Comparison between thermographic techniques for frescoes NDT

The use of infrared thermography in the architectural restoration field is examined. Three samples, made of a support of marble, brick, or tuff, covered with a layer of plaster with inclusions to simulate detachments or cracks in frescoes, are considered. Different techniques: pulse thermography, lateral heating thermography, lock-in or modulated thermography and pulse phase thermography are employed to detect the flaws artificially created; advantages and disadvantages of each technique are discussed. It is found that pulse thermography is easy and fast to use for information about the state of the art treasures, but data may be affected by non-uniform heating and local variation of thermal emission; the lateral heating can help to overcome interference effects due to non-uniform heating but it is more troublesome to use. When the evaluation regards rare art treasures the lock-in technique seems to be the only response since it is able to operate within very low increase of surface temperature; this technique is also able to give information about the material composition. The pulse phase thermography may be used to detect more in depth flaws but it needs higher temperature increase with respect to the ambient temperature and so it is recommended to control, before testing, the temperature sensitivity of the artefact.     

Laser lock-in thermography for detection of surface-breaking fatigue cracks on uncoated steel structures

This paper develops a new noncontact laser lock-in thermography (LLT) technique for detection of surface-breaking fatigue cracks on uncoated steel structures with low surface emissivity. LLT utilizes a modulated continuous (CW) wave laser as a heat source for lock-in thermography instead of commonly used flash and halogen lamps. LLT has the following merits: (1) the laser heat source can be precisely positioned at a long distance from a target structure thank to its directionality and low energy loss, (2) a large target structure can be inspected using a scanning laser heat source, (3) no special surface treatment of the target structure is necessary to generate and measure thermal wavefields, (4) thermal image noises created by arbitrary surrounding heat sources can be effectively eliminated and (5) the use of a low peak power laser makes it possible to avoid surface ablation. The LLT system is developed by integrating and synchronizing a modulated CW laser, a galvanometer and an infrared camera. Then, a fatigue crack evaluation algorithm based on a holder exponent analysis is proposed. The performance of the proposed LLT technique is validated through thermal wavefield imaging and fatigue crack evaluation tests on an uncoated steel plate with emissivity of 0.8 and a welded T-shape joint with emissivity of 0.7. Test results confirm that thermal wavefield images are effectively captured even when surface-reflected background noises and laser-generated thermal waves coexist, and surface-breaking cracks are successfully evaluated without any special surface treatment.     

Characterization and spatial resolution of cracks using lock-in vibrothermography

We present a method to characterize vertical cracks that combines lock-in vibrothermography experiments and a stabilized inversion algorithm. We analyze its capability to retrieve the shape and location of square flaws buried at increasing depths and its ability to resolve two square buried cracks by inverting synthetic data. The predictions on the reconstruction of single and double cracks are checked by inverting experimental data obtained from metallic samples containing calibrated cracks at low excitation powers. Experimental results confirm that the method is able to reconstruct square heat sources whose depth is twice the lateral dimension.     

Determination of the modulation frequency for thermographic non-destructive testing

We propose polynomial solutions of the inverse heat conduction problem to design thermographic non-destructive tests for detecting defects in composite and multi-layer materials. Inverse heat conduction in a multi-layer material slab with periodic temperature excitation is considered, and analytical quadrupole representation is used to derive a lumped parameters formulation. Predictions of the proposed polynomial representations are experimentally validated by detecting machined defects on thermally excited panels. For modulation frequencies outside the predicted detection range, defects appear on thermal images as blurry and unstructured; conversely, for modulation frequencies within the predicted range, defects are correctly represented on thermal images.     

Viscous material-filled synthetic defects for vibrothermography

This paper describes a method to create viscous material-filled (VMF) synthetic defects for vibrothermography that, like cracks, delaminations, or disbonds, generate heat when exposed to vibration. Standard synthetic defects used in nondestructive evaluation, such as EDM notches and flat-bottom holes, are not suitable for vibrothermography since they do not generate a measurable amount of heat when vibrated. An array of viscous material-filled synthetic defects can be used to evaluate the vibration field in a structure due to a specific source to determine if local vibration amplitudes are sufficient to detect cracks or other defects in different locations in a structure. These synthetic defects can be used to resolve the long-standing problem of testing vibration coverage in a specimen for vibrothermography. Such defects can be manufactured in a wide range of materials and can aid in probability of detection studies.     

新兴的无损检测技术——红外热波成像检测

针对红外热波成像检测技术，重点介绍了其理论基础、检测原理、红外探测器、各种不同的主流检测方法及其检测机理和优缺点。综述了国内外红外热波成像检测进展及所取得的最新应用成果，最后给出其技术特点，指出了该技术发展存在的问题和发展方向。     

Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons

For thin (< 200 μm) air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) ceramic thermal barrier coatings (TBCs), some non-destructive techniques indicate damage at the bond coat-TBC interface during either ageing or cyclic oxidation tests. However, no technique is available for thick (> 200 μm) APS TBCs.In this work, a semi-quantitative estimation of cracks at the interface of APS TBCs thicker than 300 μm is obtained from thermal diffusivity values measured by using a single side thermographic technique on coupons subjected to thermal cycling.In fact, during thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity.The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface.     

Non-uniformity correction and sound zone detection in pulse thermographic nondestructive evaluation

An important requirement in the thermographic nondestructive evaluation is the identification of actual sound zone or the base line with which the defective areas are compared to determine the actual temperature contrast and the corresponding defect severity. In a part under inspection, the actual sound zone is not known a priori and various approximations have been used in the past to serve as this unknown base line. Determination of actual sound zone in a defective test object is still a challenge. A related issue before the identification of this base line pixel is in the elimination of non-uniformity in the temperature distribution across the specimen surface which is unrelated to the actual defects. This spurious contrast is often introduced by the limitations of the instrumentation or the test procedure and it has to be eliminated before pixels in the sound zone can be located. This paper presents an automated procedure for simultaneously eliminating spurious contrast and locating sound zone pixels, directly from experimental data in a thermographic nondestructive evaluation. Location of actual sound zone pixels facilitates accurate thermal contrast computations, extraction of thermal properties such as break time and thermal diffusivity. In addition, based on the actual sound zone temperature profile it is possible to normalize experimental thermographic results in such a way that they can be directly correlated with results from numerical simulations.     

Defect detection in aircraft composites by using a neural approach in the analysis of thermographic images

Safety in aeronautics could be improved if continuous checks were guaranteed during the in-service inspection of aircraft. However, until now, the maintenance costs of so doing have proved prohibitive. For this reason there is a great interest for the development of low cost non-destructive inspection techniques that can be applied during normal routine tests. The analysis of the internal defects (not detectable by a visual inspection) of the aircraft composite materials is a difficult task unless invasive techniques are applied. In this paper, we have addressed the problem of inspecting composite materials by using automatic analysis of thermographic techniques. The analysis of the time/space variations in a sequence of thermographic images allows the identification of internal defects in composite materials that otherwise could not be detected. A neural network was trained to extract the information that characterises a range of internal defects in different types of composite materials. After the training phase the same neural network was applied to all the points of a sequence of thermographic images. The experimental results demonstrate the ability of the method to recognize regions containing defects but also to identify the contour regions that cannot be associated either with a defective or with a sound region.     

Simultaneous measurement of the in-plane and in-depth thermal diffusivity of solids using pulsed infrared thermography with focused illumination

We extend the flash method to retrieve simultaneously the principal in-plane and the in-depth thermal diffusivities of anisotropic solids using focused Gaussian illumination. A complete theoretical model allows calculating the temperature rise of an anisotropic and semitransparent sample. The surface temperature distribution has a Gaussian shape along the principal axes, whose radii give the principal in-plane thermal diffusivities. On the other hand, the time evolution of the spatially averaged surface temperature gives the principal in-depth thermal diffusivity. Measurements performed on opaque and semitransparent samples, covering a wide range of thermal diffusivities, validate the method. It is especially suited to characterize the principal components of the thermal diffusivity tensor of anisotropic plates from a single and fast measurement.     

红外热波无损检测技术及其进展

综述了红外热波无损检测技术的基本原理、技术特点，给出了一些典型的应用试验结果，介绍了国内外相应研究的发展状况和进展。     

Approach to identify cracking in asphalt pavement using GPR and infrared thermographic methods: Preliminary findings

Pavement condition is a factor of major interest due to its direct contribution to safety and comfort of the users of the road. Consequently, road inspections imply the evaluation of different parameters such as roughness of the pavement, skid resistance, and presence and condition of cracks. Although the first two parameters can be quantitatively evaluated with different sensors, the latter is usually only qualitatively assessed by visual inspection. This paper deals with this drawback through the combined application of Ground Penetrating Radar and Infrared Thermography to the detection and characterization of cracks in pavement and their origins.     

红外热波技术在飞机复合材料损伤检测中的应用

红外热波无损检测基于物体的热辐射特性，利用主动加热技术，通过相关的检测系统记录试件表面缺陷和基体材料由于不同热特性引起的温度差异，进而判定飞机复合材料表面及内部的损伤。较之于常规检测方法，红外热波无损检测具有非接触、快速、直观、准确等优点。     

激光扫描红外热波成像技术在无损检测中的应用

对激光扫描热波成像技术与传统的闪光灯激励热波技术进行了比较,介绍了一种基于激光扫描热波成像技术的新型红外无损检测设备,通过试验对所建立的2-D理论模型进行验证,试验结果表明,当激光扫描速度在一定范围内,样品表面温度场的变化服从一维热传导模式,主要表现为厚样品的温度-时间曲线在双对数坐标中为斜率-0.5的直线,与理论模型的结果相符合。并对两种特殊涂层的人工样品进行检测,验证了激光扫描红外热波成像设备的有效性。     

Defect detection in thermal image for nondestructive evaluation of petrochemical equipments

This paper proposes a method for segmenting defects depicted in a thermal image of petrochemical equipments by means of passive thermography. The technique first enhances the contrast of the defects based on local neighborhood pixel intensity operation. This local intensity operation works in two modes, either brightening the pixels for detecting hot spots or darkening the pixels for detecting cold spots. The next step is to segment the defects using simple histogram-based thresholding techniques. We propose three thresholding methods: mean absolute thresholding (MAT), mean relative thresholding (MRT), and minimum frequency thresholding (MFT). Compared to existing techniques, we found that our proposed methods have better detection and success rate.     

The infrared thermography control of the laser welding of amorphous polymers

In laser welding technique, a real-time control of temperature distribution inside the irradiated materials is essential when attempting to optimize the process. For all laser welding methods that operate by the transmission principle, the difficulty of recording the developed temperature at the interface derives from the fact that materials to be welded are in contact throughout the entire process. In the present study, in order to overcome this issue, a contact-free method such the infrared thermography is used for surface temperature measurement. Corroborating this data with a numerical simulation of the temperature field evolution inside the components, an assessment of optimal process parameters is possible. The experimental investigations are made on amorphous polymers, in a typical configuration for through-transmission laser welding. The fine agreement obtained between the experimentally and calculated data, validate the infrared thermography as a non-destructive method for real-time monitoring of the welding process.     

Tool steel and copper coatings by friction surfacing - A thermography study

Infrared thermography was used to record thermal profiles during friction surfacing. Thermal profiles for different sets of consumable rod/substrates (tool steel/steel; copper/steel and copper/copper) were recorded and analyzed. The thermal profiles showed distinct stages of plastic deformation with respect to temperature. The mechanism of bonding or no-bonding was discussed based on thermal profile data. It was found that a metallurgically bonded coating can be obtained if the flow stress of the plasticized material is comparable with the localized stress developed due to axial loading.     

Optimized inspection design for the thermographic characterization of sub-pixel sized through cracks

Non-destructive evaluation of structural components is critical for reducing costs from unnecessary replacements and maintenance. We study the utility of a non-contact modality for the inspection of thin metal plates for the presence of through cracks. Sensitivity to early stages of deterioration allows for simpler and less expensive repair than if a flaw propagates and becomes more damaging. Hence, we focus on the characterization of very small cracks with a thermal imaging technique. Through cracks interact with the flow of heat within a component, so that the characterization of cracks from a thermal image amounts to solving an inverse problem to discover unknown parameters that describe the crack.We consider cracks with length of less than a millimeter, falling under the pixel resolution of the recording thermal camera. Although these flaws are not directly visible from imaging data, the well-understood theory of heat conduction can be used in inference of crack properties. Herein we present a method to design an inspection modality that yields optimal data for such inference. Numerical experiments are performed to compare our optimized inspection setup to previous thermographic inspection scenarios found in the literature. Our design is found to produce the same quality of inference as these previous experiments which require much more expensive equipment (e.g. more powerful lasers and more sensitive IR cameras).     

New algorithm based on the Hough transform for the analysis of pulsed thermographic sequences

The automatic detection of subsurface defects has become a desired goal in the application of non-destructive techniques. A new algorithm based on the Hough transform is proposed to reduce human intervention to a minimum by pulsed thermography. The final result provided by this algorithm is an image showing the different defects without having attended to parameters so determinant in other algorithms as the delayed time of the first image or any subjective point of view in the analysis.     

Segmentation of thermal images for non-destructive evaluation of bridge decks

The objective of the project presented in this paper is to automate the detection of subsurface defects in concrete bridge decks using infrared thermography. The algorithm developed for this purpose is based on the region growing approach, which segments the image and identifies the voids without human interference or prior knowledge of the conditions. The segmentation algorithm starts with the hottest pixels in the image as seed points, and then regions are grown around them based on a neighborhood selection criterion. The algorithm was tested on images collected from concrete bridge deck specimens containing various man-made defects and also on a defect-free control model. The experimental work successfully identified defects in concrete bridge decks up to 3 in. below surface using thermograph imaging.