Limits of Spatial Resolution for Thermography and Other Non-destructive Imaging Methods Based on Diffusion Waves

In this work the measured variable, such as temperature, is a random variable showing fluctuations. The loss of information caused by diffusion waves in non-destructive testing can be described by stochastic processes. In non-destructive imaging, the information about the spatial pattern of a samples interior has to be transferred to the sample surface by certain waves, e.g., thermal waves. At the sample surface these waves can be detected and the interior structure is reconstructed from the measured signals. The amount of information about the interior of the sample, which can be gained from the detected waves on the sample surface, is essentially influenced by the propagation from its excitation to the surface. Diffusion causes entropy production and information loss for the propagating waves. Mandelis has developed a unifying framework for treating diverse diffusion-related periodic phenomena under the global mathematical label of diffusion-wave fields, such as thermal waves. Thermography uses the time-dependent diffusion of heat (either pulsed or modulated periodically) which goes along with entropy production and a loss of information. Several attempts have been made to compensate for this diffusive effect to get a higher resolution for the reconstructed images of the samples interior. In this work it is shown that fluctuations limit this compensation. Therefore, the spatial resolution for non-destructive imaging at a certain depth is also limited by theory.     

Non-destructive Testing by Infrared Thermography Under Random Excitation and ARMA Analysis

Photothermal thermography is a non-destructive testing (NDT) method, which has many applications in the field of control and characterization of thin materials. This technique is usually implemented under CW or flash excitation. Such excitations are not adapted for control of fragile materials or for multi-frequency analysis. To allow these analyses, in this article, the use of a new control mode is proposed: infrared thermography under random excitation and auto regressive moving average analysis. First, the principle of this NDT method is presented. Then, the method is shown to permit detection, with low energy constraints, of detachments situated in mural paintings.     

Non-destructive Imaging of Standard Cracks of Railway by Photoacoustic Piezoelectric Technology

The photoacoustic piezoelectric (PAPE) technique is an effective non-destructive testing technique for detecting defects in materials. In this paper, Chinese national standard railway cracks have been detected by thermal wave imaging based on the PAPE technique. First, the theory of the PAPE technique has been introduced and the corresponding imaging principle has been analyzed. Second, the corresponding experimental system has been setup, and the imaging tests have been carried out. Third, two kinds of standard cracks have been examined by the imaging system. The results show that thermal wave imaging based on the PAPE technique can effectively image and identify the cracks at different depths, which lays a foundation for practical application to the detection of rail cracks.     

An Investigation of the Bias Caused by Surface Coatings on Material Loss Evaluation by Quantitative Thermography

The effect of a surface coating on the response of a metallic substrate to pulsed thermal excitation is examined with the view to assessing its impact on the efficacy of quantitative thermographic evaluation as applied to the problem of material loss evaluation. An analytical model describing the response of a layered structure to a surface thermal excitation has been developed and its predictions are shown to be in excellent qualitative agreement with experimental observations. The results indicate that for materials with high thermal diffusivity, a surface paint layer bears important influence on the development of the surface thermal response and should be taken into account when undertaking quantitative assessments of thermographic data.     

基于表面超声波无损评价激光熔覆层厚度的实验研究

针对目前激光熔覆层厚度无损评价的重要性,本工作结合表面超声波在介质中的传播特性对其进行测量,并在互相关函数基础上对熔覆层厚度进行了表征和评价。结果表明:随着激光熔覆层厚度的增加,表面超声波在其中的传播速率逐渐变小,在互相关函数计算微小时间差的基础上,建立了可用于评价激光熔覆层厚度的厚度-时间差关系曲线,并采用三次多项式对其进行拟合,进而得到可用于评价激光熔覆层厚度的拟合公式。最后采用不同厚度的激光熔覆层试样对拟合结果进行了验证,并同时对影响计算结果的因素进行了分析。结果表明:采用该方法对激光熔覆层厚度进行评价是可行的。     

海面波合成孔径雷达成像仿真研究

目的：研究合成孔径雷达（SAR）对海面波成像的机理和海面波对雷波截面的调制机理。方法；选用速度聚束模型，采用不同参数仿真SAR对海面波成像的情况。结果：由于海面波的运动，使得SAR对海面波成像过程成为非线性，使SAR图像在方位向发生移位和模糊。结论：用这种仿真方法研究SAR对海面波成像是一种有效途径。     

海面波合成孔径雷达成象仿真研究

目的研究合成孔径雷达(SAR)对海面波成象的机理和海面波对雷达截面的调制机理. 方法选用速度聚束模型, 采用不同参数仿真 SAR 对海面波成象的情况. 结果由于海面波的运动, 使得 SAR 对海面波成象过程成为非线性,使 SAR 图象在方位向发生移位和模糊. 结论用这种仿真方法研究 SAR 对海面波成象是一种有效途径.     

Discovering the Defects in Paintings Using Non-destructive Testing (NDT) Techniques and Passing Through Measurements of Deformation

The present study is focused on two topics. The former is a mathematical model useful to understand the deformation of paintings, which uses straining devices, adjustable and micrometrically controlled through a pin implanted in a hollow cylinder. Strains were analyzed by holographic interferometry (HI) technique using an appropriate frame. The latter concerns the need to improve the conservator’s knowledge about the defect’s detection and defect’s propagation in acrylic painting characterized by underdrawings and pentimenti. To accomplish this task, a sample was manufactured to clarify the several uncertainties inherent the influence of external factors on their conservation. Subsurface anomalies were also retrieved by near-infrared reflectography and transmittography techniques, using LED lamps and several narrow-band filters mounted on a CMOS camera, working at different wavelengths and in combination with UV imaging. In addition, a sponge glued on the rear side of the canvas was impregnated with a precise amount of water by means of a syringe to verify the stretcher effect by digital speckle photography (DSP) technique (using MatPIV). The same effect also affects the sharp transition of the canvas at the stretcher’s edge. In this case, the direct mechanical contact between stretcher and canvas was investigated by HI technique. Finally, advanced algorithms were successfully applied to the square pulse thermography data to detect three Mylar \(^{\textregistered }\) inserts simulating different types of defects. These fabricated defects were also identified by optical techniques: DSP and laser speckle imaging.     

Experimental Determination of the Thermal Parameters of Carbon Fiber-Composite Materials Exposed to Fire by Infrared Imaging Pulse Thermography

A new procedure adapted from the classical one performed by Parker et al. has been developed to extend it to insulating plates (carbon fiber-reinforced composites). The measurement system consists of an infrared camera synchronized with a flash lamp. This method has been implemented to obtain the thermal parameters of the samples in different states of degradation by fire. The method is based on experimental-theory adjustment to obtain the intrinsic thermal parameters: thermal diffusivity, thermal conductivity, and volumetric heat capacity. In addition, the method has required development of a theoretical model accounting for the cooling losses significant for insulating plates. The results have been validated by comparison between the experimental data and those provided by a heat transfer model.     

超声波清洗噪声的研究

通过对超声波清洗中空化现象的研究,分析了空化噪声产生的原因和影响因素,提出一些减小空化噪声对环境影响的方法.     

爆轰波反应区结构的数字化学发光成像研究

利用数字化学发光成像系统对氢氧系统气相爆轰反应区结构进行直接实验观察. 实验采用 ICCD(Intensified Charge-Coupled Device)、 DG5335 延时器和中心波长 312 nm、半带宽 15 nm 的滤光片,精确设置爆轰波、 ICCD 之间的同步控制,得到不同初压、不同氩稀释度的爆轰 OH 自发辐射光图像. 结果表明: 反应区在空间上不均匀,其阵面形状不是平面且随时间不断改变. 马赫杆后的 OH 自发辐射光强较入射激波后大. 随着初压升高,OH 自发辐射光强增大,反应阵面也近似趋于平面. 随着氩稀释度的增大,OH 自发辐射光强明显衰减.     

Experimental Investigation on Flash Evaporation of Saltwater Droplets Released into Vacuum

In this paper, the flash evaporation process of saltwater droplets released into vacuum is experimentally investigated. During the experiment, a saltwater (NaCl) droplet was suspended on a thermocouple junction, which was used to measure the temperature evolution. The droplet surface temperature was captured by an infrared thermal imager, and the shape variation was recorded by a high speed camera. According to the experimental results, the component and solution concentration has great influence on the evaporation process. With a rise of salt concentration in water, the evaporation rate decreases. The shape of temperature transition curve also depends on the salt concentration in solution, no matter whether it is higher or lower than the eutectic point (22.4%). The effects of environmental pressure, initial droplet temperature and initial droplet diameter on the temperature transition of droplets were also summarized based on the experimental data.     

Non-destructive Flaw Mapping of Steel Surfaces by the Continuous Magnetic Barkhausen Noise Method: Detection of Plastic Deformation

This paper reports the use of a non-destructive scanning technique to identify plastic deformation defects generated in steel. The technique is based on measurement of continuous magnetic Barkhausen noise (CMBN). In the experiments described here, surfaces with plastic deformations produced by crushing stresses in a 1070 steel are scanned, and the influence of probe configuration, coil type, scanner speed, applied magnetic field and the frequency band used for the analysis on the effectiveness of the technique is studied. A moving smoothing window based on a second order statistical moment is used to analyze the time signal. The results show that the method can detect the position of plastic deformation defects and distinguish between their amplitudes.     

乳化炸药水中爆炸冲击波传播规律试验研究

在水下工程爆破中,通常采用裸露药包爆破法,进行水下金属构件的切割与拆除、水下可疑爆炸物的诱爆等,当爆破区域附近有养殖区、野生保护水生物时,就需考虑炸药爆炸产生的水中冲击波影响问题.对水中冲击波传播规律的公式形式进行了探讨,通过爆破试验以及对监测资料的分析,得到了高能、普通乳化炸药的水中爆炸冲击波传播规律.     

浅析医用超声源计量检定中基波与谐波对成像的影响

论述基波与谐波成像对医用超声源计量检定的影响.     

利用泄漏瑞利波成像检测陶瓷表面缺陷

介绍了利用泄漏瑞利波成像检测陶瓷表面缺陷的方法，并通过实例讨论了采用高频聚集探头激励泄漏瑞利波的特性以及泄漏瑞利波成像的规律。     

管道内水雾对冲击波衰减作用的实验研究

采用直径200 mm立式激波管,进行了管道内水雾对冲击波衰减作用的实验研究.实验结果表明,冲击波衰减率与水雾密度、雾滴直径和雾区厚度有关.相同雾滴直径时,水雾密度越大,冲击波的衰减率越大相同水雾密度时,水雾的雾滴直径越小,冲击波的衰减率越大.     

纳米复合薄膜吸波材料的研究进展

介绍纳米吸波剂的吸波剂机理,综述其研究现状,并总结各种薄膜吸波剂的优、缺点.最后对制备强、宽、轻、薄的纳米复合吸波剂进行展望.     

Design and Characterisation of Titanium Nitride Subarrays of Kinetic Inductance Detectors for Passive Terahertz Imaging

We report on the investigation of titanium nitride (TiN) thin films deposited via atomic layer deposition (ALD) for microwave kinetic inductance detectors (MKID). Using our in-house ALD process, we have grown a sequence of TiN thin films (thickness 15, 30, 60 nm). The films have been characterised in terms of superconducting transition temperature \(T_\mathrm{c}\), sheet resistance \(R_\mathrm{s}\) and microstructure. We have fabricated test resonator structures and characterised them at a temperature of 300 mK. At 350 GHz, we report an optical noise equivalent power \(\hbox {NEP}_\mathrm{opt} \approx 2.3\times 10^{-15}~\hbox {W}/\sqrt{\hbox {Hz}}\), which is promising for passive terahertz imaging applications.     

Imaging of Barely Visible Impact Damage on a Complex Composite Stiffened Panel Using a Nonlinear Ultrasound Stimulated Thermography Approach

Thermosonics, also known as ultrasonic stimulated thermography, is a rapid non-destructive evaluation technique that uses an infrared camera to visualise material defects by detecting the frictional heating at crack surfaces when a part under inspection is vibrated. These vibrations are usually produced by an ultrasonic horn being pressed against the surface of the test sample, which result in uncontrolled generations of frequency components and excitation amplitude. This makes thermosonics highly non-reproducible and unreliable. This paper presents a novel thermographic method, here named as nonlinear ultrasound stimulated thermography, for the detection and imaging of real material defects such as impact damage on a complex composite stiffener panel. This technique combines nonlinear ultrasonic techniques with thermography. A nonlinear ultrasonic approach was used as signature for a reliable frequency-selective excitation of material defects, while an infrared camera was employed to reveal the damage location and severity. A nonlinear narrow sweep excitation method was employed to efficiently excite the local resonance frequencies of the damaged region in order to give rise to the highest nonlinear harmonic response in the material leading to a high heat generation at the crack surface. The experimental tests were carried out with a laser vibrometer in order to better understand the interaction of elastic waves with nonlinear scattering. An ad-hoc nonlinear thermal-structural finite element and crack model was developed to study the heat generation caused by the movement of the crack surfaces when elastic waves with a particular frequency impinges on the crack interphase with good agreement with the experimental results. The proposed new method allows to detect single and multiple barely visible impact damage in a quick, reliable and reproducible manner and overcomes the main limitations of classical thermosonics.