Investigation of Infrared Thermography NDE Techniques for Use in Power Station Environments

Three active thermal methods capable of detecting surface breaking cracks in metals are considered in this Thesis. The three thermal methods exploit different means of excitation, each with practical advantages and varying abilities to detect specific types of crack morphology. Thermosonics uses a broadband, high power ultrasonic input to vibrate the test-piece. Defects damp the vibrational energy into heat which is imaged by a thermal camera. Laser-spot thermography uses a short laser pulse to spot heat the surface of the test-piece, and the subsequent radial heat diffusion is then observed. Defects can cause both increased emission of infrared and localised increases in thermal impedance, both effects causing distortion of the radial heat diffusion. Eddy-current induced thermography uses a high power magnetic field to induce a flow of current inside the test-piece. Defects create a localised increase in electrical impedance, diverting the electric field around the defect. This diversion of current flow causes neighbouring regions of high and low current density, the corresponding Joule heating imaged by a thermal camera. In this Thesis the three methods are explored experimentally. For laser-spot thermography and eddy-current induced thermography the physical phenomena are characterised and experimental best-practice for short pulse excitation determined. The effect of crack opening on each of the three methods is found to give insight into which applications the methods are most suited. It was found that the relationship between crack opening and detectability was complex for thermosonics, relatively linear for laser-spot thermography, and that eddy-current induced thermography is largely insensitive to crack opening. The methods are tested for the feasibility of detecting cracks in Inconel buried beneath metallic and ceramic coatings typical of gas turbine blades, with thermosonics and eddy-current induced thermography found to be viable methods. A study of the detectability of a large number of cracks in steel, titanium and Waspaloy by eddy-current induced thermography is detailed, and from this data the probability of detection is established. Eddy-current thermography is shown to be an extremely sensitive method capable of detecting fatigue cracks of approximately 0.25 mm in steel and 0.50-0.75 mm in titanium and Waspaloy. The practicality of the thermal methods is discussed, and the methods put into the context of the wider field of NDE. Based on the works in this Thesis it was found that for most applications eddy-current induced thermography is the most appealing thermal method since it is highly sensitive, rapid, non-contacting and relatively easy to validate. However, both thermosonics and laser-spot thermography remain useful alternative inspections for more niche applications

Recent advances in active infrared thermography for non-destructive testing of aerospace components

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters’ primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites

Radio frequency non-destructive testing and evaluation of defects under insulation

PhD ThesisThe use of insulation such as paint coatings has grown rapidly over the past decades. However, defects and corrosion under insulation (CUI) still present challenges for current non-destructive testing and evaluation (NDT&E) techniques. One of such challenges is the large lift-off introduced by thick insulation layer. Inaccessibility due to insulation leads corrosion and defects to be undetected, which can lead to catastrophic failure. Furthermore, lift-off effects due to the insulation layers reduce the sensitivities. The limitations of existing NDT&E techniques heighten the need for novel approaches to the characterisation of corrosion and defects under insulation. This research project is conducted in collaboration with International Paint®, and a radio frequency non-destructive evaluation for monitoring structural condition is proposed. High frequency (HF) passive RFID in conjunction with microwave NDT is proposed for monitoring and imaging under insulation. The small-size, battery-free and cost-efficient nature of RFID makes it attractive for long-term condition monitoring. To overcome the limitations of RFID-based sensing system such as effective monitoring area and lift-off tolerance, microwave NDT is proposed for the imaging of larger areas under thick insulation layers. Experimental studies are carried out in conjunction with specially designed mild steel sample sets to demonstrate the detection capabilities of the proposed systems. The contributions of this research can be summarised as follows. Corrosion detection using HF passive RFID-based sensing and microwave NDT is demonstrated in experimental feasibility studies considering variance in surface roughness, conductivity and permeability. The lift-off effects introduced by insulation layers are reduced by applying feature extraction with principal component analysis and non-negative matrix factorisation. The problem of thick insulation layers is overcome by employing a linear sweep frequency with PCA to improve the sensitivity and resolution of microwave NDT-based imaging. Finally, the merits of microwave NDT are identified for imaging defects under thick insulation in a realistic test scenario. In conclusion, HF passive RFID can be adapted for long term corrosion monitoring of steel under insulation, but sensing area and lift-off tolerance are limited. In contrast, the microwave NDT&E has shown greater potential and capability for monitoring corrosion and defects under insulation

Eddy current pulsed thermography for non-destructive evaluation of carbon fibre reinforced plastic for wind turbine blades

PhD ThesisThe use of Renewable energy such as wind power has grown rapidly over the past ten years. However, the poor reliability and high lifecycle costs of wind energy can limit power generation. Wind turbine blades suffer from relatively high failure rates resulting in long downtimes. The motivation of this research is to improve the reliability of wind turbine blades via non-destructive evaluation (NDE) for the early warning of faults and condition-based maintenance. Failure in wind turbine blades can be categorised as three types of major defect in carbon fibre reinforced plastic (CFRP), which are cracks, delaminations and impact damages. To detect and characterise those defects in their early stages, this thesis proposes eddy current pulsed thermography (ECPT) NDE method for CFRP-based wind turbine blades. The ECPT system is a redesigned extension of previous work. Directional excitation is applied to overcome the problems of non-homogeneous and anisotropic properties of composites in both numerical and experimental studies. Through the investigation of the multiple-physical phenomena of electromagnetic-thermal interaction, defects can be detected, classified and characterised via numerical simulation and experimental studies. An integrative multiple-physical ECPT system can provide transient thermal responses under eddy current heating inside a sample. It is applied for the measurement and characterisation of different samples. Samples with surface defects such as cracks are detected from hot-spots in thermal images, whereas internal defects, like delamination and impact damage, are detected through thermal or heat flow patterns. For quantitative NDE, defect detection, characterisation and classification are carried out at different levels to deal with various defect locations and fibre textures. Different approaches for different applications are tested and compared via samples with crack, delamination and impact damage. Comprehensive transient feature extraction at the three different levels of the pixel, local area and pattern are developed and implemented with respect to defect location in terms of the thickness and complexity of fibre texture. Three types of defects are detected and classified at those three levels. The transient responses at pixel level, flow patterns at local area level, and principal or independent components at pattern level are derived for defect classification. Features at the pixel and local area levels are extracted in order to gain quantitative information about the defects. Through comparison of the performance of evaluations at those three levels, the pixel level is shown to be good at evaluating surface defects, in particular within uni- directional fibres. Meanwhile the local area level has advantages for detecting deeper defects such as delamination and impact damage, and in specimens with multiple fibre orientations, the pattern level is useful for the separation of defective patterns and fibre texture, as well as in distinguishing multiple defects.Engineering and Physical Sciences Research Council(EPSRC), Frame Programme 7(FP7

Multi-layer carbon fiber reinforced plastic characterization and reconstruction using eddy current pulsed thermography

Ph. D. Thesis.Carbon fibre composite materials are widely used in high-value, high-profit applications, such as aerospace manufacturing and shipbuilding – due to their low density, high mechanical strength, and flexibility. Existing NDT techniques such as eddy current testing suffers from electrical anisotropy in CFRP (carbon fibre reinforced plastics). Ultrasonic is limited by substantial attenuation of signal caused by the multilayer structure. The eddy current pulsed thermography has previously been applied for composites NDE (non-destructive evaluation)such as impact damage, which has the ability for quick and accurate QNDE(quantitative non-destructive evaluation) inspection but can be challenging for detection and evaluation of sub-surface defects, e.g., delamination and debonding in multiple layer structures. Developing QNDE solutions using eddy current thermography for addressing subsurface defects evaluation in multi-layer and anisotropic CFRP is urgently required. This thesis proposes the application of eddy current pulsed thermography (ECPT) and ECPuCT (eddy current pulse compression thermography) for tackling the challenges of anisotropic properties and the multi-layer structure of CFRP using feature-based and reconstruction-based QNDE and material characterisation. The major merit for eddy current heating CFRP is the volumetric heating nature enabling subsurface defect detectability. Therefore, the thesis proposes the investigation of different ECPT and their features for QNDE of various defects, including delamination and debonding. Based on the proposed systems and QNDE approach, three case studies are implemented for delamination QNDE, debonding QNDE, conductivity estimation and orientation inverse reconstruction using the two different ECPT systems and features, e.g., a pulse compression approach to increase the capability of the current ECPT system, the feature-based QNDE approach for defect detection and quantification, and reconstruction-based approach for conductivity estimation and inversion. The three case studies include 1) investigation of delamination with different depths in terms of delamination location, and depth quantification using K-PCA, proposed temporal feature-crossing point feature and ECPuCT system; 2) investigation of debonding with different electrical and thermal properties in terms of non-uniform heating pattern removal and properties QNDE using PLS approaches, impulse response based feature

Nondestructive testing of metals and composite materials using ultrasound thermography : comparison with pulse-echo ultrasonics

La thermographie stimulée par ultrasons (TU) est une méthode de contrôle non destructif qui a été inventée en 1979 mais qui a s'est répandue à la fin des années 90. L'idée de cette méthode est d'exciter le matériau à inspecter avec des ondes mécaniques à des fréquences allant de 20kHz à 40kHz et d'observer ensuite leur température de surface avec une caméra infrarouge. TU est une méthode de thermographie active; les autres méthodes les plus connues sont la thermographie optique et celle stimulée par courants de Foucault. Son habilité à révéler des défauts dans des cas où les autres techniques échouent, fait d'elle une méthode pertinente ou complémentaire. L'inconvénient de la TU est que beaucoup de conditions expérimentales doivent être respectées pour obtenir des résultats adéquats incluant quelques paramètres qui doivent être bien choisis. Le but de ce projet est d'explorer les capacités, les avantages et les limites de la TU. Pour comparer la performance de la TU à celle des ultrasons conventionnels, des tests ultrasons de type C-Scan ont été réalisés pour quelques échantillons. Quatre matériaux différents avec quatre types de défauts ont été investigués afin de mieux définir les conditions optimales pour améliorer la détection des défauts. Les résultats bruts obtenus étaient traités dans chaque cas afin de mieux visualiser les contrastes thermiques causés par les discontinuités cachées

Transient thermography for detection of micro-defects in multilayer thin films

Delamination and cracks within the multilayer structure are typical failure modes observed in microelectronic and micro electro mechanical system (MEMS) devices and packages. As destructive detection methods consume large numbers of devices during reliability tests, non-destructive techniques (NDT) are critical for measuring the size and position of internal defects throughout such tests. There are several established NDT methods; however, some of them have significant disadvantages for detecting defects within multilayer structures such as those found in MEMS devices. This thesis presents research into the application of transient infrared thermography as a non-destructive method for detecting and measuring internal defects, such as delamination and cracks, in the multilayer structure of MEMS devices. This technique works through the use of an infrared imaging system to map the changing temperature distribution over the surface of a target object following a sudden change in the boundary conditions, such as the application of a heat source to an external surface. It has previously been utilised in various applications, such as damage assessment in aerospace composites and verification of printed circuit board solder joint manufacture, but little research of its applicability to MEMS structures has previously been reported. In this work, the thermal behaviour of a multilayer structure containing defects was first numerically analysed. A multilayer structure was then successfully modelled using COMSOL finite element analysis (FEA) software with pulse heating on the bottom surface and observing the resulting time varying temperature distribution on the top. The optimum detecting conditions such as the pulse heating energy, pulse duration and heating method were determined and applied in the simulation. The influences of thermal properties of materials, physical dimensions of film, substrate and defect and other factors that will influence the surface temperature gradients were analytically evaluated. Furthermore, a functional relationship between the defect size and the resulting surface temperature was obtained to improve the accuracy of estimating the physical dimensions and location of the internal defect in detection. Corresponding experiments on specimens containing artificially created defects in macro-scale revealed the ability of the thermographic method to detect the internal defect. The precision of the established model was confirmed by contrasting the experimental results and numerical simulations

Investigation of Tx-Rx mutual inductance eddy current system for high lift-off inspection

PhD ThesisEddy current (EC) testing is a popular inspection technique due to its harsh environment tolerance and cost-effectiveness. Despite the immense research in EC inspection, defect detection at high lift-off still poses a challenge. The weakening mutual coupling of EC probe and sample due to the increase in lift-off degrades signal strength and thus reduces the detection sensitivity. Although signal processing can be used to mitigate lift-off influence, it is laborious and time consuming. Therefore, in this study, a Tx-Rx probe system is proposed to deal with high lift-off inspection. The parts of the study of the Tx-Rx EC system includes optimisation of probe configuration, improvement of signal conditioning circuit and comparative study of excitation modes. In optimisation of probe configuration, lift-off and coil gap are optimized to mitigate the offset caused by the direct coupling of Tx-Rx coils. The optimum coil gaps of Tx-Rx probe for different lift-offs are found by observing the highest signal strength. The optimisation of coil gap against lift-off extends the detection sensitivity of the EC system to a lift-off of about 30 mm which is by far higher than 5 mm lift-off limit of a single-coil EC probe. In signal conditioning aspect, a modified Maxwell bridge circuit is designed to remove the offset due to self- impedance of the Rx coil. The proposed circuit mitigates the influence of the self-impedance of Rx coil and improves signal-to- noise ratio SNR. In the excitation mode, pulse and sweep frequency signals are compared to study detection sensitivity, SNR and crack quantification capability. The result of the comparative study reveals that pulse excitation is good for crack sizing while sweep frequency excitation is better for crack detection. Simulations and experimental studies are carried out to show the efficacy of the Tx-Rx EC system in high lift-off crack detection

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