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.     

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.     

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。     

金属材料次表面缺陷成像检测系统及其应用

介绍了一种基于光热光偏转检测技术的原理,可以检测金属材料次表面缺陷空间分布的激光热波探测系统.该系统具有结构紧凑,调试简便,运行稳定,测试结果可靠的特点,它用半导体激光作泵浦光源和探测光源,采用电源调制的方式对泵浦光束进行调制,有频率稳定、噪音小的优点,同时系统可以用低压直流电源供电,为仪器化和便携化研究创造了条件.利用该系统对金属铝片次表面不同深度的凹槽作了检测,得到了与实际样品一致的检测结果.     

亚表面圆柱体对热波的多重散射问题

极端条件下的传热问题是工程热物理研究中的重要课题。基于非傅里叶热传导定律,采用波函数展开法,研究了含圆柱缺陷非透明体中热波散射问题。基于热传导波动模型给出了物体中热波多重散射问题的一般解。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。具体分析了几何参量、各物理参量对温度分布的影响,特别分析了热波波长对温度变化的作用,并给出了温度变化的数值结果。研究结果可望为红外辐射技术、热波成像等问题的分析提供理论基础和参考数据。在检测金属材料缺陷空间分布的激光热波探测系统中应用。     

Nonlinear modulation technique for NDE with air-coupled ultrasound

The present study is aimed at expanding flexibility and application area of nonlinear acoustic modulation (NAM-) technique by combining the benefits of noncontact ultrasound excitation (remote locating and imaging of defects) with sensitivity of nonlinear methods in a new air-coupled NAM-version. A pair of focused air-coupled transducers was used to generate and receive (high-frequency) longitudinal or flexural waves in plate-like samples. Low-frequency (LF-) vibrations were excited with a shaker or a loudspeaker. Temporal and spectral analysis of the output signal revealed an extremely efficient nonlinear amplitude modulation and multiple frequency side-bands for sound transmission and flexural wave propagation through cracked defects. On the contrary, a negligible modulation was observed for large and medium scale inclusions and material inhomogeneities (linear defects). A new subharmonic mode of the NAM was observed at high excitation levels. It was also shown for the first time that nonlinear vibrations of cracks resulted in radiation of a very high-order harmonics (well above 100) of the driving excitation in air that enabled imaging of cracks remotely by registration their highly nonlinear "acoustic emission" with air-coupled transducers.     

Frequency modulated infrared imaging for thermal characterization of nanomaterials

The paper presents frequency modulated thermal wave imaging (FMTWI) as a fast and efficient non-contact technique for in-plane thermal characterization of thin plate nanomaterials. A novel excitation signal in the form of an up-chirp is applied and the thermal response is monitored using an infrared (IR) thermography based temperature sensing system. The in-plane thermal diffusivity of any sample can be measured using the multiple phase information extracted from a single run of the experiment. This feature provides a time efficient approach for thermal measurements using infrared thermography techniques. The theoretical background and experimental details of the technique are discussed, with practical measurement of thermal diffusivity of an empty anodic alumina (AAO) template in direction perpendicular to the nanochannel axis, in support.     

一种计算分层半空间上方导线瞬态响应的新方法

提出了一种将时域积分方程(time domain integral equation, TDIE)方法和时域有限差分(finite differnce time domain, FDTD)方法相结合计算分层有耗半空间上方导线瞬态电磁响应的新方法.其中,一维FDTD方法用于计算入射电磁波经分层半空间反射的时域波形.TDIE用于求解细导线在加入两个激励源(直接入射电磁波和经分层半空间反射的电磁波)时的瞬态响应.相关计算理论和数值模拟结果说明了本文方法是一种解决了分层有耗介质上方水平放置导线瞬态响应的高效解决方案. 关键词： 时域积分方程 时域有限差分 细导线 分层半空间     

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.     

太赫兹激励的红外热波检测技术

本文的目的在于探索一种新的适用于红外热波检测技术的热激励方式——太赫兹(THz)热激励. 文中介绍了THz波周期性热激励的热传导理论模型; 尝试利用返波振荡器(返波管backward wave oscillator, BWO)太赫兹源对一块碳纤维基底吸波涂层板进行周期性THz热激励, 红外热像仪连续观测和记录试件表面温场变化, Canny边缘算法处理热图像显示缺陷; 检测结果与闪光灯脉冲激励的结果进行比较, 讨论了太赫兹波激励红外热波检测技术可能的优势. 实现了THz技术与红外热波无损检测技术的结合.     

Lock-in thermography for nondestructive evaluation of materials

Photothermal radiometry allows for remote measurement of local harmonic heat transport where the phase angle (between remote optical energy deposition and resulting temperature modulation) is sensitive to subsurface features or defects. Phase sensitive modulation thermography (or ‘lock-in thermography’) combines the advantages of photothermal radiometry with the fast technique of infrared imaging thereby revealing hidden defects in a short time. In this paper the principle and various applications are described and analyzed. While this lock-in thermography is based on remote optical heating of the whole area of interest, one can heat defects selectively with modulated ultrasound which is converted into heat by the mechanical loss angle effect which is enhanced in defect regions. This ‘ultrasonic lock-in thermography’ provides images showing defects in a way that is similar to dark field imaging in optical microscopy.     

声表面波检测铝板表面微缺陷深度实验研究

为了探究声表面波与不同深度微裂纹缺陷相互作用的关系,将脉冲激光作用于一系列不同缺陷的试件铝板上进行线光源激励,激发激光超声波。用超声传感器接收在铝板中传播的激光超声信号,通过数字荧光示波器采集激光超声在铝板中的传播数据。对采集到的反射波数据进行分离谱分离过程得到的铝板中激光超声的时域分布和透射波数据进行频域分析。实验发现:缺陷深度影响着反射回波两峰值特征点到达时间差,两者之间近似线性关系,也影响着透射波的截止频率且二者呈现递减关系。     

Quinary excitation method for pulse compression ultrasound measurements

A novel switched excitation method for linear frequency modulated excitation of ultrasonic transducers in pulse compression systems is presented that is simple to realise, yet provides reduced signal sidelobes at the output of the matched filter compared to bipolar pseudo-chirp excitation. Pulse compression signal sidelobes are reduced through the use of simple amplitude tapering at the beginning and end of the excitation duration. Amplitude tapering using switched excitation is realised through the use of intermediate voltage switching levels, half that of the main excitation voltages. In total five excitation voltages are used creating a quinary excitation system. The absence of analogue signal generation and power amplifiers renders the excitation method attractive for applications with requirements such as a high channel count or low cost per channel. A systematic study of switched linear frequency modulated excitation methods with simulated and laboratory based experimental verification is presented for 2.25 MHz non-destructive testing immersion transducers. The signal to sidelobe noise level of compressed waveforms generated using quinary and bipolar pseudo-chirp excitation are investigated for transmission through a 0.5 m water and kaolin slurry channel. Quinary linear frequency modulated excitation consistently reduces signal sidelobe power compared to bipolar excitation methods. Experimental results for transmission between two 2.25 MHz transducers separated by a 0.5 m channel of water and 5% kaolin suspension shows improvements in signal to sidelobe noise power in the order of 7–8 dB. The reported quinary switched method for linear frequency modulated excitation provides improved performance compared to pseudo-chirp excitation without the need for high performance excitation amplifiers.     

Performance of laser-ultrasonic F-SAFT imaging

The resolution and signal-to-noise ratio of laser-ultrasonics to detect small and buried defects can be greatly enhanced by using the synthetic aperture focusing technique (SAFT). Originally developed in the time domain, SAFT can also be implemented in the frequency domain (F-SAFT) using the angular spectrum approach for a significant reduction in processing time. In this paper, an F-SAFT based data processing method especially adapted to laser-ultrasonic data is presented. This method allows for further significant improvements towards laser-ultrasonic imaging of small defects. It includes temporal deconvolution of the waveform data, control for an optimal aperture and frequency bandwidth as well as spatial interpolation of the subsurface images. All the above operations are well adapted to the frequency domain calculations and embedded in the F-SAFT data processing. Also, the aperture control and spatial interpolation allow a reduction of sampling requirements to further decrease both inspection and processing times. The above improvements are illustrated using laser-ultrasonic data taken from an aluminum sample with flat-bottom holes.     

半无限体内双圆柱亚表面热波散射问题的研究

本文基于非傅里叶导热定律,采用波函数展开法,研究了含双圆柱缺陷非透明半无限体中热传播问题,给出了基于热传导波动模型的热波散射问题的一般解．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,分析了各物理参数对温度分布的影响,特别是热波波长对温度变化的影响,并给出了温度变化的数值结果。     

接触缺陷的振动调制超声导波检测技术研究

针对常规线性超声检测方法无法实现板结构接触类缺陷(如微裂纹、脱粘等)检测问题,将超声导波技术与振动声调制技术相结合,利用稀疏分布传感器发展了一种板结构中接触缺陷非线性超声检测方法。通过低频振动改变缺陷的接触状况,使得通过接触面的高频导波信号的相位和幅值受到调制。对受低频振动调制的超声导波二维时间序列进行时频分析,由于接触类缺陷的存在,在振动调制超声导波序列的时频分布上出现明显的低频振动频率分量。利用提取出的低频振动频率下的超声导波信号,进行了结构接触缺陷成像处理。检测试验表明,基于振动声调制的超声导波缺陷成像方法可以实现结构中的接触类缺陷检测。      

Noninvasive generation and measurement of propagating waves in arterial walls

Arterial wall stiffness can be associated with various diseases. Arteriosclerosis involves the buildup of plaques within artery walls that stiffen the arteries. The stiffness of an artery can be assessed by measurement of the pulse wave velocity (PWV). Usually, PWV is estimated using the foot-to-foot method. However, the foot of the pressure wave is not very clear due to reflected waves. Also, the blood pressure wave generated by the heart is normally a low frequency wave, hence the time resolution is low. PWV is an average indicator of artery stiffness between the two measuring positions, and therefore cannot easily identify local stiffness. In this paper a force on the arterial wall is generated noninvasively by the radiation force of ultrasound. Techniques for measuring the propagating wave due to this localized force are studied. The excitation force can be either a very short pulse or a modulated cw signal of a few hundred hertz. The temporal resolution of this method, which is in the range of microseconds, is much higher than the conventional pressure PWV method, and therefore allows the wave velocity to be measured accurately over short distances of a few millimeters.     

调幅波的单模激光线性模型随机共振

对单模激光增益模型的光强方程加入调幅波，用线性化近似方法计算了以δ函数形式关联的两白噪声驱动下光强的输出功率谱及信噪比. 结果表明，信噪比不但随着抽运噪声和量子噪声强度的变化出现随机共振，而且随着高频载波信号频率和低频调制信号频率的变化也出现了随机共振. 关键词： 抽运噪声 单模激光 随机共振 调幅波     

A calibration technique for frequency domain photothermoacoustics

Photoacoustic (or more precisely, photothermoacoustic) signals are generated by the absorption of photons, expressed as acoustic waves, and can be related to the incident laser fluence rate. Here a calibration procedure for frequency domain (FD) photoacoustics is presented which utilizes the relationship between photoacoustic (PA) signal amplitude and laser fluence rate. The sample is irradiated with a continuous wave laser which is amplitude modulated at varying frequencies. Applying the calibration procedure to the PA detected signals, a linear relationship is obtained between the fluence rate and the signal amplitude.     

Fourier-domain methodology for depth-selective photothermoacoustic imaging of tissue chromophores

Development of a novel photothermoacoustic (PTA) imaging technique utilizing a frequency-modulated (chirped) optical excitation and Fourier-domain methodology for depth-selective imaging of tissue chromophores is presented. Use of frequency-domain signal detection rather than short-pulse time-resolved measurements of pressure transients give an advantage of higher SNR typical to coherent detection techniques. Additionally, we introduce chirped optical excitation to generate linear frequency modulated PTA response which enables unambiguous and precise depth measurements of tissue chromophores. In order to obtain depth profilometric information from the frequency-domain PTA (FD-PTA) measurements, we describe implementation of two signal processing algorithms: matched filter compression and heterodyne mixing with coherent detection. We show that direct relationship between chromophore depth and spectrum of photothermo acoustic signals can be established to enable depth-selective tissue imaging. Application of amplitude and phase FD-PTA imaging is demonstrated in experiments with light-scattering phantoms and chicken breast tissues containing subsurface inclusions. The potential of the FD-PTA method for noninvasive tissue tomography and molecular imaging is discussed.