The experimental study of fatigue crack detection using scanning laser point source technique

For the purpose of better understanding the interaction of Rayleigh wave and the fatigue crack in a metallic sample, a set of experimental setups is built, based on the scanning laser source (SLS) technique, utilizing a point source to take place of the line source to generate surface acoustic waves (SAWs), and an interferometer is to detect the SAWs signal. The information of the crack (such as position and length) can be obtained by utilizing a two-dimensional scan of the material surface. This paper focuses on the detection of visible and invisible fatigue crack by using this point-source-based scanning laser source technique, and comparing the results with those of conventional pitch-catch technique. The result shows that with two-dimensional scanning, and analyzing the amplitude of the generated SAWs, not only the visible fatigue can be identified, but also the invisible fatigue crack can be discriminated. As a result, the sensitivity of the scanning point laser source technique is higher than the conventional pitch-catch technique.     

A laser beam deflection technique for the quantitative detection of ultrasonic Lamb waves

Ultrasonic Lamb waves have been measured with a quantitative optical beam deflection (OBD) technique. Calibration of the technique is derived for two cases of laser probe beam cross-section, one with a uniform beam and the other with a Gaussian beam intensity distribution. Expressions for angular beam deflection angle are derived for both cases in terms of the total light intensity falling on the photodetector, assumed to have a square-law voltage response. Using a Gaussian beam intensity distribution from a HeNe laser, the OBD measurements were compared with those from a Michelson interferometer to show that measurements were self-consistent.     

Mass spring lattice modeling of the scanning laser source technique

The scanning laser source (SLS) technique is a promising new laser ultrasonic tool for the detection of small surface-breaking defects. The SLS approach is based on monitoring the changes in laser generated ultrasound as a laser source is scanned over a defect. Changes in amplitude and frequency content have been observed for ultrasound generated by the laser over uniform and defective areas. In this paper, the SLS technique is simulated numerically using the mass spring lattice model. Thermoelastic laser generation of ultrasound in an elastic material is modeled using a shear dipole distribution. The spatial and temporal energy distribution profiles of typical pulsed laser sources are used to model the laser source. The amplitude and spectral variations in the laser generated ultrasound as the SLS scans over a large aluminum block containing a small surface-breaking crack are observed. The experimentally observed SLS amplitude and spectral signatures are shown to be captured very well by the model. In addition, the possibility of utilizing the SLS technique to size surface-breaking cracks that are sub-wavelength in depth is explored.     

超声波波型转换与表面波的检测

分析了超声波纵波、横波、表面波3种波型的产生条件和声速关系,检测了超声波在固体表面的波型转换,并测量了声表面波的速度,讨论了声表面波在超声延迟线方面的应用.     

Image scanning using acousto-optic interaction with surface waves

The scanning of a one-dimensional light intensity distribution was accomplished by employing the acousto-optic interaction of surface waves on LiNbO₃. The acoustic signal was 200 nano-seconds long with a center frequency of 100 MHz. To produce a large interaction length, the light propagates through the crystal parallel to the surface on which the acoustic surface wave is launched. The detected diffraction signal yields a temporal representation of the spatial intensity distribution.     

激光扫描在工件表面检测中的应用方法

为了获取磨削工件表面特征信息，提出一种基于激光扫描的磨削工件表面检测方法。利用机械臂带动激光传感器扫描放置在激光测量平面中的磨削工件，从而获得工件在激光测量平面中的三维坐标信息，通过相邻2个扫描点之间的高度变化求出工件边界点的三维坐标信息，结合x轴和y轴坐标的极值点利用最小二乘法拟合出工件边界在激光测量平面中的解析式，进一步求出附着在工件上的坐标系相对于激光测量坐标系的位姿，最后利用工件在激光测量坐标系中的位置矢量信息得出其表面特征信息。实验结果表明，利用该方法对工件表面进行检测，得到工件表面检测误差为0.11 mm，检测平均时间在1 s内，满足工件表面特征检测要求。     

Application of laser ultrasonic technique for non-contact detection of drilling-induced delamination in aeronautical composite components

The detection of drilling-induced delamination in composite components is a vital and challenging task in aviation industry. Numerous key components of aircrafts are made of composite materials, and drilling is often a final operation during assembly. Drilling-induced delamination is a very serious defect that significantly reduces the structural reliability, but it is rather difficult to be detected effectively due to its special location. A novel application of laser ultrasonic technique for the detection of drilling-induced delamination in composites is presented in this paper. A carbon fiber reinforced plastic laminate with drilling holes was made as specimen. A laser ultrasonic system was constructed and experiments were performed to detect the drilling-induced delamination, based on propagation characteristic of ultrasonic waves generated by pulse laser with a wavelength of 1064 nm and pulse duration of 10 ns. A laser interferometer based on two wave mixing is used to measure ultrasonic wave signals, and the morphology features of the delamination are imaged clearly by laser ultrasonic C-scan testing. The results proved that the laser ultrasonic technique is a feasible and effective method for the detection of drilling-induced delamination in composite components.     

Frequency dependence of images in scanning laser source technique for a plate

Defect imaging using scanning laser source technique has been investigated for a plate with rounded defects and notch-type defects in our previous studies. This paper examines frequency dependence of the defect images with both calculations and experiments in order to acquire clearer images. Both calculation and experimental results for a straight notch revealed that clearer images of notch-type defects can be obtained in the range of low frequency-thickness product below about 200 kHz mm. Moreover, images of the defects of various shapes were obtained by synthesizing images from eight receiving transducers, and similarly to the case of the straight notches, they became more clearly in the low frequency range.     

Observation of wedge waves and their mode transformation by laser ultrasonic technique

Wedge waves (WWs) in wedges, including their dispersion characteristics and mode transformation, are investigated using the laser ultrasound technique. Pulsed laser excitation and optical deflection beam method for detection are used to record WWs. Numerous WWs are detected by scanning the excitation laser along the wedge tip. Dispersions of WWs are obtained by using the two-dimensional (2D) Fourier transformation method, and different WW orders are revealed on the wedges. Mode transformation is determined by fixing the distance between the excitation and detection position, as well as by scanning the samples along the normal direction of the wedge tip.     

Acoustic source identification using a scanning laser Doppler vibrometer

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to identify acoustical sources. This is achieved by measuring the changes in the optical path induced by local fluctuation of the air refraction index to which the LDV is sensitive. The acoustical signal used is sinusoidal and may be recovered by scanning at a uniform rate over a subject area (continuous scan) parallel to the source axis and demodulating this signal. Due to the fact that the measured scan area is in fact a line integral over a measurement volume between the laser head and a rigid object needed to reflect the laser beam, multiple view planes around the axis of the acoustic source are usually measured. These are then passed through a tomographic algorithm, thereby reconstructing the full sound field. In this article however, only one view plane is measured, but the acoustic source is placed on a rotating surface with fixed rotational frequency, thereby imposing a modulation on the measured spectrum. Demodulation will allow reconstruction of the three-dimensional sound field.     

Damage detection in composite panels based on mode-converted Lamb waves sensed using 3D laser scanning vibrometer

This paper introduces damage identification approach based on guided ultrasonic waves and 3D laser Doppler vibrometry. The method is based on the fact that the symmetric and antisymmetric Lamb wave modes differ in amplitude of the in-plane and out-of-plane vibrations. Moreover, the modes differ also in group velocities and normally they are well separated in time. For a given time window both modes can occur simultaneously only close to the wave source or to a defect that leads to mode conversion. By making the comparison between the in-plane and out-of-plane wave vector components the detection of mode conversion is possible, allowing for superior and reliable damage detection. Experimental verification of the proposed damage identification procedure is performed on fuel tank elements of Reusable Launch Vehicles designed for space exploration. Lamb waves are excited using low-profile, surface-bonded piezoceramic transducers and 3D scanning laser Doppler vibrometer is used to characterize the Lamb wave propagation field. The paper presents theoretical background of the proposed damage identification technique as well as experimental arrangements and results.     

激光多普勒技术在固体测量方面的运用

介绍了激光多普勒测量技术的原理和后散射型固体表面测量方式。通过声光调制器的偏频后 ,能够确定物体的运动方向。对多普勒信号进行光学外差处理并对多普勒信号进行测相 ,可以提高测量的精度。用差动微分多普勒技术进行测量 ,可以消除由固体表面特性和激光光强的波动引起的误差。     

Barker码激励超声导波在长骨检测中的应用

单脉冲激励的超声导波在长骨中传播时,信号的衰减大,导致接收信号的幅度很小,且噪声严重。Barker码激励能有效增大接收信号幅度,提高信噪比(SNR)。将其应用到超声导波长骨检测中,进行仿真和长骨实验,得到的信号分别用加权匹配滤波器和有限冲激响应-最小均方误差(FIR-LS)逆滤波器进行压缩,并与单脉冲激励的结果进行了对比。结果表明,对于13位的Barker码,采用加权匹配滤波器进行解码时,压缩信号幅度是单脉冲激励接收信号的13倍;而FIR-LS逆滤波器则达到-63.59 dB的峰值旁瓣水平(PSL),更好地抑制噪声。说明可以将Barker码激励超声导波应用于长骨的检测中。     

Narrow band laser-generated surface acoustic waves using a formed source in the ablative regime

A narrow band laser-generated acoustic signal was created using a 4-element lenticular array. This arrangement of the array produces an acoustical signal with frequency content that is compatible with the response of a noncontact and remote broadband receiver, such as a capacitive air-coupled transducer. To support the experimental observations, a simplified concept is presented to explain the effect of a line array source on the frequency content of a surface acoustic wave. The analytical model solution for the wave front shape is derived from the point load solution of Lamb's problem that represents the displacement of a surface acoustic wave generated by an ablative line array. The distribution function, which was used for the model to represent the laser light energy, was tailored to depict the actual energy distribution that illuminates the lenticular array. Filtering functions are applied to the resultant surface displacement function to retain frequencies similar to those detected by the broadband 50 kHz-2.25 MHz receiver. The theoretical model showed good agreement with experimental results.     

Broadband light source shearing interferometer using Savart plate and angular scanning technique

A shearing interferometer based on using broadband light source, Savart plate, and angular scanning technique is proposed for slope contour measurements in this Letter. Of which, the Savart plate divides the wavefront reflected from the detected surface into two laterally displaced ones, the interference pattern generated by the interference of the divided wavefronts is modulated by an envelope function, and the slope contour of the detected surface is determined by examining the shifting of the darkest fringe as the shear plate is angularly scanned. A setup for realizing the interferometer is constructed. The experimental results of using this setup agree the validity and feasibility of the proposed interferometer.     

Photoacoustic detection of microcracks induced in BK7 glass by focused laser pulses

An experimental investigation on the induced damage by accumulative pulses generated by a Nd:YAG laser beam focused into the bulk of the BK7 glass is reported in this work. The laser was operated at the single-pulse damage energy threshold of the sample. The optical detonation generates a shock wave emission and microcrack formation. The induced photoacoustic wave emerging from the sample was monitored by piezoelectric detection. These signals provide a simple, reliable and highly sensitive indication of damage, processes involved, and the most appropriate laser parameters for two- and three-dimensional engraving.     

Thermo-optical modulation of ultrasonic surface waves for NDE

The well-known thermo-elastic effect of laser irradiation can be exploited to produce strong localized stresses when an expanded, long pulse, low-intensity laser beam is used to irradiate the specimen. These stresses will produce a parametric modulation of the received ultrasonic signals, that is somewhat similar to the acousto-elastic effect often used in nonlinear ultrasonic studies. It is shown in this paper that otherwise hidden small cracks in fatigue-damaged aluminum and titanium specimens can be readily detected by exploiting this optically induced thermo-elastic modulation during ultrasonic surface wave inspection since they are susceptible to crack closure and therefore exhibit strong parametric modulation. The temporal and spatial variations of the ultrasonic signals due to laser irradiation were evaluated numerically and experimentally. Based on these results, the direct temperature modulation of the ultrasonic velocity can be separated from the thermo-elastic stress modulation present only in cracked specimens. It was found that this method can be used to selectively increase the sensitivity of ultrasonic flaw detection to small fatigue cracks by more than one order of magnitude.     

Sensitivity of the ultrasonic CARI technique for breast tumor detection using a FETD scheme

The CARI (clinical amplitude/velocity reconstruction imaging) technique is a new ultrasonography for better detection of the breast cancer. The method uses the mammography-like positioning of the breast and a reference structure to assess the changes of sound velocity in the screened region. In this work, we present a first attempt to simulate the CARI technique using finite element time-domain (FETD) wave propagation. The CARI sensitivity to the size, shape and location of the tumors is investigated via the simulations on 2D and 3D breast models. Small lesions can be detectable under a suitable spatial resolution. 3D simulations agree with the quantitative results of the 2D case. Moreover, the FETD approximation is proving to be a simple, but robust tool in the CARI simulation.     

Influence of laser beam profile on the generation of ultrasonic waves

The different ultrasonic fields generated in metallic materials by a laser beam with flat and Gaussian profile are investigated experimentally and using the finite element method (FEM). A high power laser beam irradiating a solid surface produces elastic waves with a mechanics that depends on many parameters, including the profile of the laser beam. The influence of the beam profile is investigated with the FEM analysis, considering the temperature dependence of material properties.     

表面脉冲超声波沿曲面爬行的可视化研究

为了研究超声表面脉冲波在钢、铁等不透明曲面材料中的传播规律,我们采用和金属材料声速比较接近的光学玻璃作样品,利用研制的数字化动态光弹系统,分析了表面波和次表面波沿玻璃圆棒侧面传播时的行为,记录了不同时刻的横截面上的脉冲声场传播图像,求出了表面波和次表面波沿曲面爬行的传播速度。