A defect shape reconstruction algorithm for pulsed thermography

An algorithm for the defect shape reconstruction from pulsed thermography data was developed. The aim was to reconstruct the defect shape in a test sample with known thermal properties. The algorithm consists of a defect shape correction unit and a simulation unit. The defect shape correction unit is designed to extract the defect shape roughly and refine it sequentially while the simulation unit models the heat conduction process in the inspected sample. The developed iterative algorithm is able to reconstruct 2D as well as 3D defect shapes. The algorithm was tested using experimental data obtained on a plate-shaped steel sample with a wall thickness profile. Robust defect shape reconstruction results were demonstrated.     

Research on the quantitative analysis of subsurface defects for non-destructive testing by lock-in thermography

This paper describes the quantitative analysis of the boundary, location and depth of subsurface defects by lock-in thermography. The phase difference between defective areas and non-defective areas illustrates the qualitative analysis of the boundary and the location of the subsurface defect. In order to accurately estimate the size, location and depth of the defects, the phase is normalised, the heat transfer partial differential equation (PDE) model is used to filter the noise of normalised phase image and the differential normalised phase profile is employed to determine the boundary and location of the defect. The profile of the differential normalised phase distribution has maximum, minimum and zero points that help to quantitatively determine the boundary and location of the subsurface defect. An artificial neural network (ANN) is proposed to determine the depth of the subsurface defect. Experimental results for a steel plate, a carbon fibre-reinforced polymer (CFRP) sheet-foam sandwich, and honeycomb structure composites with artificial subsurface defects show good agreement with the actual values.     

Determination of thermal diffusivity distribution for three types of materials by transient thermography

In many component applications knowledge of the two-dimensional distribution of material properties within the part is highly desirable as a quality assessment tool. The contour plotting of material properties allows identification of local regions of unacceptable variation for rejection, repair or further analysis. Transient thermography is a rapid, inexpensive, on-line, nondestructive technique that can be used to determine the two-dimensional distribution of material properties. In this paper the transient method of pulsed video thermography (PVT) is studied. Procedures for the parameter normalization and numerical method are combined with experimental data to accurately map the thermal diffusivity distribution over the entire surface of a specimen. Materials whose diffusivity ranged over three orders of magnitude were tested. The accuracy of the PVT results was evaluated by comparing local values with those obtained by standard laser flash diffusivity testing.     

金属表面缺陷的电涡流脉冲热成像检测

利用搭建的电涡流脉冲热成像检测平台,对常用金属的表面缺陷进行了检测.为了去除热成像图中的噪声,提高金属试件表面缺陷的对比度,采用独立成分分析法分析了表面缺陷的热响应特征.结果 表明,独立成分分析法可有效地识别出缺陷、线圈、缺陷及线圈附近、试件边缘等区域的热响应信号特征,缺陷区域成像清晰,实现了金属构件表面线性缺陷和微小...     

Optimization of pulsed thermography inspection by partial least-squares regression

This paper introduces and tests a statistical correlation method for the optimization of the pulsed thermography inspection. The method is based on partial least squares regression, which decomposes the thermographic PT data sequence obtained during the cooling regime into a set of latent variables. The regression method is applied to experimental PT data from a carbon fiber-reinforced composite with simulated defects. The performance of the regression technique is evaluated in terms of the signal-to-noise ratio. The results showed an increase in the SNRs for 96% of the defects after processing the original sequence with PLSR.     

Characterization of defects of pulsed thermography inspections by orthogonal polynomial decomposition

Methods in infrared nondestructive testing such as normalized thermal contrast (NTC) and thermal signal reconstruction (TSR) are based on a pixel-by-pixel time domain analysis that ignores spatial correlation. Other techniques in the frequency domain such as pulsed phase thermography (PPT) present the same disadvantage. In this paper, we propose a method to evaluate defects in composite specimens based on image decomposition into a 2D orthogonal space. We compare NTC, TSR, PPT, and principal component thermography (PCT) with the new approach by inspecting three samples of composite anisotropic materials. Without defining a sound area our method estimates the depths of defects up to 1.2 mm in carbon and glass reinforced plastic specimens. An implementation of the proposed method in Matlab is available at https://github.com/charlielito/2DOrthogonal_Polynomial_Decomposition.     

基于视频定位的焊缝缺陷超声检测技术

研究了一种基于USB摄像头定位的焊缝缺陷手动超声检测系统,通过USB摄像头实时获得超声波探头相对于焊缝的平面二维位置,结合探头传感的焊缝缺陷深度信息,以三维投影成像方法表征缺陷三维信息,进而直观的对焊缝缺陷进行定位定性定量综合分析诊断,适用于在役焊接结构的现场检测.利用该系统对裂纹实际焊接缺陷进行了超声波无损检测.结果表明,采用该方法可以方便快捷,且较直观的给出焊缝缺陷的位置、大小、分布等特征,有助于对不同类焊接缺陷的定性识别.     

Surface roughness evaluation of marble by 3D laser profilometry and pulsed thermography

Three dimensional (3D) laser profilometry (LP) and pulsed thermography (PT) were used in the inspection of Pentelic marble. Quarry Pentelic marble samples, after processed with different roughness treatments (i.e. 60, 80, 100, 220, 400 and 600 mesh), were evaluated in the laboratory. Furthermore, different surface cleaning treatments were applied to a Pentelic marble surface in situ and then representative samples were collected and evaluated in the laboratory by the means of these two non-destructive techniques. Quantitative analysis of all samples was performed. The surface roughness parameter Rq at a specific length scale was estimated by the use of the LP scanning approach. Furthermore, 3D micro-topography plots from the laser scans were attained. PT, through the formation of temperature—time plots that display the intensity of pixels as a function of time on the obtained thermal images, was able to distinguish the influence of the applied roughness treatments. Results indicate that these two non-contact and non-destructive techniques can be used for the assessment of surface roughness.     

Defect characterization in pulsed thermography: a statistical method compared with Kohonen and Perceptron neural networks

Pulsed thermography is a popular NDE technique. In this paper, a novel statistical processing method is proposed and compared in term of defect detection and characterization with two common neural network architectures (Perceptron and Kohonen). Interest is on characterization of aluminum corrosion. The statistical method and neural network architectures use temperature, phase and amplitude data with phase and amplitude data coming from the so-called pulsed phase thermography approach. The statistical method reveals interesting performance over tested neural networks, especially in the ’interference technique,’ a combined ’two-step’ approach: detection with phase and characterization with amplitude. Theory is discussed and examples of results are presented.     

Comparison between pulsed and modulated thermography in glass–epoxy laminates

Infrared (IR) thermography is a two-dimensional, non-contact technique which can be usefully employed in non-destructive evaluation of materials. Basically, two different approaches are possible: traditional pulse thermography (PT) and modulated (or lock-in) thermography (MT). The attention of the present work is focused on the peculiar aspects, which characterise the two different techniques. Tests are carried out by considering glass–epoxy specimens and the results obtained, by employing either PT or MT, are compared. The capability of each technique, to detect a defect and give precise information about size, depth and thermal resistance of the defect, is analysed. The advantages and disadvantages of using these techniques are discussed in order to assess the fundamental requirements for the most appropriate choice in quality control processes.     

Investigation into eddy current pulsed thermography for rolling contact fatigue detection and characterization

This paper reports on the use of eddy current pulsed thermography (ECPT) for detection and characterization of rolling contact fatigue (RCF). Detection mechanisms with eddy currents and heat propagation effects were discussed with RCF modeled as a simple angled defect. Two different angled defects were studied through numerical simulations and experimentally by using uniform magnetic field (UMF) excited by Helmholtz coils. Finally, a rail sample with RCF defects was inspected using UMF excitation. It is shown that ECPT with UMF excitation provides an efficient and robust method to detect angled defects, compared with nonuniform magnetic field (NUMF) excitation.     

Local-stress-induced thermal conductivity anisotropy analysis using non-destructive photo-thermo-mechanical lock-in thermography (PTM-LIT) imaging

When tensile loading is exerted on a sample, internal stress will be generated to counterbalance the external force. The accumulation of local stress results in the change of thermal properties and the sample becomes locally anisotropic, wherein lies the significance of correlation between mechanical properties and thermal parameters. In this research, anisotropic thermal conductivity was considered as the direct result of tensile loading and was interpreted by a theoretical three-dimensional anisotropic diffusion-wave model. Photo-thermo-mechanical lock-in thermography (PTM-LIT) was introduced and used with a focused laser beam and a mid-infrared camera viewing an aluminum alloy sample fixed on a dynamic home-made tensile rig. A numerical two-dimensional Fourier transform was used to compute the thermal-wave field and the effects of several important factors were investigated. Both theoretical and experimental images were analyzed with an isothermal conductivity anisotropy contour fitting approach. It was demonstrated that PTM-LIT can qualitatively and quantitatively reveal otherwise hard-to- measure mechanical property behavior of materials subjected to tensile loading from the stress-free state to its ultimate level of mechanical strength before fracture.     

Investigation on eddy current pulsed thermography to detect hidden cracks on corroded metal surface

This paper explores the feasibility of eddy current pulsed thermography (ECPT) to detect hidden cracks on corroded metal surface without removing the corrosion layer. The detection mechanisms are analyzed by using electromagnetic and heat conduction theories. Experiments are conducted on a metallic bar with three hidden cracks and the validity of ECPT is verified with the analysis of IR images and thermal responses. In order to further improve the detection sensitivity of ECPT, the principal component analysis (PCA) is applied to enhance the features of hidden cracks in the raw IR images by eliminating the effects of uneven corrosion and non-uniform heating. It is found that ECPT combined with PCA provides a convenient and effective way to detect hidden cracks on corroded metal surface.     

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.     

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.     

Electro-thermal modelling and analysis for estimation of defect parameters by stepped infrared thermography

This paper exploits a resistance–capacitance (RC) model of transient heat conduction for estimation of defect depth, and more significantly of the defect area, from single point surface temperature evolutions over the defect and non-defect regions of material subjected to step heating. For the initial time behaviour of the surface temperature, a distributed RC model has been presented and analysed for estimation of defect depth and area. In contrast, a lumped RC model forms the basis for obtaining an analytical expression for defect area from the later part of the surface temperature evolution. The derived analytical expression estimates the defect area from the defect depth, the saturation value of the absolute thermal contrast and the slopes of the initial and final surface temperature evolution. The presented method does not require knowledge of material thermal properties or the value of the incident heat flux for estimation of defect parameters. The method has been validated by estimating the parameters of defects of various sizes and at different depths from experimental and simulated surface temperature evolutions.     

Ultrasonic analysis and lock-in thermography for debonding evaluation of composite adhesive joints

Glass-fiber reinforced thermosetting plastic adhesive joints were characterized through ultrasonic imaging and lock-in thermographic analysis for assessing the adhesion quality before being subjected to static tensile mechanical tests and to accelerated aging cycles.The mapping of each sample has been obtained. Visual testing were performed on all specimens after the mechanical tests in order to obtain a comparison with ultrasonic and lock-in thermography technique.A quantitative analysis has been carried out to evaluate the ability of lock-in thermography in investigating inadequate bonding and obtaining the validation of the technique by the consistency of the results with the well-established ultrasonic testing.     

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.     

Pulsed phase thermography imaging of fatigue-loaded composite adhesively bonded joints

We applied pulsed phase thermography to image and size damage in adhesively bonded joints. Specifically, the initiation and propagation of fatigue-induced damage in single lap joints with carbon fiber epoxy adherends was investigated. Lap joint specimens with various levels of manufacturing defects were fabricated and loaded in low-cycle fatigue. A calibration specimen with artificial defects was used to design a threshold algorithm for sizing of the damaged regions. The dominant failure mode in specimens without manufacturing defects was fiber-failure, whereas joints failing prematurely demonstrated adhesive failure. Imaging of the lap joints after regular number of fatigue cycles revealed that manufacturing defects could be detected and the resulting, imminent adhesive failure could be identified prior to joint failure. Additionally, the extent of this damage could be accurately estimated through the sizing algorithm. Due to the brittle nature of fiber-failure, it could not be detected prior to failure of the joint, however this was not critical, as the goal was to identify premature failure of the adhesively bonded joint.