Corrosion and erosion monitoring in plates and pipes using constant group velocity Lamb wave inspection

Recent improvements in tomographic reconstruction techniques generated a renewed interest in short-range ultrasonic guided wave inspection for real-time monitoring of internal corrosion and erosion in pipes and other plate-like structures. Emerging evidence suggests that in most cases the fundamental asymmetric A₀ mode holds a distinct advantage over the earlier market leader fundamental symmetric S₀ mode. Most existing A₀ mode inspections operate at relatively low inspection frequencies where the mode is highly dispersive therefore very sensitive to variations in wall thickness. This paper examines the potential advantages of increasing the inspection frequency to the so-called constant group velocity (CGV) point where the group velocity remains essentially constant over a wide range of wall thickness variation, but the phase velocity is still dispersive enough to allow accurate wall thickness assessment from phase angle measurements. This paper shows that in the CGV region the crucial issue of temperature correction becomes especially simple, which is particularly beneficial when higher-order helical modes are also exploited for tomography. One disadvantage of working at such relatively high inspection frequency is that, as the slower A₀ mode becomes faster and less dispersive, the competing faster S₀ mode becomes slower and more dispersive. At higher inspection frequencies these modes cannot be separated any longer based on their vibration polarization only, which is mostly tangential for the S₀ mode while mostly normal for the A₀ at low frequencies, as the two modes become more similar as the frequency increases. Therefore, we propose a novel method for suppressing the unwanted S₀ mode based on the Poisson effect of the material by optimizing the angle of inclination of the equivalent transduction force of the Electromagnetic Acoustic Transducers (EMATs) used for generation and detection purposes.     

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.     

Hidden corrosion detection in aircraft aluminum structures using laser ultrasonics and wavelet transform signal analysis

Preliminary results of hidden corrosion detection in aircraft aluminum structures using a noncontact laser based ultrasonic technique are presented. A short laser pulse focused to a line spot is used as a broadband source of ultrasonic guided waves in an aluminum 2024 sample cut from an aircraft structure and prepared with artificially corroded circular areas on its back surface. The out of plane surface displacements produced by the propagating ultrasonic waves were detected with a heterodyne Mach-Zehnder interferometer. Time-frequency analysis of the signals using a continuous wavelet transform allowed the identification of the generated Lamb modes by comparison with the calculated dispersion curves. The presence of back surface corrosion was detected by noting the loss of the S(1) mode near its cutoff frequency. This method is applicable to fast scanning inspection techniques and it is particularly suited for early corrosion detection.     

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.     

Hyperbolic heat equations in laser generated ultrasound models

The generation of ultrasound by pulsed lasers is a thermoelastic process, and in the literature on general thermoelasticity there are presently three different forms of the heat equation in popular use, the classical, Lord-Schulman (LS), and Green-Lindsay (GL) heat equations. The question may thus arise as to which heat equation should be used to model laser generated ultrasound. The purpose of this work is to summarize the current rationale for using these different heat equations, in order to provide a basis for choosing one of the forms. A review of the classical, LS and GL theories is given, and the potential advantages of hyperbolic heat flow theories in laser ultrasonics are discussed. A numerical example is given that clearly shows the small time differences these theories predict, and also points out potential problems with using hyperbolic heat equations on small time scales.     

Transient deformation of thin metal sheets during pulsed laser forming

The transient deformation of thin grade 304 stainless steel metal sheets heated by a single pulse of a CO₂ laser beam is simulated in this paper. The laser beam is assumed to be line-shaped and the problem is treated as three-dimensional thermo-elastoplastic. The temperature field, deformation pattern, stress–strain states and the residual stress distribution of the specimens have been calculated numerically and the transient response of the bending angle has been validated by experiments. Good agreement has been obtained between the numerical simulation and the experiments under various operating conditions. The numerical study reveals that a high temperature gradient exists for a positive bending angle and a low one for a negative angle. It transpires that the mechanisms of pulsed laser forming are dependent mainly upon the laser power, the heating time, the clamping arrangement, as well as the geometry, the thermal properties and the original stress states of the specimen.     

Study of characteristics of ultrasound pulse generated by a laser pulse

I.IntroductionThegenerationofacousticpulsebylaserirradiationofametalsurfacewasfirstsuggestcdbyWhitein1963[1l.SincethatdateLaserU1trasoundtechniquchasbeendcvclopedrapidly.Becausethistechniquehasanumberoftechnicalfcatures,suchasnon-contact,highbandwidth,highhme-spacia1resolution,quantitativeteshng,generationoflongitudinal,shcarandRay1cighwaves(simu1taniously),andsoon,ithasbccnwidelyapp1icdtomcasurementsofmatcrialproperties,detectionofdefects,andcalibrationoftransd.ccrsl'-'o].Inordertodcve1opth…     

Simultaneous measurement of the phase and group velocities of Lamb waves in a laser-generation based imaging method

This paper describes a novel approach to the simultaneous measurement of the phase and group velocities of Lamb waves based on images of their propagation. The laser-generation based imaging method was first introduced to obtain images of Lamb wave propagation. The time series of snapshot images is used to make a position-time diagram, and the velocities can be estimated based on the slopes of the position curves. Thus, the phase and group velocities can be obtained by measuring the phase advance and energy flow of the Lamb wave, respectively. Details of the principle of simultaneous measurement are presented herein. Experimental verification was also performed in the range of 0.2-3.0 MHz-mm using aluminum plates. The average errors between experiment and theory in the phase and group velocities were 3.31% and 5.68%, respectively.     

固体中脉冲激光激发声表面波的理论研究

本文运用本征函数展开的方法对固体材料中脉冲激光激发的声表面波进行了理论研究，在考虑热弹激发的条件下，利用三维的轴对称模型，得到了脉冲激光光源的脉冲宽度以及聚焦半径对固体材料中声表面波信号的时域及频域的影响，对进行超短脉冲激光激发超声的研究具有指导意义。     

Effect of optical penetration on laser generatedthermoelastic ultrasound in solids

I.IntroductionTbegenerationofu1trasoundbytheirradiationofpulsedlaseratso1idsurfacehasbeenwidelystlldied,boththeorctica1lyandexpcrimentallyI1-8].Intheprocessoflaserthermoelasticg6nerationofu1trasound,temperatureriseinduccdbytheabsorptionoflaserenergyproducestherma1expansion,andthcnanultrasonicsourceiscreated.Sofar,thestudyof1asergenera-honofultrasoundinso1idsconcentratesmain1yonmeta1s,anduntilrecent1y,1ittleattentionhasbenpaidtonon-mctals.Inametal,1aserenergyisabsorbedonlyatthesurfaceofthesamp…     

多通道超声兰姆波检测板状结构中的裂纹

超声兰姆(Lamb)波在结构缺陷检测方面愈来愈受到重视,但目前Lamb波的应用局限于单信号源激励,单通道接收的方法,容易受到噪声干扰,其后续的信号分析处理也比较复杂。本文旨在采用多通道Lamb波对板状结构中的裂纹进行定量分析与诊断。在铝板的表面凿刻出不同深度的凹槽作为裂纹,通过线阵换能器采集在一定传播距离内的多通道Lamb波信号,并采用二维傅里叶变换分析在不同深度的裂纹下,Lamb波模式能量的变化规律。结果表明,相对于完好铝板中的Lamb波信号,裂纹的存在会使Lamb波发生模式转换现象,并且转换模式能量百分比随裂纹深度的增加而线性增加。其结果为Lamb波评价板状结构中的裂纹状况提供了一种可能的方法。     

Imaging anisotropic elastic properties of an orthotropic paper sheet using photorefractive dynamic holography

An important material property in the paper industry is the anisotropic stiffness distribution due to the fibrous microstructure of paper and to processing procedures. Ultrasonic methods offer a means of determining the stiffness of sheets of paper from the anisotropic propagation characteristics of elastic Lamb waves along the machine direction and the cross direction. That is, along and perpendicular to the direction of paper production. Currently, piezoelectric ultrasonic methods are employed in the industry to measure the elastic polar diagram of paper through multiple contacting measurements made in all directions. This paper describes a new approach utilizing the INEEL Laser Ultrasonic Camera to provide a complete image of the elastic waves traveling in all directions in the plane of the paper sheet. This approach is based on optical dynamic holographic methods that record the out of plane ultrasonic motion over the entire paper surface simultaneously without scanning. The full-field imaging technique offers great potential for increasing the speed of the measurement and it ultimately provides a substantial amount of information concerning local property variations and flaws in the paper. This report shows the success of the method and the manner in which it yields the elastic polar diagram for the paper from the dispersive flexural or antisymmetric Lamb wave.     

Effect of plasma confinement on laser shock microforming of thin metal sheets

Laser shock forming is conceived as a non-thermal laser forming method of thin metal sheets using the shock wave induced by laser irradiation to modify the target curvature. The plastic deformation induced by the shock wave and the direct plasma pressure applied on the material generate a residual stress distribution in the material finally leading to its bending. Using water as a confinement medium for the plasma the pressure can be increased around 10 times and the final deformation has a dramatic increase.The effect can be made clearly apparent in thin specimens (up to 1 mm). In the present study thin (100 μm) stainless steel (AISI 316) strips (1 mm long and 300 μm wide) in single and double pinned configurations have been investigated.A Nd:YAG Laser (1064 nm) with 10 ns of pulse length (FWHM) and an energy of 21 mJ per pulse is focused in the strip (spot diameter of the spot = 500 μm).Experimental and numerical studies of the influence of plasma confinement in the process and number of applied pulses are presented.The study shows that the final bending of the specimens can be controlled on a relative wide range by a stable quasi-proportional relation to the number of applied pulses and, what is considered as of major importance, that plasma confinement increases the generated pressure and thus the bending in the target.Laser shock microforming in confined configuration is considered as a technique allowing the successful processing of components in a medium range of miniaturization.     

Dynamics of a pulsed laser generated tin plasma expanding in an oxygen atmosphere

Semiconducting tin oxide can be successfully deposited by means of the laser ablation technique. In particular by ablating metallic tin in a controlled oxygen atmosphere, thin films of SnO _x have been deposited. The partial oxygen pressure at which the films are deposited strongly influences both the stoichiometry and the structural properties of the films. In this work, we present a study of the expansion dynamics of the plasma generated by ablating a tin target by means of a pulsed laser using time and space resolved optical emission spectroscopy and fast photography imaging of the expanding plasma. Both Sn I and Sn II optical emission lines have been observed from the time-integrated spectroscopy. Time resolved-measurements revealed the dynamics of the expanding plasma in the ambient oxygen atmosphere. Stoichiometry of the films has been determined by means of X-ray photoelectron spectroscopy and correlated to the expansion dynamics of the plasma.     

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.     

Metal nanoparticle production by pulsed laser nanostructuring of thin metal films

The controllable nanostructuring of thin metal films by nanosecond UV laser pulses is introduced as a novel technique for the production of metal nanoparticles supported on a range of different oxide substrates, including indium tin oxide. This processing is performed at low macroscopic temperatures. The physical mechanisms underlying the nanostructuring are discussed and applications for these nanoparticle films, including as catalysts for nanotube/nanowire growth and in surface enhanced Raman spectroscopy measurements, are introduced.     

Simulation of laser generated ultrasound with application to defect detection

Laser generated ultrasound holds substantial promise for use as a tool for defect detection in remote inspection thanks to its ability to produce frequencies in the MHz range, enabling fine spatial resolution of defects. Despite the potential impact of laser generated ultrasound in many areas of science and industry, robust tools for studying the phenomenon are lacking and thus limit the design and optimization of non-destructive testing and evaluation techniques. The laser generated ultrasound propagation in complex structures is an intricate phenomenon and is extremely hard to analyze. Only simple geometries can be studied analytically. Numerical techniques found in the literature have proved to be limited in their applicability, by the frequencies in the MHz range and very short wavelengths. The objective of this research is to prove that by using an explicit integration rule together with diagonal element mass matrices, instead of the almost universally adopted implicit integration rule to integrate the equations of motion in a dynamic analysis, it is possible to efficiently and accurately solve ultrasound wave propagation problems with frequencies in the MHz range travelling in relatively large bodies. Presented results on NDE testing of rails demonstrate that the proposed FE technique can provide a valuable tool for studying the laser generated ultrasound propagation. PACS 02.70.Dh; 43.35.+d; 42.62.-b     

Micromould based laser shock embossing of thin metal sheets for MEMS applications

Laser shock forming is a new material processing technology. Micro-channel with dimension of 260 μm × 59 μm was successfully fabricated on metallic foil surface using laser-generated shock wave. The work piece has a high spatial resolution at the micron-level. A series of experiments was conducted to validate the finite element model. An analysis procedure including dynamic analysis performed by ANSYS/LS-DYNA and static analysis performed by ANSYS is presented in detail to attain the simulation of laser shock embossing to predict the surface deformation. Micromould based laser shock embossing holds promise for achieving precise, well-controlled, low-cost, high efficiency of three-dimensional metallic microstructures. In addition, this technique can fabricate complex 3D microstructures directly by single pulse.     

A theoretical study on vertical finite cracks detection using pulsed laser spot thermography (PLST)

A theoretical study on vertical finite cracks detection in low carbon steel (A36) blocks using pulsed laser spot thermography (PLST) was carried out in this paper. The basic principle of PLST was described. The 3D heat conduction model of flux transfer under pulsed laser spot excitation was established and calculated using finite element method (FEM). The effect of laser-crack distance to temperature abrupt jump across the vertical finite cracks was analyzed, and results show that the optimum laser-crack distance is the radius of laser spot. The relationship between geometric parameters of cracks and temperature abrupt jump was studied, and results show larger geometric size of cracks leads to greater peak values of temperature abrupt jump, and tends to stable values at certain size.