Flash Thermography Research Articles

Detection of dis-bond between honeycomb and composite facesheet of an Inner Fixed Structure bond panel of a jet engine nacelle using infrared thermographic techniques

ABSTRACT The Inner Fixed Structure (IFS) bond panel is a honeycomb sandwich panel with CFRP facesheet and a heat shield on one side, and a perforated CFRP facesheet on the other side, of a jet engine nacelle. It is subjected to extreme temperature on both sides which damages the inner epoxy adhesive bond between the facesheet and the honeycomb core. Accessibility to this layer for non-destructive evaluation is extremely challenging using conventional methods. This work proposes active thermography techniques such as flash thermography and induction thermography for accessing the inner layer. The infrared camera utilises the perforations in the facesheet of the IFS bond panel, which is used for attenuating the engine noise, for imaging the defects. However, flash thermography requires the removal of the thermal insulation layer for the inspection, whereas induction thermography can be performed without any modifications to the structure. The minimum detectable dis-bond size using these techniques is restricted to the spacing between the perforations on the facesheet. A numerical model has developed for induction thermography to optimise the excitation frequency that can produce reasonable thermal contrast at the inner facesheet and minimum temperature rise on the intermediate stainless-steel thin sheet that covers the thermal insulation layer.

Multi-scale gapped smoothing algorithm for robust baseline-free damage detection in optical infrared thermography

Flash thermography is a promising technique to perform rapid non-destructive testing of composite materials. However, it is well known that several difficulties are inherently paired with this approach, such as non-uniform heating, measurement noise and lateral heat diffusion effects. Hence, advanced signal-processing techniques are indispensable in order to analyze the recorded dataset. One such processing technique is Gapped Smoothing Algorithm, which predicts a gapped pixel's value in its sound state from a measurement in the defected state by evaluating only its neighboring pixels. However, the standard Gapped Smoothing Algorithm uses a fixed spatial gap size, which induces issues to detect variable defect sizes in a noisy dataset.In this paper, a Multi-Scale Gapped Smoothing Algorithm (MSGSA) is introduced as a baseline-free image processing technique and an extension to the standard Gapped Smoothing Algorithm. The MSGSA makes use of the evaluation of a wide range of spatial gap sizes so that defects of highly different dimensions are identified. Moreover, it is shown that a weighted combination of all assessed spatial gap sizes significantly improves the detectability of defects and results in an (almost) zero-reference background. The technique thus effectively suppresses the measurement noise and excitation non-uniformity. The efficiency of the MSGSA technique is evaluated and confirmed through numerical simulation and an experimental procedure of flash thermography on carbon fiber reinforced polymers with various defect sizes.

New methods for art object conservation and restoration – practical experience in the field: the experience of The True Image Solution Ltd

Previously, scientific examination of works of art was almost exclusively carried out in a specialist laboratory, major gallery or institution. Moving the artwork nearly always involved associated risks and transportation and insurance costs. Founded in 2009, The True Image Solution (TIS) had a vision to bring non-destructive technology to the artwork and to extend the possible range of evaluation by applying other imaging techniques, including ultrasound, flash thermography, X-ray spectroscopy and microwave imaging. In conjunction with English Heritage, these techniques were applied to a wide variety of objects. The results demonstrated that all of these techniques could be successfully carried out in situ and the risk of transportation damage, as well as associated transport and insurance costs, could be eliminated.

Thermal stresses applied on helicopter blades useful to retrieve defects by means of infrared thermography and speckle patterns

This work presents a non-destructive analysis based on infrared thermography (IRT) and speckle inspection methods performed on a helicopter blade. In both cases, thermal stresses are needed in order to provoke the visualization of defects. The sample possess fabricated defects, and three techniques were selected, namely, long-pulse thermography, flash thermography, and digital speckle photography (DSP) to retrieve their positions. The first two techniques belong to the group of infrared imaging, that are inherent to the analysis of the infrared thermal patterns to detect internal anomalies in the material, whilst the last one corresponds to the optical imaging group that requires visible light to measure the material response under a thermal stimulus. The active approach, useful to produce a gradient in either, thermal and/or displacement field of the material, was used. In all cases, heat lamps were used to generate the required thermal stresses.Post-processing algorithms were applied to raw data in order to improve the defect detection. Results are finally compared to evaluate pros and cons of each method.

Thermal Diffusivity Measurement of Laser-Deposited AISI H13 Tool Steel and Impact on Cooling Performance of Hot Stamping Tools

Additive manufacturing is a technology that enables the repair and coating of high-added-value parts. In applications such as hot stamping, the thermal behavior of the material is essential to ensure the proper operation of the manufactured part. Therefore, the effective thermal diffusivity of the material needs to be evaluated. In the present work, the thermal diffusivity of laser-deposited AISI H13 is measured experimentally using flash and lock-in thermography. Because of the fast cooling rate that characterizes the additive process and the associated grain refinement, the effective thermal diffusivity of the laser-deposited AISI H13 is approximately 15% lower than the reference value of the cast AISI H13. Despite the directional nature of the process, the laser-deposited material’s thermal diffusivity behavior is found to be isotropic. The paper also presents a case study that illustrates the impact of considering the effective thermal conductivity of the deposited material on the hot stamping process.

Comparative Study on Fatigue Life of CFRP Composites with Damages

Abstract In this work, the compressive residual strength tests results, Compression After Impact (CAI), are presented. The specimens were made of carbon-epoxy prepreg E722-02 UHS 130-14. Two variants of specimens were tested: samples undamaged and samples with damage that was centrally introduced by a drop-weight impact, as per the ASTM D7136/7136M standard. An impactor with potential energy equal to 15J and the type of support required by the standard were used. The size of impacted damages, defined as an area of damage on a plane perpendicular to the impact direction, and the equivalent diameter were specified using the flash thermography method. The tests were performed using the fixtures manufactured according to the ASTM D7137/7137M standard. The specimens were compressed to determine the residual strength. This value was afterwards used to specify the force levels for the fatigue tests. The fatigue tests were carried out under force control – with a sinusoidal shape, stress ratio R equal to 0.1 and frequency f 1Hz. Maximum force in a loading cycle Pmax was being increased after each thousand of cycles N until its value was close to the residual strength determined in the previously mentioned tests. In this work, the following relationships were presented: force-displacement P-δ for both static and fatigue tests and displacement-loading cycles δ-N for fatigue tests. A method of conducting the fatigue tests of CFRP composite was proposed, in which both the CAI specimens and CAI fixture were used. This allowed researchers to accelerate making initial comparisons between the two groups of specimens with damages – grouped relative to the way of conditioning.

Automated non-destructive inspection of Fused Filament Fabrication components using Thermographic Signal Reconstruction

Manufacturers struggle to produce low-cost, robust and intricate components in small batches. Additive processes like Fused Filament Fabrication (FFF) inexpensively generate such complex geometries, but potential defects may limit these components’ viability in critical applications. We present a high-accuracy, high-throughput and low-cost approach to automated non-destructive testing (NDT) for FFF interlayer delamination. This Artificially Intelligent (AI) approach utilizes Flash Thermography (FT) data processed with Thermographic Signal Reconstruction (TSR). A Deep Neural Network (DNN) attains 95.4% per-pixel accuracy when differentiating four delamination severities 5 mm below the surface in PolyLactic Acid (PLA) widgets, and 98.6% accuracy in differentiating acceptable from unacceptable states for the same components. Automation supports time- and cost-efficient inspection for delamination defects in 100% of widgets, supporting FFF's use in critical and lot-size one applications.

Edge disbond detection of carbon/epoxy repair patch on aluminum using thermography

The composite patches are used to repair the damaged metal and composite structures in different industries, especially in the aerospace industry. One of the most dangerous types of defects are the disbond defects in repaired structures because of cutting the flow of stress to the patch. Disbond defects are often initiated at the edge of the repair patch and propagated to the center by degradation in service. Flash thermography inspection at the edge of repair patch is a challenging issue due to the edge effect.In the present study, the disbonds at the edges of composite repair patch have been investigated by step heating thermography method. Since, raw thermal data are usually not suitable for quantitative defects evaluation and so, different image processing methods, including Fourier transform (FT), one- and two-dimensional Daubechie's wavelet transforms have been performed. Furthermore, for the first time, the three dimensional wavelet transform has been used to optimize the results in terms of signal to noise ratio and time processing. Moreover, the simulation of the thermography inspection process has been carried out using finite element modeling (FEM). The model enables a better understanding of the heat transfer process and provides a means of establishing the experimental set-up parameters required for inspecting appropriately the edge of the repair patch on metal structures. Results provided some suggestions in selecting more suitable post processing for interrogating of adhesive bonded composite with different states.

Quantitative evaluation of eggs freshness using flash thermography

ABSTRACT The paper examines the topic of identifying eggs freshness through an original approach based on pulsed thermography. The proposed method relies on a short thermal stimulation performed by means of a xenon flash, which rises up the temperature of the egg of less than 1°C. The entire process requires about 1 s, thus providing an effective quality control tool for assessing the freshness of eggs intended for human consumption. The identification and evaluation procedure has been automated by developing a dedicated image processing algorithm, based on a morphological operator with ‘white top hat’ transformation. The software performs a series of preliminary operations on the acquired thermograms and returns an estimation of the air chamber projections, that are correlated to egg degradation and weight reduction.

Enhanced pre-processing of thermal data in long pulse thermography using the Levenberg-Marquardt algorithm

Long pulse thermography (LPT) has recently been proposed as a new promising and cost-effective active thermographic technique over traditional flash thermography. However, LPT can be affected by uneven heating noise when two or multiple optical heat excitation sources are used. Signal pre-progressing algorithms such as the background subtraction method are typically used to remove uneven heating noise. In this study, a novel signal pre-progressing approach to remove uneven heating noise for LPT is presented, which relies on the Levenberg-Marquardt (LM) algorithm. Unlike the Ordinary Least Squares (OLS) method, LM operates an iterative optimization process to fitting the raw thermal data that allows selecting thermal signals contained only in the sound area. Results showed that the LM algorithm provided higher efficiency than the OLS algorithm to denoise thermal data. Moreover, the LM method combined with principal component analysis further enhanced the capability of LPT to visualize material damage.

Evaluation of an ancient cast-iron Buddha head by step-heating infrared thermography

The authors report on the development of a step-heating infrared thermography method for nondestructive evaluation of a cast-iron Buddha head made in the Song Dynasty (AD 960–1279) in China. In particular, this method measures the wall-thickness distribution, which can be used as an indicator for the condition of the object. Unlike other areas of analysis, inspection of cultural relics requires special attention to safety. To avoid the heat shock associated with traditional flash thermography, a step-heating method with a mild heating process is considered to be more acceptable for evaluating cultural relics made from various materials. Before application to inspection of the Buddha head, a numerical analysis and experimental tests were performed to verify the accuracy of this step-heating thickness measurement method. Finally, thickness images of the Buddha head were acquired and analyzed. The step-heating thickness measurement method has a high accuracy and can be safely used for inspecting cultural relics.

Nondestructive Testing of a Complex Aluminium-CFRP Hybrid Structure with EMAT and Thermography

A new concept of an aluminium-CFRP (carbon-fibre-reinforced-polymer) hybrid structure with integrated thermoplastic layer is introduced in this work. The occurring interfaces between the three materials are characterised with flash thermography and ultrasonic testing with electromagnetic acoustic transducer (EMAT). Thermography is able to detect artificial integrated defects in inner layers of the CFRP component and to characterise the bonding quality of the interface between the CFRP and the thermoplastic layer. Additionally a POD model (probability of detection) provides the smallest detectable size a90/95 of gapping defects in the CFRP with 5.184 mm diameter. This work shows that a characterisation of the deeper interface between the aluminium and the thermoplastic layer is possible despite the small and complex geometry. Furthermore the quality of this bonding can be investigated independently of the EMAT’s position. By combining thermography and EMAT the whole complex hybrid structure with its interfaces can be characterised.

Absorption Coefficient Dispersion in Flash Thermography of Semitransparent Solids

Pulse and flash thermography are experimental techniques which are widely used in the field of non-destructive testing for materials characterization and defect detection. We recently showed that it is possible to determine quantitatively the thickness of semitransparent polymeric solids by fitting of results of an analytical model to experimental flash thermography data, for both transmission and reflection configuration. However, depending on the chosen experimental configuration, different effective optical absorption coefficients had to be used in the model to properly fit the respective experimental data, although the material was always the same. Here, we show that this effect can be explained by the wavelength dependency of the absorption coefficient of the sample material if a polychromatic light source, such as a flash lamp, is used. We present an extension of the analytical model to describe the decay of the heating irradiance by two instead of only one effective absorption coefficient, greatly extending its applicability. We show that using this extended model, the experimental results from both measurement configurations and for different sample thicknesses can be fitted by a single set of parameters. Additionally, the deviations between experimental and modeled surface temperatures are reduced compared to a single optimized effective absorption coefficient.

Ceramic Matrix Composite Materials for Engine Exhaust Systems on Next-Generation Vertical Lift Vehicles

Future gas turbine engines will operate at significantly higher temperatures (∼1800 °C) than current engines (∼1400 °C) for improved efficiency and power density. As a result, the current set of metallic components (titanium-based and nickel-based superalloys) will be replaced with ceramics and ceramic matrix composites (CMCs). These materials can survive the higher operating temperatures of future engines at significant weight savings over the current metallic components, i.e., advanced ceramic components will facilitate more powerful engines. While oxide-based CMCs may not be suitable candidates for hot-section components, they may be suitable for structural and/or exhaust components. However, a more thorough understanding of the performance under relevant environment of these materials is needed. To this end, this work investigates the high-temperature durability of a family of oxide–oxide CMCs (Ox–Ox CMCs) under an engine-relevant environment. Flat Ox–Ox CMC panels were cyclically exposed to temperatures up to 1150 °C, within 240 m/s (∼0.3 M) gas flows and hot sand impingement. Front and backside surface temperatures were monitored by a single-wavelength (SW) pyrometer and thermocouple, respectively. In addition, an infrared (IR) camera was used to evaluate the damage evolution of the samples during testing. Flash thermography nondestructive evaluation (NDE) was used to elucidate defects present before and after thermal exposure.

Nonlinear air-coupled thermosonics for fatigue micro-damage detection and localisation

Over the years, traditional active infrared thermography has played a pivotal role in ensuring that a component is free of any damage. However, whilst optical thermography is still not sufficiently sensitive to the presence of material micro-flaws, current vibro-thermography requires inspection solutions that are not always feasible, such as the use of coupling materials between the sensing probe and the monitored structure. This paper presents a valid alternative to current thermographic systems by developing and experimentally validating nonlinear air-coupled thermosonics for a contactless, rapid and accurate detection of fatigue micro-cracks in an aero-engine turbine blade. The proposed thermographic method combines the high sensitivity to micro-damage of nonlinear ultrasonic techniques with non-contact air-coupled ultrasonic transducers and thermographic equipment. Narrowband frequency sweeps were performed to identify local damage resonance frequencies in order to generate large vibrational amplitudes at the damage location and compensate for signal losses caused by the high acoustic impedance mismatch between the air and the sample. An infrared camera was then used to acquire the thermal response generated by frictional heat at the crack interfaces. Moreover, an image processing method based on a combination of morphological opening and a Savitzky-Golay smoothing filter was employed to enhance the quality of thermal images affected by anisotropic heating and thermal noise effects. Nonlinear air-coupled thermosonics experiments were validated with laser Doppler vibrometry scan measurements and compared with both flash and pulsed phase thermography. Thermal imaging results showed that the proposed nonlinear air-coupled thermosonics was the only thermographic technique able to detect fatigue micro-cracks, thus demonstrating its potential as an efficient and sensitive inspection tool for micro-damage detection in geometrically complex components.

Evaluation of Different Techniques of Active Thermography for Quantification of Artificial Defects in Fiber-Reinforced Composites Using Thermal and Phase Contrast Data Analysis

For assuring the safety and reliability of components and constructions in energy applications made of fiber-reinforced polymers (e.g., blades of wind turbines and tidal power plants, engine chassis, flexible oil and gas pipelines) innovative non-destructive testing methods are required. Within the European project VITCEA complementary methods (shearography, microwave, ultrasonics and thermography) have been further developed and validated. Together with partners from the industry, test specimens have been constructed and selected on-site containing different artificial and natural defect artefacts. As base materials, carbon and glass fibers in different orientations and layering embedded in different matrix materials (epoxy, polyamide) have been considered. In this contribution, the validation of flash and lock-in thermography to these testing problems is presented. Data analysis is based on thermal contrasts and phase evaluation techniques. Experimental data are compared to analytical and numerical models. Among others, the influence of two different types of artificial defects (flat bottom holes and delaminations) with varying diameters and depths and of two different materials (CFRP and GFRP) with unidirectional and quasi-isotropic fiber alignment is discussed.

Continuous and Laplace transformable approximation for the temporal pulse shape of Xe-flash lamps for flash thermography

Flash thermography is widely used in non-destructive testing and material characterisation. The use of analytical modelling utilising the Laplace transform allows one to calculate the temperature transients of flash-heated samples and therefore characterise them by fitting of the results of model calculations to experimental data. However, for samples with high thermal diffusivity or very thin samples, the temperature transient is strongly influenced by the temporal shape of the heating pulse, especially in reflection configuration. To incorporate this into the model, the temporal shape of the heating pulse and its Laplace transform have to be known. Here we present a close phenomenological approximation of the temporal shape of pulses of Xe-flash lamps. It is a non-stitched solution, has a simple Laplace transform and is suitable for different lamps and energy settings. As an example for a practical application of the pulse shape approximation, we use it to determine the thickness of polymer samples with thicknesses down to 80 μm by means of flash thermography, both in transmission and reflection configuration. Using a rectangular pulse shape or a delayed Dirac pulse shape, the thickness results are very sensitive to the start time of the fit and an additional calibration is needed.

Comparative study of thermal contrast and contrast in thermal signal derivatives in pulse thermography

For detection of subsurface defects, flash thermography results are often evaluated by selecting the thermal image with the largest contrast between the sound and defective area. Another possibility is to calculate the logarithmic derivatives of the thermal signal, and evaluate the 1st or 2nd derivative images. In this paper, it is investigated how the contrast in these images depend on the defect size and defect depth. Analytical calculations and FEM simulations are presented, and based on these results equations for the contrast maxima are derived. Several advantages of the derivative images are shown; for example a defect with an aspect ratio around unity can be reliably detected in the derivative image, but not in the thermal image. To demonstrate these results experimental data are also presented.

Thickness determination of semitransparent solids using flash thermography and an analytical model

As groundwork for thickness determination of polymeric surface protection systems for concrete, we present a method for measuring the thickness of isolated semitransparent solids using flash thermography both in transmission and reflection configuration. Since standard models do not capture semitransparency, an advanced analytical model by Salazar et al. is applied. Physical material parameters are deduced by fitting experimental data from samples of well-known thickness. Using those, the thickness of samples of the material can be obtained by fitting, as demonstrated for different semitransparent polymer materials.

Non-destructive Investigation of Paintings on Canvas by Continuous Wave Terahertz Imaging and Flash Thermography

Terahertz (THz) imaging is increasingly used in the cultural heritage field. In particular, continuous wave (CW) and low frequency THz is attracting more attention. The first application of the THz technique inherent to the cultural heritage field dates back 10 years ago. Since 2006, tangible improvements have been conducted in the refinement of the technique, with the aim to produce clear maps useful for any art restorer. In this paper, a CW THz (0.1 THz) imaging system was used to inspect paintings on canvas both in reflection and in transmission modes. In particular, two paintings were analyzed: in the first one, similar materials and painting execution of the original artwork were used, while in the second one, the canvas layer is slightly different. Flash thermography was used herein together with the THz method in order to observe the differences in results for the textile support materials. A possible application of this method for the detection of artwork forgery requires some parameterization and analysis of various materials or thickness influence which will be addressed in a future study. In this work, advanced image processing techniques including principal component thermography (PCT) and partial least squares thermography (PLST) were used to process the infrared data. Finally, a comparison of CW THz and thermographic results was conducted.