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.     

Quantitative determination of a subsurface defect of reference specimen by lock-in infrared thermography

This paper describes the quantitative determination of the sizes and locations of subsurface defects using lock-in infrared thermography. A phase (or temperature) difference between the defect area and the healthy area indicates the qualitative location and size of the defect. To accurately estimate these parameters, the shearing-phase technique has been employed, where the inspected image is shifted by a certain number of pixels to obtain a shifted image while subtraction of one image from the other gives the shearing-phase distribution. The shearing-phase distribution has maximum, minimum, and zero points that help determine quantitatively the size and location of the subsurface defect. Experimental results for a steel plate with artificial subsurface defects show good agreement with actual values.     

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.     

锁相热成像无损检测方法的基础实验研究

红外热成像技术是一种新型的无损检测技术,能够成功应用于各种类型材料缺陷检测,但随着无损检测要求的不断提高,特别是对于新型航空航天材料的缺陷检测,如微小尺度缺陷和疲劳损伤等,因此需要加强各种无损检测技术在检测微弱信号方面的能力.锁相热成像技术是一种高灵敏度的热成像技术,在此主要探讨了锁相热成像技术的原理、实现方法以及实验中注意的问题.     

Remote inspection of surface cracks in metallic structures with fiber-guided laser array spots thermography

In this work, a fiber-guided laser array spots thermography (LAST) system with good flexibility is proposed for the inspection of surface cracks in metallic structural objects with difficult access. To generate an array of laser spots on the target surface, an optical head with fiber delivery is designed and fabricated at first, which has good adaptability and robustness. Second, a numerical simulation model is developed to optimize the spatial distribution of the laser spots. In addition, an improved image processing method with use of multiple background free images is developed to enhance the crack detectability from the LAST images. Finally, cracks of different types in stainless steel specimens are successfully detected from the thermal images obtained using the fiber-guided LAST system.     

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.     

铂的反演势的构建与研究

通过第一性原理计算得到了铂的晶格内聚曲线,而后利用陈式晶格反演方法,得到了反演势曲线。利用提出的双指数函数,拟合得到了精确的铂原子的反演势函数。利用反演势数据、EAM(嵌入式原子法)势理论和第一原理分别计算声子谱,验证了反演势的可靠性。提出了将玻尔兹曼统计方程与晶格粘聚能曲线精确拟合相结合的热膨胀系数计算方法。此外,还计算了室温下的体积模量和格鲁内森常数。结果与实验结果吻合较好,说明计算得到的铂的反演势是有效的、准确的。     

Automated detection and quantification of hidden voids in triplex bonding layers using active lock-in thermography

This paper presents an automated hidden void detection and quantification technique for inspecting triplex bonding layers in liquefied natural gas (LNG) carriers using active lock-in thermography. Hidden voids are first detected and visualized by an amplitude image and a series of binary image processing. Then, the sizes of the detected voids are quantified using an empirical mapping function, relating the detected void sizes to the void sizes obtained by an independent X-ray testing. The performance of the proposed technique is blind tested using two triplex specimens. The experimental results reveal that the hidden voids can be successfully detected and quantified.     

铜-钢反向凝固复合层厚度变化的数值模拟

根据铜—钢反向凝固复合试验的实际情况，建立了温度场的计算模型，应用ANSYS软件模拟了钢带厚度和预热温度、浸渍复合时间以及钢液温度对钢凝固复合层厚度变化的影响。     

铝合金电阻点焊熔核截面反演成像

将反演理论及正则化方法用于熔核横截面电流反演成像,以反演图像直观判断焊点质量,扩大铝合金电阻点焊在对焊点质量要求较高领域中的应用.首先,建立熔核截面电流分布和周围磁感应强度关系的正演模型,并通过试验验证模型的可靠性;其次,建立反演模型,通过数值试验验证反演模型的可行性;最后,点焊成像试验,将测量的磁场信号反演为熔核截面的电流分布信息,并以图像形式显示.结果表明,反演图像反映了熔核截面电流分布特点,能够反应熔核质量的好坏,可以作为判断熔核质量的依据;正则化参数的选择影响反演图像的质量,过大或过小,反演图像都将偏离实际情况.     

弧焊电源用全桥逆变电路的偏磁控制及仿真

大功率弧焊逆变器主电路的理想结构是全桥式逆变电路,但其突出问题是变压器偏磁引起的磁饱和及逆变颠覆。作者通过对全桥逆变电路变压器磁饱和原理的研究,提出了采用双环反馈控制法解决该问题的方案。经过电路设计、仿真和波形分析,从实验的角度证明了该方案的可行性和有效性。     

微观相场法反演L12结构的原子间相互作用势

基于微观相场理论,根据Khachaturyan的占位几率和能量关系方程,推导出L12结构的第一近邻原子间相互作用势与长程序参数关系的反演方程-4W1·η/kBT=1n(1-η)[1-c(1+3·η)]/(1+3·η)[1-c(1-η)],根据该方程,只需输入根据相图中L12结构相变点的温度和原子浓度,就可以计算出不同温度和原子浓度下的L12结构的第1近邻原子间相互作用势,计算结果与第1性原理及其他方法计算的值接近;同时在计算过程中发现了L12结构的第1近邻原子间相互作用势在析出L12相的相图范围内随温度和原子浓度变化.     

On the suitability of induction coils for crack detection and sizing in metals by the surface magnetic field measurement technique

The surface magnetic field measurement (SMFM) technique has proved to be an accurate means for crack detection and sizing cracks in ferrous metals. The technique involves the use of two U-shaped current-carrying wires of sufficiently high frequency while measuring the discontinuity in the resultant magnetic field at the crack edge with an appropriate magnetic field sensor. In this work, we describe a mathematical algorithm to obtain the crack signal from the output of an induction coil used in a SMFM probe. We also discuss the measurement errors due to the coil size and shape. To reduce the measurement errors, we present an algorithm in which the crack signal is recovered by appropriate deconvolution of the coil output signal and its spatial transfer function. The algorithm is then used to recover crack signals for various coil shapes and sizes. The study of the results demonstrates the effectiveness of the algorithm in the case of large coils.     

Diagnosis of the leakage point on a structure surface using infrared thermography in near ambient conditions

It is important to check for leakage points of fluids on a surface to maintain and manage various structures satisfactorily. In particular, it is necessary that the leaking walls of a bank, dam, tunnel, pipeline and so on are diagnosed appropriately using some remote sensing technique and that they should be repaired immediately. Recently, infrared thermography for measuring temperature has become attractive in a wide variety of engineering applications. Infrared thermography in near ambient conditions is introduced to check the leakage point and its applicability is estimated by visualizing the temperature field generated around the point. The generated temperature around the leakage point is not usually consistent with the surrounding temperature. When using infrared thermography, we can detect the leakage point at the location where a marked temperature difference occurs.     

微观相场法反演Ni0.75AlxV0.25-x合金的原子间相互作用势

采用微观相场法,利用Khachaturyan所给原子间相互作用势与长程序参数关系方程,计算出Ni_(0.75)Al_xV_(0.25-x)合金L1_0、L1_2和DO_(22)相第一近邻原子间相互作用势,并用计算的原子间相互作用势模拟了Ni_(0.75)Al_xV_(0.25-x)合金沉淀过程以及最终形貌。计算结果表明,L1_0、L1_2和DO_(22)相第一近邻原子间相互作用势随温度增大而增大,随浓度增大而增大,且计算得到的随温度和浓度变化的原子间相互作用势与之前的实验值符合较好。计算的原子间相互作用势的模拟结果能依次得到预析出相L1_0、稳定相L1_2和第二相DO_(22),且合金沉淀形貌与实验结果吻合。相场法反演原子间相互作用势,拓宽了相场法在合金设计中的应用范围。     

Wavelet analysis based deconvolution to improve the resolution of scanning acoustic microscope images for the inspection of thin die layer in semiconductor

Scanning acoustic microscopy (SAM) is used as an important non-destructive test tool in semiconductor reliability evaluation and failure analysis. However, inspection of thin die layer has proven difficult as the reflected signals from the die top and bottom are superimposed. Conventional deconvolution techniques have been used for the improvement of time resolution in A-scan signal, however, they are not effective for SAM signal because the waveform of the reflected echo is quite different from the incident waveform due to the frequency dependent attenuation and the focal effect. In this paper, in order to overcome this difficulty, a new signal processing method, wavelet analysis based deconvolution technique is proposed. Its validity is approved by computer simulations and practical performances are demonstrated by experiments for the fabricated semiconductor sample.     

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.     

Possible role of two iron sites in the metamagnetic transition observed in LaFe12 B6

Fixed spin moment calculations within TB-LMTO-ASA method using LSDA were performed for the compound LaFe₁₂ B₆ and for lattice constants taken from experimental data. Minima corresponding to the observed low moment and high moment states were found. However, their relative stability is not in agreement with experimental data. Fixed spin moment calculations on model compounds where Fe atoms at either crystallographic sites were replaced by Cu atoms were also performed. It was found that the stability of magnetic moment of Fe atoms at 18(h) site is much more dependent on environment than the stability of magnetic moment of Fe atoms at the 18(g) site.     

光笔式视觉测量中被测特征点像面坐标提取方法的研究

针对光笔式视觉测量中被测特征点像面坐标的提取问题进行了研究。依据数字图像处理理论并结合本系统中被测特征点的成像特点，给出了图像处理过程及实际处理结果；针对椭圆形光斑中心点像面坐标的获取提出了椭圆曲线拟合法和灰度质心法，分析结果表明，灰度质心法应用于本测量系统中是可行性的。     

IR thermography study of the effect of moulding parameters on impact resistance in E-glass/polypropylene commingled laminates

Infrared thermography has often been used to characterise impact damage on polymeric composite materials. In this study, the impact properties of commingled E-glass polypropylene composites (Twintex®) moulded using different tool temperatures (60, 80 and 100 °C) and different preheating time are compared. With this aim the material, a balanced weave laminate (1:1 Twintex) with 60% wt glass content, compression moulded at different tool temperatures and using different preheating time, is impacted with impact energies ranging from 15 to 55 J. After that, the impact-damaged areas on the laminates are observed and characterised using a long pulse active thermography approach. The positive effect of an increasing tool temperature and of an increased preheating time on impact damage tolerance is clear from the analysis of thermographic images.