Pulsed Thermography Research Articles

Defect detectability based on square wave lock-in thermography.

A lock-in thermography technique based on a periodical square wave is used to detect stainless steel plates with defects. Combining a neural network with lock-in thermography, an image processing technique is proposed, and the results are compared with traditional image processing methods. A full-field defect reconstruction technology is proposed that combines pulsed phase thermography, threshold segmentation technology, and lock-in thermography technology to reconstruct the full-field depth image. This method has fast processing speed and high detection accuracy. Finally, the effects of excitation frequency and duty cycle on thermal image quality, defect detection range, and defect detection accuracy are investigated through extensive experiments to arrive at the optimal excitation frequency and duty cycle.

Tilt- and warpage measurement as inline quality assessment tool

The tilt of semiconductor dies is a common issue during assembly in power electronics as, e.g. die bonding during silver sintering caused by inhomogeneous thicknesses of applied die-attach material, problems with a homogeneous force application, uneven substrates etc. This tilt usually leads to reliability problems later during testing or operation in the field. A weakly sintered layer will be formed with voids larger than 1–2 μm in major numbers and more importantly will lead to insignificantly developed interfaces. None of them can be detected by SoA failure analytical techniques like scanning acoustic microscopy (SAM) or pulsed infrared thermography (PIRT), since both will still provide a sufficient phonon coupling, indistinguishable to well sintered layers. However, the undetected weak interfaces will lead to severe reliability problems and can only be detected indirectly by thickness measurement without a cross section. In this work, we present a scanning system based on confocal polychromatic distance sensors, which can scan a fully loaded tray of bonded dies on the substrate for industry-grade inline integration. We demonstrate the novel system on different bonded power dies on active metal braze substrate (AMB) under industrial production conditions. We achieve this in the required accuracy of about 2–3 μm in an environment with strong vibration influence. The in-line capability has been proven by using a setup, which is independent of mechanical vibrations and uses only optical fibers to transfer measuring signals within future smart production environments.

3D photothermal reconstruction of subsurface defects in notched multidirectional CFRP laminates due to tensile fatigue loading

This paper presents results from 3D photothermal reconstruction of real subsurface defects in carbon fiber-reinforced polymers using the virtual wave concept. We apply pulsed thermography during and after interrupted fatigue loading in tensile-tensile configuration for notched multidirectional laminates. In addition, we discuss the influence of the ambient conditions from in-situ photothermal testing on the virtual wave signal. The results obtained from 3D photothermal reconstruction are directly compared with those obtained from laser ultrasound testing. To summarize, we demonstrate a practicable and fast 3D photothermal evaluation of subsurface defect evolution utilizing the virtual wave concept.

Papier-Mâché Puppets’ Characterization by Infrared Imaging Techniques

Among the different forms of art, the puppet theatre constitutes a long-standing and often little-known tradition. The use of puppets as support for acting dates back to the Greek age, and it was mainly developed during the modern period. The reason for such a large diffusion was due to the possibility of using affordable materials, such as papier-mâché, for the puppets’ manufacture. In this paper, a method based on the combined use of pulsed thermography (PT) and mid-wave infrared reflectography (MIR) is, for the first time, proposed for the characterization of papier-mâché artworks. In particular, some puppets belonging to the collection of the Museo delle Civiltà in Rome and made by Olga Lampe Minelli, a 20th-century puppet master, were investigated in order to detect damaged areas, such as those affected by insect attacks, and, consequently, to specifically plan suitable restoration works. Finally, the investigations were also carried out after the restoration to evaluate the effectiveness of the adopted treatments.

Spot Weld Inspections Using Active Thermography

Spot welds have a significant part in the creation of automotive vehicles. Since the integrity of, for example, a car, is dependent on the performance of multiple welds, it is important to ensure the quality of each spot weld. Several attempts have been made in order to determine the quality of spot welds, but most of them do not focus on the applicability in the manufacturing process. Spot weld inspections are often performed using back heating. However, during manufacturing, robotic inspections are desired, and since the bodywork of a car is a complex shape, the accessibility from the inside of the vehicle is minor. Therefore, inspections using front heating are more suitable. In this manuscript, multiple excitation methods are compared as well as different post-processing techniques. The used excitation techniques can be divided into light heating and inductive heating. Light heating is further divided in lock-in thermography and pulse thermography. The used post-processing techniques are principle component analysis and fast Fourier transform. Inductive heating turns out to be the most suitable measurement technique since it is fast and can be performed as front and back heating. Both investigated post-processing techniques deliver suitable information, such as relief images and information of the internal structure of the spot weld.

The Study of Inspection on Thin Film Resistance Strain Gauge Contact Failure by Electrical Excitation Thermal-Wave Imaging

Contact failure as one of primary failures, which occurs between thin film resistance strain gauge (FRSG) and metal measured objective substrate, seriously influences the measurement validity and precision of the strain gauge. In this article, an electrical excitation thermal-wave imaging (EE-TWI) method is proposed for the inspection of contact failure. Equivalent circuits of the contact resistance under an external electrical source are used to identify the location of the contact failure. Long pulse thermography and lock-in thermography (LIT) are experimentally studied on the detection of contact failure. Pulsed phase thermography and principal component thermography in long pulse thermography as well as the amplitude, quadrature component in LIT are desirable to form the EE-TWI feature images, respectively. A graph-based segmentation method is employed to distinguish and localize the contact failure from EE-TWI images. EE-TWI provides a powerful tool for the contact failure evaluation of integrated thin film resistance strain gauge (FRSG) and has a significant potential to ensure the thin FRSG sticking performance and quality with application of mechanical properties measurements.

Characterisation and evaluation of paint-coated marine corrosion in carbon steel using eddy current pulsed thermography

Marine atmospheric corrosion is a major challenge for carbon steel. Eddy current pulsed thermography (ECPT) can characterise defects under coating, but it is necessary to consider samples' surface geometry. This paper proposes a statistical approach for paint-coated S275 steel corrosion evaluation based on ECPT. The thermal response curve skewness of each point is utilised to reveal the corrosion region. The normalised skewness avoids the influence of emissivity anisotropy and surface geometry; corrosion can be evaluated by taking excitation coil skewness as a reference. In the results, the differential skewness drops monotonously with corrosion aggravation in 6 months, decreasing more than 90% from the uncorroded zone to the 3-month corrosion sample. The approach provides high sensitivity and detectability in paint-coated corrosion evaluation.

Analysis of the RCF crack detection phenomenon based on induction thermography.

Eddy current pulsed thermography (ECPT) is used to detect rolling contact fatigue (RCF) cracks on the rail. It is observed that some of the cracks disappear in the thermal image with the increase of heating time. Based on the finite element method, with double cracks as the basic unit, three different crack models are established, and the mutual disturbance relationship between the double cracks is discussed based on the eddy current distribution and thermal diffusion process. The simulation and experimental results show that different crack models are affected by thermal diffusion in different heating stages to different degrees, and the time of the crack thermal image disappearance is obtained. According to the above conclusions, the RCF cracks are extracted and classified based on the influence of thermal diffusion. The possibility of rail condition assessment and maintenance based on the disappearance time is explained.

Non-destructive evaluation of uneven coating thickness based on active long pulse thermography

Non-destructive detection of coating thickness is important after coating deposition and during service. The present work is to establish a measurement method for fast inspection, large area detection of coating thickness with an active long pulse thermography. The measurement method was theoretically analyzed based on the equation of 1D heat transfer in the depth direction, accordingly coating thickness can be quantitively evaluated by recording the temperature decay curve in the cooling stage and computing the time at the minimum of its 2nd derivative. Then, the proposed method was experimentally validated by uniform coating specimen with different thicknesses. Furthermore, the thickness measurement method was successfully applied to measure thickness of an uneven coating specimen within a relative error of 5% with sufficient sampling rate. Finally, some key techniques were discussed for accurate measurements including thermal excitation, sampling rate of thermography, thickness ratio of coating and substrate, etc. The results verify that active long pulse thermography is a powerful tool for non-contact and full-field measurement of coating thickness with merits of safe and easy to implement.

Analyzing probability of detection as a function of defect size and depth in pulsed IR thermography

This study introduces a novel approach to the presentation of the probability of detection (POD) function in infrared (IR) thermographic nondestructive testing. The modified POD is suggested as a function of two defect parameters, namely, defect depth and lateral size. The proposed approach is based on calculating theoretical values of maximum temperature contrast for many defect size/depth combinations by using an appropriate analytical model. Furthermore, these values are used for the quantification of defects to produce predicted POD curves by applying a signal/response method. The results appear as the POD maps illustrating detectability of defects with various size/depth combinations. By setting a particular POD threshold, for example, 90%, the detectability limit contours can be obtained. These contours illustrate the limiting combinations of the depth and diameter of the defects, which can be detected with a required probability of correct detection under a particular temperature signal threshold. The proposed methodology is illustrated with an example of using the POD approach in pulsed IR thermographic inspection of a 3D printed specimen with artificial sphere-like defects. Such an approach allows predicting the detectability of defects in a vast range of depth/size ratios by using an analytical model and a limited number of experiments.

Latent Low Rank Representation Applied to Pulsed Thermography Data For Carbon Fibre Reinforced Polymer Inspection

ABSTRACT This paper explores the implementation of Latent Low-Rank Representation (LatLRR) on pulsed thermographic data. LatLRR decomposes an image in the form of a linear association of three types of information: observed, unobserved and noise. This information is then used in order to separate the salient and principal features. This study has found that when used as a post-processing method prior to the application of state-of-the-art signal processing techniques, such as principal component thermography (PCT) and pulsed phase thermography (PPT), LatLRR significantly improves defect detection: 18% for PCT and 92% for PPT. Nevertheless, no noticeable improvement was measured when LatLRR was used to reconstruct a noiseless version of each image of a dataset, before processing it with a state-of-the-art algorithm. The investigations conducted on each type of feature returned by the LatLRR have also failed to provide results regarding the detection of defects.

Experimental Procedure to Assess Depth and Size of Defects with Pulsed Thermography

Subsurface defects can be detected by the pulsed thermography (PT) technique analysing the raw thermal data with the application of different post-processing algorithms. In this regard, different methods, based on one-dimensional models, are used in the literature to estimate the depth and size of defects. Two of the most established methods are the thermal signal reconstruction (TSR) and the pulsed phase thermography (PPT) algorithms. These latter require a careful set up of the testing parameters such as the frame rate, the truncation window size, and the energy density to obtain an accurate estimation of both depths and sizes of defects. Even if some works have already investigated the issue of defect characterization, there are few works in which the correct procedures to obtain both the size and depth were deeply explained, above all for real components with real defects. The aim of this work is to propose a new empirical procedure to obtain depth and size estimation of the defects using the pulsed thermography technique and in particular the principal component thermography (PCT) algorithm. The proposed procedure is based on the experimental observation that exists a linear correlation between the defect contrasts and the relative aspect ratios. In this way, by means of a master specimen, a calibration curve can be obtained considering a suitable truncation window of the analysed data. Then, the size and depth of defects have been retrieved imposing threshold criteria. The procedure is quite general, and it can be also tested with other algorithms. Different experimental tests have been carried out on two materials, aluminium and glass fiber reinforced polymer (GFRP) and then the procedure has been applied and validated both on simulated (flat bottom holes) and real defects.

Application of blind image quality assessment metrics to pulsed thermography

ABSTRACT This paper explores the application of Blind Image Quality Assessment (BIQA) metrics to pulsed thermography. Two BIQA were used to subsample a sequence of images acquired using Pulse Thermography (PT). The experiments show that the sequences subsampled using BIQA significantly improve the results when applied to metallic samples but fail to capture informative features on composite materials. On metallic samples, when the PCT is applied, an average improvement of 139% of the Contrast to Noise Ratio (CNR) score is observed, on the sequences subsampled, compared with the entire sequence. Nonetheless, the CNR shows that processing the entire data sequence offers 240% improvement compared with subsampled sequences on composite materials.

Development, Validation and Preliminary Experiments of a Measuring Technique for Eggs Aging Estimation Based on Pulse Phase Thermography.

Assessment of the freshness of hen eggs destinated to human consumption is an extremely important goal for the modern food industry and sale chains, as eggs show a rapid natural aging which also depends on the storage conditions. Traditional techniques, such as candling and visual observation, have some practical limitations related to the subjective and qualitative nature of the analysis. The main objective of this paper is to propose a robust and automated approach, based on the use of pulsed phase thermography (PPT) and image processing, that can be used as an effective quality control tool to evaluate the freshness of eggs. As many studies show that the air chamber size is proportional to the egg freshness, the technique relies on the monitoring of the air chamber parameters to infer egg aging over time. The raw and phase infrared images are acquired and then post-processed by a dedicated algorithm which has been designed to automatically measure the size of the air chamber, in terms of normalized area and volume. The robustness of the method is firstly assessed through repeatability and reproducibility tests, which demonstrate that the uncertainty in the measure of the air chamber size never exceeds 5%. Then, an experimental campaign on a larger sample of 30 eggs, equally divided into three size categories (M, L, XL), is conducted. For each egg, the main sizes of the air chamber are measured with the proposed method and their evolution over time is investigated. Results have revealed, for all the egg categories, the existence of an analytic relationship and a high degree of correlation (R2 > 0.95) between the geometric data of the air chamber and the weight loss, which is a well-known marker of egg aging.

Evaluation of defect depth in CFRP composites by long pulse thermography

Long pulse thermography (LPT) is an emerging non-destructive technique that has been used for rapid, cost-effective and accurate damage assessment of composite materials. This paper proposes three analytical models for the in-depth evaluation of defects in a carbon fibre-reinforced plastic (CFRP) specimen by long pulse thermography. The first two models rely on the estimate of the peak temperature-contrast and the peak temperature-contrast slope from the thermal data recorded by an infrared (IR) camera. Both the analytical and experimental results showed that the arriving time of the peak temperature-contrast depends on both the depth and the diameter of defects, whilst the contrast time slope is mainly proportional to the square of the damage depth. The third model here proposed consists of a two-dimensional (2D) axisymmetric heat conduction model based on virtual heat sources. Compared to previously reported analytical models, the proposed 2D one showed higher accuracy on the depth estimation.

Quantitative mapping of depth profile of fatigue cracks using eddy current pulsed thermography assisted by PCA and 2D wavelet transformation

Fatigue cracks are widely generated in numerous engineering components under alternating loads, whose sudden brittle fracture would seriously shorten the service life and cause significant hazards. A large number of nondestructive testing (NDT) techniques have been utilized to quantitatively evaluate the fatigue cracks, however, most of these methods are difficult to be used for a full characterization of the cracks’ depth profile. The eddy current pulsed thermography (ECPT) as a promising NDT technique shows great potential for fatigue crack evaluation due to its high detection efficiency, large inspection area, and high spatial resolution. In this article, a ferrite yoke was utilized to generate a homogeneous eddy current field, and as such to enhance the detection sensitivity and to provide a wide open-view area for the camera. The hybrid processing strategy combining principal component analysis (PCA) and 2D wavelet transformation was proposed to extract spatial features of the thermographic image sequences. Firstly, numerical results demonstrate that the profiles of slot cracks can be identified directly from the thermographic images in the early heating stage. Afterward, the ECPT experiments on slot cracks with different depth profiles demonstrate the feasibility of the hybrid processing strategy. Then, three fatigue cracks were manufactured through the three-point bending method and detected by ECPT. The results of the hybrid processing method indicate that these three fatigue cracks have elliptical profiles. Finally, the authentic depth profile of a fatigue crack with an elliptical shape is extracted through a destructive experiment. The comparison results between the real depth profile and the profile characterized through the proposed strategy show great consistency and validate the superiority of the proposed strategy.

Transient thermal pattern separation and detection of conductive defects in composite insulators using eddy current pulsed thermography

Composite insulators are prone to internal conductive defects owing to their long-term high-voltage operation and outdoor aging. In this study, a method based on eddy current pulsed thermography (ECPT) combined with feature extraction transform algorithms is proposed for transient thermal pattern separation and defect detection in composite insulators with internal conductive defects. The principles of this method, including electromagnetic heating, thermal diffusion, and transient thermal response extraction, are investigated. Conductive defects in insulators have unique thermal patterns that differ from those of non-conductive defects in conductive materials. In this study, the original transient thermal responses of conductive defects are analyzed and compared with those of non-conductive defects. The effectiveness of the transient thermal pattern separation of this method is verified through experimental studies. Different feature extraction transform methods are utilized and compared to separate different thermal transient patterns and perform quantitative evaluations. It is found that ECPT combined with feature extraction transform algorithms can provide a visual and effective method to detect internal conductive defects in composite insulators.

Hyper-dimensional Visualization of Cultural Heritage: A Novel Multi-analytical Approach on 3D Pomological Models in the Collection of the University of Milan

Digital close-range photogrammetry allows us to acquire high-fidelity tridimensional models useful to document cultural heritage objects with an impressive level of detail. In addition, this technique carries a strong analytical potentiality, able to gain improved knowledge of cultural objects and their preservation conditions. This project is focused on a comprehensive diagnostic survey using 3D multispectral modeling, high-resolution digital radiography, pulsed thermography, XRF, FT-IR, and FORS spectroscopies to document and characterize from a conservative point of view the poly-material objects that belong to the “Garnier Valletti” pomological collection—a unique collection from both scientific and artistic points of view. The analytical integration of imaging techniques, 3D modeling, and spectroscopic techniques provides information from the surface, sub-surface, and innermost layers of the object, respectively, capturing both accurate morphometric, spectral, and compositional data. The article presents the results obtained on typical poly-material and multilayered objects of this collection for which the combination of the considered techniques provided important data to the technical knowledge of the realization highlighting a particular predictive ability from a conservative point of view.

Defect Detection and Depth Estimation in CFRP Through Phase of Transient Response of Flash Thermography

This article presents a new method called phase of transient response using the local reference pixel vector (PTR-LRPV) to process pulsed thermography data for defect detection and depth estimation in carbon fiber reinforced polymer specimens. Due to the use of flash as excitation source in pulsed thermography and subsequently its adaptation with the conditions of suddenly applied input, the received signal from the infrared camera can be separated into two transient and steady-state responses in the frequency domain. Defects cause local variations in the thermal spatio-temporal patterns that the transient response can fairly reveal such highly informative variations. On the other hand, the steady-state response mainly includes the intrinsic characteristics of the specimen (sound areas). In fact, by properly separating these responses and employing the phase of the transient term, a suitable distinction of defect characteristics from sound areas has been reached. The results show that the proposed PTR-LRPV is effective in both defect detection and depth estimation tasks, and also, can fairly compete with several well-known algorithms in terms of both quantitative and qualitative criteria.

Potential of Eddy Current Pulsed Thermography as a Nondestructive Testing Method

The parts of working machinery have always suffered from various working conditions in the process of use (e.g., high temperature, high humidity, and high pressure) that may generate flaws, pitting, thermal fatigue crack and other types of defects on the surface or inside. Because these tiny defects carry big safety risks, regular detection is necessary to ensure the reliability of the production equipment.