Coating thickness affects surface stress measurement of brush electro-plating nickel coating using Rayleigh wave approach

A surface ultrasonic wave approach was presented for measuring surface stress of brush electro-plating nickel coating specimen, and the influence of coating thickness on surface stress measurement was discussed. In this research, two Rayleigh wave transducers with 5 MHz frequency were employed to collect Rayleigh wave signals of coating specimen with different static tensile stresses and different coating thickness. The difference in time of flight between two Rayleigh wave signals was determined based on normalized cross correlation function. The influence of stress on propagation velocity of Rayleigh wave and the relationship between the difference in time of flight and tensile stress that corresponded to different coating thickness were discussed. Results indicate that inhomogeneous deformation of coating affects the relationship between the difference in time of flight and tensile stress, velocity of Rayleigh wave propagating in coating specimen increases with coating thickness increasing, and the variation rate reduces of difference in time of flight with tensile stress increasing as coating thickness increases.     

Stress evaluation of laser cladding coating with critically refracted longitudinal wave based on cross correlation function

Research on stress evaluation of laser cladding coating with critically refracted longitudinal wave was introduced in this paper. Two critically refracted longitudinal wave transducers with 5 MHz frequency, spacing between which was constant, were employed as signal emitter and receiver. Based on acoustoelastic equation deduction, relationship between the difference in time of flight and tensile stress is obtained. Combing with cross correlation theory, the difference in time of flight between stressed and unstressed critically refracted longitudinal signals was calculated. Results show that stress evaluation is affected by layer interface and anisotropic microstructure of laser cladding coating, precision of stress evaluation of laser cladding coating increases as step length increases until it attains one cycle. In addition, influence of waveform distortion caused by microstructure of laser cladding coating on stress evaluation is discussed. At last, verification test is carried out and the experimental result is well consistent with theoretical result.     

Determination of mechanical stresses in solids by an acoustic method

Different versions of linearized problems of the elastic wave propagation in prestressed bodies are considered. The influence of the version of the linearized elasticity theory chosen for calculating the magnitude of the acoustoelastic effect on the result of calculation is analyzed for longitudinal and shear waves and for Rayleigh surface waves. The results of theoretical calculations are compared with the experiment. Experimental data on the effect of bending stresses on the propagation velocity of Rayleigh waves are presented. These results extend the range of problems that can be solved by using the acoustoelastic effect.     

Surface phonon calculations for noble metals: Comparison with he-surface scattering experiments

We evaluate the surface phonon dispersion relations of Ag(111) and Au(111) within a force constants parametrization of the bu?k dynamics. By changing the surface force constants we are able to explain all the salient features appearing in the experimental time of flight spectra of a 7.9 meV He beam. The parallel momentum Q has been considered along the [11?0] and [112?] directions of the surface Brillouin ozne. We show that the peaks of the time of flight spectra are related to the Rayleigh wave, to the pseudo Rayleigh wave and to a new resonance of longitudinal character.     

The acoustoelastic effect on Rayleigh waves in elastic--plastic deformed layered rocks

On the basis of the acoustoelastic theory for elastic--plastic materials, the influence of statically deformed states including both the elastic and plastic deformations induced by applied uniaxial stresses on the Rayleigh wave in layered rocks is investigated by using a transfer matrix method. The acoustoelastic effects of elastic--plastic strains in rocks caused by static deformations, are discussed in detail. The Rayleigh-type and Sezawa modes exhibit similar trends in acoustoelastic effect: the acoustoelastic effect increasing rapidly with the frequency-thickness product and the phase velocity change approaching a constant value for thick layer and high frequency limit. Elastic--plastic deformations in the Castlegate layered rock obviously modify the phase velocity of the Rayleigh wave and the cutoff points for the Sezawa modes. The investigation may be useful for seismic exploration, geotechnical engineering and ultrasonic detection.     

Measurement of velocity distribution for longitudinal acoustic waves in welds by a laser optoacoustic technique

An optoacoustic technique for diagnostics of residual stress in metals is proposed. The theoretical part of the technique employs acoustoelastic relations establishing a linear relationship between the biaxial residual stress and the relative variation of the velocity of longitudinal ultrasonic waves. The experimental technique is based on laser excitation of nanosecond ultrasonic pulses at the surface of samples under investigation and their detection with a high time resolution. Distributions of the relative variation of longitudinal wave velocities due to the presence of residual stress in the samples are obtained.     

航空透明件表面波声弹效应

表面波作为超声波的一种特殊形式,由于其传播特性,使得通过改变频率检测距物体表面不同深度处的;初邕应力成为可能,因此,各种产表面波声弹理论与实验技术的研究不断受到重视。本文从弹性波和有限变形理论出发,推导了表面波在有初始应力的各同性弹性体中传播时,物体表面应力与超声波传播速度之间的一般关系。对作为收音机坐舱的航空透年ＹＢ－３有机玻璃试件进行了声弹性实验。同时还给出了由实验数据回归的三次多项式声弹公式     

双横波声速法检测单向受压混凝土构件永存应力*

该文力求寻找一种高效准确检测混凝土构件永存应力的方法。基于声弹性理论提出了一种双横波声速法检测单向受压混凝土构件永存应力的方法,该方法通过测试受力构件第一波速和第二波速,以第一波速和第二波速的平方差为基础构建综合声学参数来检测单向受压混凝土构件永存应力。所述第一波速为声波传播方向与应力方向垂直、质点振动方向与应力方向平行的横波波速,第二波速为声波传播方向及质点振动方向均与应力方向垂直的横波波速。实验结果显示,该声学参数用于检测混凝土单向受压构件永存应力的效果与使用单一声速参数相比,对应力的敏感性有所提高,受混凝土黏滞性及材质离散性的影响更小,并且测试结果基本不受温度和湿度变化影响。     

超声双折射法测试铝合金的内部应力

声各向同性的金属材料在应力作用下表现出声各向异性,这是用声弹性法分析材料内部应力的基础。本文用偏振方向平行或垂直于应力方向的超声纯横波对LY11型铝合金进行测试。实验结果表明:材料在拉、压单轴应力作用下,偏振方向平行和垂直于应力方向的超声纯横波的声速都发生了变化。实验在分析材料声各向异性的基础上计算材料声弹性双折射系数,得到测试LY11型铝合金内部应力的理论公式,并对其内部的残余应力进行评估。实验中利用双换能器回振法测量声速,时间测试精确度可达10^-11s,可精确测量声速的微小变化量。     

矩形表面波探头声场的高斯声束叠加法

利用矩形压电晶片和有机玻璃楔块折射可激励出超声表面波,广泛用于固体近表面缺陷检测和材料特性测量.由于描述表面波三维声场的理论方法还鲜有报道,因而主要采用简化的表面波二维声场模型来定量分析这类问题.高斯声束模型近些年被广泛应用于解决超声体波传播的各种复杂问题,然而,目前还没有将其扩展应用到超声表面波的声场的计算中.通过结合表面波格林方程和矩形换能器的高斯声束模型,推导出基于高斯声束叠加的表面波三维声场解析解.进一步,将该方法与点源叠加的数值解进行了分析比较,计算结果表明表面波声场的高斯声束叠加方法在具有较好计算精度的同时,还具有更快的计算效率.     

Guided ultrasonic waves for non-destructive monitoring of the stress levels in prestressed steel strands

The safety of prestressed civil structures such as bridges, dams, nuclear power plants, etc. directly involves the security of both environment and users. Health monitoring of the tensioning components, such as strands, tendons, bars, anchorage bolts, etc. is an important research topic and a challenging task bringing together the non-destructive evaluation (NDE) and civil engineering communities. This paper deals with a guided ultrasonic wave procedure for monitoring the stress levels in seven-wire steel strands (15.7 mm in diameter). The mechanical and geometrical characteristics of the prestressed strands were taken into account for optimizing the measurement configuration and then the choice of the guided ultrasonic mode at a suitable frequency. Simplified acoustoelastic formulations were derived from the acoustoelasticity theory according to either calibration test or in situ measurement. The results from acoustoelastic measurements on the seven-wire steel strands are presented and discussed in the case of calibration tests and industrially prestressed strands. They show the potential and the suitability of the proposed guided wave method for evaluating the stress levels in the tested seven-wire steel strands.     

Nonlinear elastodynamics in micro-inhomogeneous solids observed by head-wave based dynamic acoustoelastic testing

Dynamic acoustoelastic testing provides a more complete insight into the acoustic nonlinearity exhibited by micro-inhomogeneous media like granular and cracked materials. This method consists of measuring time of flight and energy modulations of pulsed ultrasonic waves induced by a low-frequency standing wave. Here pulsed ultrasonic head waves were employed to assess elastic and dissipative nonlinearities in a region near the surface of a solid. Synchronization of the ultrasound pulse sequence with the low-frequency excitation provided instantaneous variations in the elastic modulus and the attenuation as functions of the instantaneous low-frequency strain. Weak quadratic elastic nonlinearity and no dissipative nonlinearity were detected in duralumin. In limestone, distinction between tensile and compressive behaviors revealed an asymmetry in the acoustic nonlinearity and hysteresis in both the elastic modulus and the attenuation variations. Measured nonlinear acoustical parameters are in good agreement with values obtained by different techniques. Reversible acoustically induced conditioning modified the acoustic nonlinearity both quantitatively and qualitatively. It reduced tension-compression asymmetry, suggesting a nonequilibrium modification of the sources of acoustic nonlinearity. Additionally to the metrology of the acoustic nonlinearity, head wave based dynamic acoustoelastic testing may be a useful tool to monitor changes in the microstructure or the accumulation of damage in solids.     

Relationship between Rayleigh wave polarization and state of stress

This research develops an analytical model (using Stroh's formalism) to predict the affect of applied stress on the wave speed and the polarization of Rayleigh surface waves. Simulation results are then used to demonstrate that the polarization of a Rayleigh wave (which is reference-free) could be more sensitive than wave speed as an indicator of the state of stress.     

激光激发瑞利波检测表面倾斜缺陷的研究

研究激光激发瑞利波检测样品的表面倾斜缺陷。基于频域热弹耦合方程,采用有限元方法建立激光激发瑞利波检测倾斜缺陷的数值模型,研究倾斜缺陷的检测机理。数值计算含不同的长度及倾斜角度缺陷的样品中瞬态位移波形,分析瑞利波在倾斜缺陷处模式转换的过程,研究各种瑞利波的传播路径。在此基础上,研究缺陷宽度与材料黏性对瑞利波传播及缺陷检测的影响。结果表明:瑞利波在缺陷处产生的反射及透射瑞利波的到达观测点的时间可以检测缺陷位置和长度,瑞利波在缺陷的底部发生模式转换产生的切变波可以检测缺陷倾斜角度。数值结果和已有的实验结果一致,从而为表面倾斜缺陷的检测提供有效的理论依据。      

Visualization and numerical analysis of stress waves in blasting process

Visualization studies were performed both experimentally and theoretically to observe stress wave propagation in a material and its interaction with the free surface of the material in a blasting process. PMMA (polymethyl-methacrylate) plates were used as a transparent material. The stress wave was generated by initiation of an electric detonator. The stress waves in the PMMA specimen with the right-angled corner were observed by means of the shadowgraph system using a Q switched ruby laser as a light source. In addition to the experiment, a numerical analysis using smoothed particle hydrodynamics (SPH) was also carried out to clarify the dynamic behavior of stress waves in the blasting process. Transmittance and reflection of the stress wave at the free surface could be visualized by both the experiment and the numerical analysis.     

非轴向力下埋入式光纤传感器应变传递分析

利用剪滞法建立了当光纤光栅传感器的轴线和基体主应力的方向成一定角度时,光纤光栅传感器的测量应变与基体结构实际应变之间的关系,进而得出了光纤光栅传感器的平均应变传递率的一般公式。采用裸光纤光栅传感器进行实验,在倾斜角度α为30.72°时,实验所得的波长变化之比分别为:0.727,0.738,0.746;理论计算所得的波长变化之比为0.739,相对误差都在2%以内。同时分析了埋设角度偏差对测量结果的影响。研究结果表明,非轴向力作用下光纤光栅传感器的应变与结构基体应变之间的传递关系与其在轴向力作用下存在明显的区别,埋设角度的偏差会给测量结果造成一定的误差。     

Assessment of material degradation due to corrosion-fatigue using a backscattered Rayleigh surface wave

Material degradation due to corrosion-fatigue was evaluated nondestructively using backscattered Rayleigh surface wave. A corrosion-fatigue test was carried out for the specimens made of thermo-mechanically controlled process steel in 3.5 wt.% NaCl solution at 25 degrees C. The Backscattering profile, which is the amplitude variation of backscattered ultrasound according to the incident angle, of the specimens were measured in water at room temperature after the corrosion-fatigue test. The velocity of Rayleigh surface wave, determined from the incident angle at which the profile of the backscattered ultrasound became maximum, decreased for the specimen that had the large number of cycles to failure in the corrosion-fatigue test. This fact implies that the corrosion degradation occurred at specimen surface in this specific test is dominantly dependant on the time exposed to corrosion environment. The result observed in the present work demonstrates the high potential of backscattered Rayleigh surface wave as a tool for nondestructive evaluation of corrosion degradation of aged materials.     

Theoretical determination of Rayleigh wave acoustoelastic coefficients: comparison with experimental values

The characterization of stress states in materials is often necessary in some industrial application. The ultrasonic methods can be potentially convenient since stress states inside materials can be obtained even if materials are opaque. Nevertheless, the knowledge of acousto-elastic coefficients is generally necessary to estimate residual stresses by ultrasonic methods, but the experimental determination of these acousto-elastic coefficients can be difficult in some cases. In this paper, Rayleigh wave (RW) acousto-elastic coefficients of an orthotropic material are theoretically determined according to its characteristics, i.e. the density and the secondand third-order elastic constants. Then, these RW acousto-elastic coefficients are directly measured during an experimental stage and a comparison between calculated and measured coefficients is realized. This study allows on the one hand to check the theoretical development and on the other hand to show that it is possible to calculate acousto-elastic coefficients theoretically from intrinsic characteristics of the material rather than measuring them directly during a calibration phase which is sometimes long and difficult to realize.     

Laser Generation of Surface Waves on Cylinder with a Slow Coating

An analytical model of acoustic field excited by a pulsed-laser line source on a coated cylinder is presented. Surface wave dispersive behaviours for a cylinder with a slow coating are analysed and compared with that of a bare cylinder. Based on this analysis, the laser-generated transient response of the perturbed Rayleigh wave and the higher modes of steel cylinder with a zinc coating are calculated from the model using residue theory and FFT technique. The theoretical result from the superposed waveform of the perturbed Rayleigh wave and higher modes agree well with the waveform obtained in experiment. The results show that the model and numerical method provide a useful technique for quantitatively characterizing coating parameters of coated cylinder by the laser generated surface waves.