, wood defects and their position in the inner layers of the object. The integration of the above-mentioned techniques might stand as a new reference diagnostic method to evaluate conservative needs and support decisions for restoration.

Comparison of optimisation strategies for the improvement of depth detection capability of Pulse-Compression Thermography

ABSTRACT In Pulse-Compression Thermography, the impulse response of the Sample Under Test is retrieved pixelwise by applying a proper matched filter on the set of acquired thermal images obtained by stimulating the system with a heat source amplitude – modulated by a proper coded signal. Linear frequency-modulated chirp signals and binary codes are the most employed coded excitations, and to improve the detection capability of the technique, a non-linear frequency-modulated chirp signal can be used to deliver more energy to the sample in a frequency range of interest. In this work, we report the application of an exponential chirp to modulate the heating source and we compare it with a standard linear chirp excitation. To do a fair comparison, various windowing functions have been applied on the matched filters to reduce range side lobes, thus enhancing the retrieved impulse response quality. It is shown that the combined use of an exponential chirp and an appropriate matched filter obtained by exploiting the Reactance Transform window provides a faithful reconstruction of the sample impulse response and an enhanced signal-to-noise ratio with respect to the use of linear chirp. This has been demonstrated on a 3D-printed polymethyl methacrylate (PMMA) sample containing 16 flat-bottom holes of different depths.

Comparison of optimization strategies for the improvement of depth detection capability of Pulse-Compression Thermography

Comparison of optimization strategies for the improvement of depth detection capability of Pulse-Compression Thermography

Delamination Detection in Polymeric Ablative Materials Using Pulse-Compression Thermography and Air-Coupled Ultrasound.

Ablative materials are used extensively in the aerospace industry for protection against high thermal stresses and temperatures, an example being glass/silicone composites. The extreme conditions faced and the cost-risk related to the production/operating stage of such high-tech materials indicate the importance of detecting any anomaly or defect arising from the manufacturing process. In this paper, two different non-destructive testing techniques, namely active thermography and ultrasonic testing, have been used to detect a delamination in a glass/silicone composite. It is shown that a frequency modulated chirp signal and pulse-compression can successfully be used in active thermography for detecting such a delamination. Moreover, the same type of input signal and post-processing can be used to generate an image using air-coupled ultrasound, and an interesting comparison between the two can be made to further characterise the defect.

New features for delamination depth evaluation in carbon fiber reinforced plastic materials using eddy current pulse-compression thermography

The growing application of composite materials in aerospace leads to the urgent need of non-destructive testing and evaluation (NDT&E) techniques capable of detecting defects such as impact damage and delamination possibly existing in those materials. Eddy current pulsed thermography is an emerging non-destructive testing (NDT) technique capable of detecting such defects. However, characterization of delamination within composite materials is difficult to be achieved by a single pulse excitation, especially in carbon fiber reinforced plastic materials as the extraction of thermal diffusion in such multi-layered structures is challenging. To cope with this problem of signal-to-noise ratio, this paper proposes the eddy current pulse-compression thermography (ECPuCT), combining the Barker code modulated eddy current excitation and pulse-compression technique to enhance the capability of characterizing delamination on carbon fiber reinforced plastic materials. Additionally, a thermal pattern enhanced method based on kernel principal component analysis technique is used to locate the delaminated areas. Two features, including a newly proposed crossing point of impulse responses related to defective and non-defective areas and skewness of impulse responses are investigated for delamination depth evaluation. Results show that delamination can be detected within depths ranging from 0.46 mm to 2.30 mm and both the proposed features have a monotonic relationship with delamination depths.

The use of pulse-compression thermography for detecting defects in paintings

Interest in the conservation of paintings grows year by year. Their periodic inspection is essential for their conservation over the time. Thermographic non-destructive inspection is one technique useful for paintings, but it is essential to be able to detect buried defects while minimising the level of thermal stimulus. This paper describes a pulse-compression infrared thermography technique whereby defect detection is optimized while minimising the rise in temperature. To accomplish this task, LED lamps driven by a coded waveform based on a linear frequency modulated chirp signal have been used on paintings on both a wooden panel and a canvas layer. These specimens contained artificially fabricated defects. Although the physical condition of each painting was different, the experimental results show that the proposed signal processing procedure is able to detect defects using a low temperature contrast.

Comparative study between linear and non-linear frequency-modulated pulse-compression thermography.

Pulse-compression thermography is an emerging non-destructive technique whose effectiveness strictly depends on the choice of the coded excitations used to modulate the heating stimulus. In this paper, the features of frequency-modulated coded signals, i.e., chirps, have been tested for imaging thin Teflon defects embedded within a carbon fiber composite specimen. With the aim of maximizing the heat transferred within the sample, the use of several optimized non-linear chirp signals has been also investigated and their defect detection capability compared in terms of the maximum achievable signal-to-noise ratio.