Probability of detection analysis for infrared nondestructive testing and evaluation with applications including a comparison with ultrasonic testing

La fiabilité d'une technique d’Évaluation Non-Destructive (END) est l'un des aspects les plus importants dans la procédure globale de contrôle industriel. La courbe de la Probabilité de Détection (PdD) est la mesure quantitative de la fiabilité acceptée en END. Celle-ci est habituellement exprimée en fonction de la taille du défaut. Chaque expérience de fiabilité en END devrait être bien conçue pour obtenir l'ensemble de données avec une source valide, y compris la technique de Thermographie Infrarouge (TI). La gamme des valeurs du rapport de l'aspect de défaut (Dimension / profondeur) est conçue selon nos expériences expérimentales afin d’assurer qu’elle vient du rapport d’aspect non détectable jusqu’à celui-ci soit détectable au minimum et plus large ensuite. Un test préliminaire est mis en œuvre pour choisir les meilleurs paramètres de contrôle, telles que l'énergie de chauffage, le temps d'acquisition et la fréquence. Pendant le processus de traitement des images et des données, plusieurs paramètres importants influent les résultats obtenus et sont également décrits. Pour la TI active, il existe diverses sources de chauffage (optique ou ultrason), des formes différentes de chauffage (pulsé ou modulé, ainsi que des méthodes différentes de traitement des données. Diverses approches de chauffage et de traitement des données produisent des résultats d'inspection divers. Dans cette recherche, les techniques de Thermographie Pulsée (TP) et Thermographie Modulée(TM) seront impliquées dans l'analyse de PdD. Pour la TP, des courbes PdD selon différentes méthodes de traitement de données sont comparées, y compris la Transformation de Fourier, la Reconstruction du Signal thermique, la Transformation en Ondelettes, le Contraste Absolu Différentiel et les Composantes Principales en Thermographie. Des études systématiques sur l'analyse PdD pour la technique de TI sont effectuées. Par ailleurs, les courbes de PdD en TI sont comparées avec celles obtenues par d'autres approches traditionnelles d’END.The reliability of a Non-Destructive Testing and Evaluation (NDT& E) technique is one of the most important aspects of the overall industrial inspection procedure. The Probability of Detection (PoD) curve is the accepted quantitative measure of the NDT& E reliability, which is usually expressed as a function of flaw size. Every reliability experiment of the NDT& E system must be well designed to obtain a valid source data set, including the infrared thermography (IRT) technique. The range of defect aspect ratio (Dimension / depth) values is designed according to our experimental experiences to make sure it is from non-detectable to minimum detectable aspect ratio and larger. A preliminary test will be implemented to choose the best inspection parameters, such as heating energy, the acquisition time and frequency. In the data and image processing procedure, several important parameters which influence the results obtained are also described. For active IRT, there are different heating sources (optical or ultrasound), heating forms (pulsed or lock-in) and also data processing methods. Distinct heating and data processing manipulations produce different inspection results. In this research, both optical Pulsed Thermography (PT) and Lock-in Thermography (LT) techniques will be involved in the PoD analysis. For PT, PoD curves of different data processing methods are compared, including Fourier Transform (FT), 1st Derivative (1st D) after Thermal Signal Reconstruction (TSR), Wavelet Transform (WT), Differential Absolute Contrast (DAC), and Principal Component Thermography (PCT). Systematic studies on PoD analysis for IRT technique are carried out. Additionally, constructed PoD curves of IRT technique are compared with those obtained by other traditional NDT& E approaches

Quantitative subsurface defect evaluation by pulsed phase thermography: depth retrieval with the phase

La Thermographie de Phase Pulsée (TPP) est une technique d’Évaluation Non-Destructive basée sur la Transformée de Fourier pouvant être considérée comme étant le lien entre la Thermographie Pulsée, pour laquelle l’acquisition de données est rapide, et la Thermographie Modulée, pour laquelle l’extraction de la profondeur est directe. Une nouvelle technique d’inversion de la profondeur reposant sur l’équation de la longueur de diffusion thermique : μ=(α /πf)½, est proposée. Le problème se résume alors à la détermination de la fréquence de borne fb, c à d, la fréquence à laquelle un défaut à une profondeur particulière présente un contraste de phase suffisant pour être détecté dans le spectre des fréquences. Cependant, les profils de température servant d’entrée en TPP, sont des signaux non-périodiques et non-limités en fréquence pour lesquels, des paramètres d’échantillonnage Δt, et de troncature w(t), doivent être soigneusement choisis lors du processus de discrétisation du signal. Une méthodologie à quatre étapes, basée sur la Dualité Temps-Fréquence de la Transformée de Fourier discrète, est proposée pour la détermination interactive de Δt et w(t), en fonction de la profondeur du défaut. Ainsi, pourvu que l’information thermique utilisée pour alimenter l’algorithme de TPP soit correctement échantillonnée et tronquée, une solution de la forme : z=C1μ, peut être envisagée, où les valeurs expérimentales de C1 se situent typiquement entre 1.5 et 2. Bien que la détermination de fb ne soit pas possible dans le cas de données thermiques incorrectement échantillonnées, les profils de phase exhibent quoi qu’il en soit un comportement caractéristique qui peut être utilisé pour l’extraction de la profondeur. La fréquence de borne apparente f’b, peut être définie comme la fréquence de borne évaluée à un seuil de phase donné φd et peut être utilisée en combinaison avec la définition de la phase pour une onde thermique : φ=z /μ, et le diamètre normalisé Dn=D/z, pour arriver à une expression alternative. L'extraction de la profondeur dans ce cas nécessite d'une étape additionnelle pour récupérer la taille du défaut.Pulsed Phase Thermography (PPT) is a NonDestructive Testing and Evaluation (NDT& E) technique based on the Fourier Transform that can be thought as being the link between Pulsed Thermography, for which data acquisition is fast and simple; and Lock-In thermography, for which depth retrieval is straightforward. A new depth inversion technique using the phase obtained by PPT is proposed. The technique relies on the thermal diffusion length equation, i.e. μ=(α /π·f)½, in a similar manner as in Lock-In Thermography. The inversion problem reduces to the estimation of the blind frequency, i.e. the limiting frequency at which a defect at a particular depth presents enough phase contrast to be detected on the frequency spectra. However, an additional problem arises in PPT when trying to adequately establish the temporal parameters that will produce the desired frequency response. The decaying thermal profiles such as the ones serving as input in PPT, are non-periodic, non-band-limited functions for which, adequate sampling Δt, and truncation w(t), parameters should be selected during the signal discretization process. These parameters are both function of the depth of the defect and of the thermal properties of the specimen/defect system. A four-step methodology based on the Time-Frequency Duality of the discrete Fourier Transform is proposed to interactively determine Δt and w(t). Hence, provided that thermal data used to feed the PPT algorithm is correctly sampled and truncated, the inversion solution using the phase takes the form: z=C 1 μ, for which typical experimental C 1 values are between 1.5 and 2. Although determination of fb is not possible when working with badly sampled data, phase profiles still present a distinctive behavior that can be used for depth retrieval purposes. An apparent blind frequency f’b , can be defined as the blind frequency at a given phase threshold φd , and be used in combination with the phase delay definition for a thermal wave: φ=z /μ, and the normalized diameter, Dn=D/z, to derive an alternative expression. Depth extraction in this case requires an additional step to recover the size of the defect.La Termografía de Fase Pulsada (TFP) es una técnica de Evaluación No-Destructiva basada en la Transformada de Fourier y que puede ser vista como el vínculo entre la Termografía Pulsada, en la cual la adquisición de datos se efectúa de manera rápida y sencilla, y la Termografía Modulada, en la que la extracción de la profundidad es directa. Un nuevo método de inversión de la profundidad por TFP es propuesto a partir de la ecuación de la longitud de difusión térmica: μ=(α /π·f)½. El problema de inversion se reduce entonces a la determinación de la frecuencia límite fb (frecuencia a la cual un defecto de profundidad determinada presenta un contraste de fase suficiente para ser detectado en el espectro de frecuencias). Sin embargo, las curvas de temperatura utilizadas como entrada en TFP, son señales no-periódicas y no limitadas en frecuencia para las cuales, los parámetros de muestreo Δt, y de truncamiento w(t), deben ser cuidadosamente seleccionados durante el proceso de discretización de la señal. Una metodología de cuatro etapas, basada en la Dualidad Tiempo-Frecuencia de la Transformada de Fourier discreta, ha sido desarrollada para la determinación interactiva de Δt y w(t), en función de la profundidad del defecto. Así, a condición que la información de temperatura sea correctamente muestreada y truncada, el problema de inversión de la profundidad por la fase toma la forma : z=C 1 μ, donde los valores experimentales de C 1 se sitúan típicamente entre 1.5 y 2. Si bien la determinación de fb no es posible en el caso de datos térmicos incorrectamente muestreados, los perfiles de fase exhiben de cualquier manera un comportamiento característico que puede ser utilizado para la extracción de la profundidad. La frecuencia límite aparente f’b , puede ser definida como la frecuencia límite evaluada en un umbral de fase dado φd , y puede utilizarse en combinación con la definición de la fase para una onda térmica: φ=z /μ, y el diámetro normalizado Dn , para derivar una expresión alternativa. La determinación de la profundidad en este caso, requiere de una etapa adicional para recuperar el tamaño del defecto

Evaluating the industrial application of non-destructive inspection of composites using transient thermography

Thesis (MEng)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Transient thermography is a non-destructive testing method used in the detection and visualization of sub-surface flaws. Transient thermography could use one of two heating methods: step and square-pulse heating. Both these methods rely on observing the temperature rise of a surface that is subjected to a constant heat flux, while square pulse thermography also observes the subsequent thermal decay after the heat has been removed. The transient methods have not been thoroughly explored in literature with respect to the more popular methods, such as pulsed and lock-in thermography. Particular interest has been placed on investigating transient thermography on fiber-reinforced polymer (FRP) materials and its application in industry. Composites are prone to flaws such as delaminations, voids and inclusions that do not accurately represent flat-bottom holes, which are commonly evaluated in experimental work. Therefore, the inspection of thin artificial air-gaps and Teflon® delaminations were investigated. These artificial flaws can be considered to represent either a fully-separated or contacting delamination. A significant reduction in defect contrast and definition was observed for the thin delaminations, which is ascribed to the lower thermal resistance than that for flat-bottom holes. Further studies investigated the qualitative and quantitative performance of thermographic inspection on defective samples provided by an industrial partner. Experimental results demonstrated that variability in core geometry, ply arrangement, surface and sub-surface anomalies could be identiffied. The smallest detectable anomaly was found to be 1 mm wide, which was a spatial resolution limitation of the infrared camera. The investigated samples exhibited small radius and low resistance defects. It was found that current techniques to quantify defect depth are inadequate, especially if an accurate reference depth cannot be found. Thermography data is typically associated with subtle defect signatures that are strongly affected by non-uniform heating and surface variability. Advanced processing methods have been shown to help mitigate these effects. Various processing methods are reviewed from literature. Several methods were tested here for the first time, such as: multiscale retinex, matched filters, Markov error contrast and modified differential absolute contrast (IDAC) for step thermography. Transient thermography has shown to be a strong competitor amongst other thermographic methods for its simple application, relatively fast inspection times, and high thermal contrast for low defect resistance cases. It further enables the use of an entry-level infrared camera. The ndings of the artificial samples reported a maximum defect depth up to 7 mm was observed for clear Plexiglas®. The clear Plexiglas® can be considered to be the least optimal case of heating with optical excitation and has a low thermal emissivity. For the carbon and glass fibre reinforced polymers, a maximum detectable defect depth of 5 mm was observed, which is considered to be comparative or even better than pulsed thermography. The method was particularly better for low diffusivity materials, such as glass fibre composites.AFRIKAANSE OPSOMMING: Oorgangstermografie is 'n nie-destruktiewe tegniek om defekte onder die oppervlak waar te neem en te visualiseer. Oorgangstermografie kan een van twee verhittingsmetodes gebruik: stap en vierkant puls verhitting. Beide tegnieke is gebaseer op die waarneming van die temperatuur styging van 'n oppervlak onderwerp aan 'n konstante warmtelas, terwyl vierkant puls verhitting ook die temperatuur daling waarneem nadat die warmtelas verwyder is. In vergelyking met meer populêre metodes, soos gepulseerde en geslote termografie, is die oorgangsmetodes nog nie ewe deeglik beskryf in die literatuur nie. Daar is veral belangstelling in ondersoeke na oorgangstermografie vir veselversterkte polimere en die toepassing daarvan in industrie. Saamgesteldemateriale is geneig om defekte soos delaminasie, leemtes en inklusies te hê wat nie goed voorgestel word deur plat bodem gate nie, soos algemeen gebruik in eksperimentele werk. Hier is die gebruik van dun, kunsmatige, luggapings en Te on® delaminasies ondersoek. 'n Beduidende verlaging in kontras en definisie is waargeneem vir dun delaminasies wat toegeskryf kan word aan die feit dat dit 'n laer termiese weerstand het as plat bodem gate. Verdere ondersoeke na die kwalitatiewe en kwantitatiewe vermoë van die termografiese inspeksie van defektiewe onderdele voorsien deur 'n industriële vennoot is gedoen. Eksperimentele resultate het getoon dat variasies in die kern geometrie, laag oriëntasie, oppervlak en sub-oppervlak afwykings geïdenti fiseer kan word. Die kleinste, waarneembare afwyking was 1 mm wyd, wat toegeskryf word aan die beperkte ruimtelike resolusie van die infrarooikamera. Die ondersoekte voorbeelde het klein radius en lae weerstand defekte getoon. Dit is gevind dat bestaande tegnieke om defek diepte te vind deur die gebruik van inversie metodes ontoereikend is, veral wanneer 'n verwysingsdiepte nie akkuraat bepaal kan word nie. Termografiese data word dikwels geassosieer met fyn defek kenmerke wat sterk beïnvloed word deur oneweredige verhitting en oppervlakte variasies. Dit is al gevind dat gevorderde verwerkingsmetodes die effek hiervan kan verminder. Verskeie van hierdie tegnieke, soos gevind in die literatuur, is oorweeg. Nuwe metodes, soos multiskaal retinex, bypassende lters, Markov fout kontras en aangepaste differensiële absolute kontras, word ook beskryf en ge-evalueer. Die prosesseringsmetodes is geïmplimenteer in 'n oopbron sagteware pakket en is getoets met voorbeelde uit die industrie. Dit is getoon dat oorgangstermografie 'n sterk mededinger is in die versameling termografiese tegnieke vernaamlik as gevolg van die eenvoudige toepassing daarvan, relatief vinnige inspeksie tye en hoë termiese kontras vir gevalle waar die termiese weerstand van die defek laag is. Verder is dit moontlik om intreevlak infrarooikameras te gebruik met hierdie tegnieke. Gebaseer op toetse met kunsmatige defekte kon foute so diep as 7 mm onder die oppervlak gevind word in helder Plexiglas®. Helder Plexiglas® is nie 'n ideale materiaal vir hierdie tegnieke nie as gevolg van die materiaal se lae termiese emmisiwiteit. Defekte so diep as 5 mm kon gevind word in koolstof- en glasvesel versterkte polimere. Dit is vergelykbaar met en selfs beter as gepulseerde termografie. Die tegniek het veral beter resultate gelewer met materiale met lae diffusiwiteit, soos saamgeselde veselglas materiale

Three-dimensional subsurface defect reconstruction for industrial components using pulsed thermography

Pulsed thermography is a promising method for detecting subsurface defects, but most pulsed thermographic inspection results are represented in the form of 2D images. Such a representation can limit the understanding of where the defects initiate and how they grow by time, which is a key to predict the remaining use of life of component and feedback to the design to avoid such defects. Threedimensional subsurface defect visualisation is a solution that can unlock this limitation. A straightforward approach to reconstruct 3D subsurface defect is conducting two inspections on both front and rear sides. However, the deployment of this approach can be limited because 1) one side of the inspected component could be inaccessible; 2) the accuracy of measurement could be compromised if the defect thickness is very thin due to extreme closed values of defect depths from two inspections; and 3) if the defect is too deep for one side, the defect could be missed. Addressing the challenge of 3D subsurface defect reconstruction and visualisation, this thesis proposes a novel technique to measure defect depth and estimate defect thickness simultaneously through estimating the thermal wave reflection coefficient value achieved by introducing a modified heat transfer model based on a single-side inspection method. The proposed method is validated through model simulations, experimental studies, and a use case. Four composite samples with different defect types, sizes, depths and thicknesses, are used for experimental studies; a steel sample with a ‘s’ shape triangular air-gap inside is used for a use case. The simulation results show that under the noise level of 25 dB, the percentage error of the developed depth measurement method is 0.25% whilst the minimum error of the best existing method is 2.25%. From the experimental study results, the averaged percentage error of the defect thickness estimation is less than 10% if the defect thickness is no more than 3 mm. For the use case, the reconstructed defect shape is similar to the X-ray image.Manufacturin

Three-dimensional eddy current pulsed thermography and its applications

Ph. D. Thesis.The measurement and quantification of defects is a challenge for Non-DestructiveTesting and Evaluation (NDT&E). Such challenges include the precise localisation and detection of surface and sub-surface defects, as well as the quantification of such defects. This work first reports a three-dimensional (3D) Eddy Current Pulsed Thermography (ECPT) system via integration with an RGB-D camera. Then, various quantitative measurements and analyses of defects are carried out based on the 3D ECPT system. The ECPT system at Newcastle University has been prooven to be an effective nondestructive testing (NDT) method in surface and sub-surface detection over the past few years. Based on the different numerical or analytical models, it has achieved precise defect detection on the rail tracks, wind turbines, carbon fibre reinforced plastic (CFRP) and so on. The ECPT system has the advantage of fast inspection and a large lift-off range. However, it involves a trade-off between detectable defect size and inspection area compared with other NDT methods. In addition, there are challenges of defect detection in a complex structure. Thus, the quantification of defects gives a higher requirement of the measurement the object geometry information. Furthermore, the analysis of thermal diffusion requires a precise 3D model. For this reason, a 3D ECPT system is proposed that adds each heat pixel with an exact X-Y-Z coordinate. In this work, first, the 3D ECPT system is built. A feature-based automatic calibration of the infrared camera and the RGB-D camera is proposed. Second, the software platform is built. A fast 3D visualization is completed with multi-threading technology and the Point Cloud Library. Lastly, various studies of defect localization, quantification and thermal tomography reconstruction are carried ou

Liquid crystal thermography for the thermal analysis of gas turbine blades internal cooling systems

The present work focus on the analysis of the transient liquid crystal thermography, which is employed to accomplish spatially resolved heat transfer performance on cooling channel of gas turbine blades. This methodology has already been implemented to its early stage in the rotating channel test facility of the Turbomachinery and Energy Systems Laboratory of the University of Udine; however, several aspects are still unsettled. Therefore, the main objectives of this thesis is to address the accuracy and validation of the transient thermography technique with the particular approach developed at the University of Udine. With these aims, transient thermography tests are carried out in a ribbed cooling channel on both static and rotating conditions. Even if a very common channel geometry has been chosen as a study case, no reliable experimental data were found in the open literature for validation purposes. In order to overcome this lack, the heat transfer data necessary to perform the comparison are achieved with the better-established liquid crystal thermography in steady-state approach. This work addresses further development and improvement of the test facility to make possible the implementation of the steady-state methodology. Moreover, a complex iterative numerical procedure is set up to estimate the heat losses that are the major cause of the lack of accuracy in the steady-state thermography measurements. Part of the work was also dedicated to the definition of the best calibration methodology to take when liquid crystals are exploited as temperature indicators in transient thermography; especially, when liquid crystals with activation temperatures below ambient one are used, as in the present case. The results clearly show that the temperature evolution approach must be preferred to the previously used calibration method (gradient temperature approach). The results for all the rotation conditions provided by the two experimental approaches are in good agreement, representing the evidence of the validation of the transient thermography. Nevertheless, this work suggests a possible method to estimate the uncertainty of the heat transfer coefficient values in transient experimental approach, and this is done by a sensitivity analysis to the variation of the most important experimental parameters. Furthermore, the influence of two uneven channel wall heating conditions on the local heat transfer distribution is investigated by means of the steady-state technique. The results show that the uneven thermal conditions have negligible impact on the stationary case, but they significantly affect the heat transfer when the rotation takes place. This can be due to the different buoyancy effects that in turns affects the secondary flow structures, and consequently, the local heat transfer. Anyway, additional investigations are required to better understand the reasons why of this behaviour