基于红外测温的试件内部缺陷的识别算法研究

内部缺陷形状位置的识别是传热逆问题的重要研究课题.本文对带有内部缺陷的试件建立了二维传热模型,通过有限元法求解了在一定加热条件下试件表面的温度分布;并通过共轭梯度法,提出了根据表面所测温度分布识别试件内部缺陷形状、位置的计算方法.数值实验证明了算法的有效性.     

Research on thermal wave processing of lock-in thermography based on analyzing image sequences for NDT

Lock-in thermography, an active IR thermography technique for NDT, is based on propagation and reflection of thermal waves which are launched from the surface into the inspected component by absorption of modulated radiation. In this paper, thermal wave image sequences were sampled by a Cedip JADE MWIR 550 FPA infrared camera. Thermal wave signal processing algorithms are investigated to obtain information on subsurface defects. The Fourier transform, four-point correlation, and digital lock-in correlation algorithms are applied to extract the amplitude and phase of thermal wave’s harmonic component. A novel method called the time constant method (TCM) is proposed to analyze subsurface defects by using lock-in thermography. The experimental results confirm the thermal wave signal processing algorithms’ efficiency on subsurface defect detection.     

二维不规则形状发热型缺陷的红外识别算法

对带有自身发热型缺陷的试件建立二维传热模型,利用有限元法进行求解,研究试件表面的温度分布规律,并根据共轭梯度法提出根据试件外表面红外测温,定量识别发热型内部缺陷边界轮廓的方法.数值实验证明了算法的有效性.对所研究缺陷类型,导热系数较小的试件识别精度较高;温度测量误差、测温点数目及缺陷轮廓初值假设对识别结果的影响可以忽略;检测表面最大温差越大,识别精度越高.     

基于反射式脉冲红外热成像法的定量测量方法研究

本文提出了一种利用反射式脉冲热成像法测量缺陷深度、热扩散系数或缺陷界面热波反射系数的方法. 首先,介绍了脉冲热成像法的基本原理以及定量测量算法.其次,利用304不锈钢制作了平底孔试件 并预埋了四种不同物质并进行了实验,给出在不同条件下对缺陷深度、热扩散系数或缺陷界面热波 反射系数测量的结果.实验结果显示实际测量值与其他方法测量值基本符合, 误差范围在±5%以内, 并讨论了影响测量精度的原因.     

亚表面双球孔洞的热波散射与温度分布

基于非Fourier热传导方程,采用波函数展开法,对含双球形孔洞缺陷的半无限体材料内部的热波散射与温度分布进行了研究,给出了材料内部任一点温度的解析解和表面温度分布的数值计算结果。分析了孔洞的几何参数和物理参数对金属材料表面温度分布的影响。结果表明:相对热扩散长度、入射波波数和埋藏深度对表面温度分布的影响比较大,当孔洞间距较大时,可以忽略孔洞之间的热波散射。     

Thermographic inspection of wind turbine rotor blade segment utilizing natural conditions as excitation source,Part II: The effect of climatic conditions on thermographic inspections – A long term outdoor experiment

The present study continues the work described in part I of this paper in evaluating a long-term-experiment, where a rotor blade segment of a wind turbine is exposed to the elements and thereby monitored with passive thermography. First, it is investigated whether subsurface features in rotor blades – mainly made of GFRP – can generally be detected with thermography from greater distances under favorable conditions. The suitability of the sun for acting as a heat source in applying active thermography has been tested in the previous study. In this study, the climatic influence on thermographic measurement is evaluated. It is demonstrated that there are favorable and unfavorable circumstances for imaging thermal contrasts which reflect inner structures and other subsurface features like potential defects. It turns out that solar radiation serves as a very effective heat source, but not at all times of day. Other environmental influences such as diurnal temperature variations also create temperature contrasts that permit conclusions on subsurface features. Particular scenarios are reconstructed with FEM-simulations in order to gain deeper insight into the driving mechanisms that produce the observed thermal contrasts. These investigations may help planning useful outdoor operations for inspecting rotor blades with thermography.     

Application of thermal wave imaging and phase shifting method for defect detection in Stainless steel

This paper presents an experimental arrangement for detection of artificial subsurface defects in a stainless steel sample by means of thermal wave imaging with lock-in thermography and consequently, the impact of excitation frequency on defect detectability. The experimental analysis was performed at several excitation frequencies to observe the sample beginning from 0.18 Hz all the way down to 0.01 Hz. The phase contrast between the defective and sound regions illustrates the qualitative and quantitative investigation of defects. The two, three, four and five-step phase shifting methods are investigated to obtain the information on defects. A contrast to noise ratio analysis was applied to each phase shifting method allowing the choice of the most appropriate one. Phase contrast with four-step phase shifting at an optimum frequency of 0.01 Hz provides excellent results. The inquiry with the effect of defect size and depth on phase contrast shows that phase contrast decreases with increase in defect depth and increases with the increase in defect size.     

含双圆柱亚表面缺陷板条材料的表面温度分布

基于非傅里叶热传导方程,采用复变函数法和镜像法,研究了含双圆柱亚表面缺陷板条材料热波散射的温度场,并给出了热波散射温度场的解析解。分析了入射波波数、热扩散长度、缺陷的埋藏深度以及板条材料的厚度等对板条表面温度分布的影响。温度波由调制光束在材料表面激发,缺陷表面的边界条件为绝热。该分析方法和数值结果可为工程材料结构的传热分析、热波成像和材料内部缺陷评估,以及热物理反问题研究提供参考。     

Experimental study of inspection on a metal plate with defect using ultrasound lock-in thermographic technique

The ultrasound lock-in thermographic technique was developed to detect interfacial defects and cracks in a metal plate. Discrete correlation method (DCM), Fourier transformation method (FTM), Short Time Fourier transformation (STF) and Discrete Wavelet Transformation (DWT) algorithms were used to extract the characteristic information of the thermal wave signal generated by ultrasonic wave modulated. It is found that STF and DWT algorithms are available for analyzing the thermal wave signal generated by ultrasonic wave modulated due to higher signal–noise ratio. Experiments were performed to investigate the effect on the amplitude contrast and phase angle contrast by the ultrasound transducer position, initial sonotrode action force and modulation frequency, respectively. Experimental results show that transducer position closed to the defect, higher initial sonotrode action force loaded and optimal modulation frequency selected are help to detect the defects of metal plate using ultrasound lock-in thermographic technique.     

Cooling analysis of welded materials for crack detection using infrared thermography

Infrared thermography offers a wide range of possibilities for the detection of flaws in welding, being the main difference among them the thermal excitation of the material. This paper analyzes the application of an inexpensive and versatile thermographic test to the detection of subsurface cracks in welding. The procedure begins with the thermal excitation of the material, following with the monitoring of the cooling process with IRT (InfraRed Thermography). The result is a sequence of frames that enables the extraction of thermal data, useful for the study of the cooling tendencies in the defect and the non-defect zone. Then, each image is subjected to a contour lines algorithm towards the definition of the morphology of the detected defects. This combination of data acquisition and processing allows the differentiation between two types of cracks: toe crack and subsuperficial crack, as defined in the quality standards.     

超声导波管道缺陷定位方法研究*

本文提出了一种基于多换能器的超声导波管道缺陷定位方法,利用两个环形换能器阵列分别接收与缺陷相关的反射信号和透射信号,实现管道缺陷轴向及周向定位。首先利用变分模态分解法对信号进行滤波,在滤除噪声的基础上对信号进行小波变换,获得缺陷反射的导波时频信息,根据反射波的渡越时间计算缺陷在管道轴向的位置。在缺陷的周向定位上,提出一种损伤指数迭代方法,根据透过缺陷后导波的损伤指数来计算缺陷的周向位置及周向长度。仿真和实验结果表明,本文所采用的方法能够较为准确的测量出管道上缺陷的位置及周向长度。     

基于小波变换的红外热波无损检测融合算法

在红外热波无损检测中获取的热像序列存在着背景噪声大、缺陷边缘模糊、对比度低等特点。为了提高由红外热像序列重构的数字图像的缺陷显示能力,以小波变换为热像处理工具,采用基于像素级和特征级的图像融合算法对热像序列进行了处理,并采用基于统计学的图像评估标准对处理效果作了定量评价。通过对铝合金试件的检测实验说明该方法可用于材料内部缺陷的红外热波无损检测。研究结果表明,此种图像融合算法可对不同深度缺陷所对应的两幅最佳热像进行有效地融合,在一幅融合图像中直观地反映出全部缺陷,并能有效地减少加热不均和背景噪声对缺陷识别的不利影响。     

Size determination of subsurface defect by active thermography – Simulation research

This paper presents the results of 3D numerical simulations of thermal phenomena in uniform material with subsurface defect located at some depth. The material is aimed to be tested by means of IR active thermography. The temperature field of the front surface of material tested is observed and analysed. The results of modelling are suitable for step heating as an external thermal stimulation – heat flux. The heating duration needed for accurately defect sizing is discussed. Also, an effect of two defects with different thermal properties and simultaneously occurring in the material tested is analysed.     

Filtered thermal contrast based technique for testing of material by infrared thermography

A new kind of thermal contrast, called “filtered contrast” is presented, which allows detecting and characterizing material defects using active thermography under some assumptions on physical and thermal parameters of materials. In opposition to known definitions of the thermal contrast, knowledge about defect-free area is not necessary and this contrast is less sensitive to nonuniformity of heat disposal to the material surface. The measurements were performed on an experimental setup equipped with a ThermaCAM PM 595 infrared camera and frame grabber. The step heating was chosen as heat excitation. The results demonstrate usefulness of the 1D model of heat transfer used for determination of depth of subsurface defects. The influence of the parameter of the smoothing filter, required for filtered contrast implementation, thermal parameters of the tested material and defect on expanded uncertainty of determination of defect depth is also presented. Due to significant complexity of the model of heat transfer, the conditions for the “law of propagation of uncertainty” were not fulfilled and a numerical method, i.e., Monte Carlo simulation is applied for the propagation of distributions.     

亚表面异质缺陷对功能梯度材料表面温度场的影响

基于双曲型热传导方程,采用镜像法和波函数展开法,求解了含亚表面异质圆柱缺陷的半无限功能梯度材料的表面温度场,给出了功能梯度材料中热波散射的一般解.分析了亚表面异质圆柱缺陷的几何参数(如埋藏深度)和热物理参数(如导热系数、热扩散长度、热扩散率及热弛豫时间等)对功能梯度材料表面温度场的影响.温度波由调制的超短脉冲激光在功能梯度材料表面激发,异质圆柱缺陷表面的边界条件为导热边界.研究结果可望为功能梯度材料的红外热波无损检测、导热反问题提供计算方法和参考数据.     

Experimental study of inspection on SiC coated high-temperature alloy plates with defects using pulsed thermographic technique

The pulsed thermographic technique was used to detect flat-bottomed hole defects in SiC coated high-temperature alloy plates. Temperature–time Logarithm Fitting Method (TtLFM), Principal Component Analysis (PCA) and Pulsed Phase Thermography (PPT) were used to extract the characteristic information of the thermal wave signal generated by thermal pulse. It is found that PCA algorithm is available for analyzing the thermal wave signal generated by thermal pulse due to higher signal–noise ratio. Experiments were performed to investigate the effect on the thermal contrast by the pulse intensity, frame rate and analysis moment, respectively. Experimental results show that larger pulse intensity and frame rate are benefit to get larger thermal contrast thermogram, and optimal analysis moment selected is helpful to detect the defects of SiC coated high-temperature alloy plates using pulsed thermographic technique.     

Depth profile reconstruction of the thermal conductivity of inhomogeneous solids: application to laser-hardened Al alloys

Photoacoustic measurement techniques can be used to determine thermal properties on and below the sample’s surface, thus subsurface thermal inhomogeneities, such as continuous profiles of thermal parameters, become measurable by photoacoustic methods. In this paper, the study is focused on the quantitative characterization of material modifications in subsurface layers of laser-hardened Al alloy samples. The variation of surface temperature is measured by PA technique. Then a new numerical algorithm, carried out by employing the pulsed spectrum technique and the regularization method, is used to reconstruct thermal conductivity depth profiles. The experiment results demonstrate that the experiment and the algorithm are very effective for microstructure depth profile reconstruction by nondestructive method. Received: 18 February 2000 / Accepted: 28 February 2000 / Published online: 30 June 2000     

Quantitative determination of subsurface defects in a reference specimen made of Plexiglas by means of lock-in and pulse phase infrared thermography

Lock-in and pulse phase infrared thermography measurement techniques have been exploited for quantitative assessment of subsurface defects in a reference specimen made of Plexiglas. Radiometric thermal images were post-processed using a contrast approach in the frequency domain, allowing defect depth to be resolved with a combined standard uncertainty of about 5% for thicknesses up to 3.6 mm. Conversely, significant radial heat diffusion next to the boundary of the discontinuities made accurate sizing of deeper subsurface defects more difficult, resulting in a combined standard uncertainty of about 17% for a 10 mm diameter flat-bottomed hole of 3.6 mm deep. The obtained results demonstrate the potentiality of active thermography as a fast, powerful contactless NDE measurement tool.     

利用矢量海洋环境噪声提取声场格林函数

考虑到矢量水听器在垂直方向上具有8字形指向性,能够有效抑制远方非平稳噪声源的干扰,提出了一种矢量环境噪声相关函数(NCF)提取声场时域格林函数(TDGF)的方法。基于简正波理论建立了声压和垂直振速垂直相关性模型。在此基础上,给出了声压和垂直振速相关函数提取声场纵向格林函数的过程.数值仿真对比和实验数据分析表明,相对于声压提取方法,垂直振速提取方法能够有效消除直达波前出现的亮纹与亮区干扰。此外,对于同等时间长度噪声序列,声压提取方法只提取到直达波路径,而垂直振速提取方法还提取到了我们更为关心的海底反射路径。利用直达波与海底反射波到达时延差估计的海深与实测海深吻合较好。      

A novel compression algorithm for infrared thermal image sequence based on K-means method

High resolution in space and time is becoming the new trend of thermographic inspection of equipments, therefore, the development of a fast and precise processing and data store technique of high resolution thermal image should be well studied. This article will propose a novel global compression algorithm, which will provide an effective way to improve the precision and processing speed of thermal image data. This new algorithm is based on the decay of the temperature of thermograph and the feature of thermal image morphology. Firstly, it will sort the data in space according to K-means method. Then it will employ classic fitting calculation to fit all the typical temperature decay curves. At last, it will use the fitting parameters of the curves as the parameters for compression and reconstruction of thermal image sequence to achieve the method for which the thermal image sequence can be compressed in space and time simultaneously. To validate the proposed new algorithm, the authors used two embedded defective specimens made of different materials to do the experiment. The results show that the proposed infrared thermal image sequence compression processing algorithm is an effective solution with high speed and high precision. Compared to the conventional method, the global compression algorithm is not only noise resistant but also can improve the computing speed in hundreds of times.