An ultrasound method for the acoustoelastic evaluation of simple bending stresses

Ultrasonic waves constitute a privileged tool for investigation in the analysis and characterisation of mechanical stress states. Ultrasonic wave velocities depend on many physical properties of the propagation medium such as the second-, third- and higher-order elastic constants, the volume density and the strain. They also depend on whether the wave is longitudinal or transversal. In this last case, they also depend on the wave polarisation direction.

The present paper deals with the classical acoustoelasticity in stressed and elastically deformed media when they are submitted to bending stresses. A numerical and an experimental evaluation of the resulting change in the ultrasound velocities as a function of bending loads are described. Some results have been obtained on the variation of propagation velocities of the longitudinal and transversal polarised waves as a function of mechanical bending loads applied on samples made of S 185 steel. The acoustoelastic evaluations have been achieved in three zones under bending stress (compressed, central and extended fibres) in the case where the path of probing, longitudinal and linearly polarised shear waves are parallel to the sample axis. As additional investigation on acoustoelastic behaviour on bending load, we propose some acoustoelastic responses simulated numerically on materials of known macroscopic properties.

The experimental study was achieved by means of a measurement set-up composed of an ultrasound bench and a mechanical test machine. In order to measure the variations of the propagation velocity, a correlation technique has been used to obtain an accurate estimation of the propagation time. The velocity measurements have taken into account the elastic deformation of the samples made of carbon steel (C 35).     

Measurements of local elastic moduli by amplitude and phase acoustic microscope

A new method is suggested for the nondestructive measurement of elastic moduli in a localized area, 100–400 μm in diameter, by the complex V(z) curve using an amplitude and phase acoustic microscope. The inverse Fourier transform of the complex V(z) curve contains the reflectance function of a liquid-specimen interface. Therefore, the longitudinal, transverse and Rayleigh wave velocities for the specimen are simultaneously obtained by the inversion of the complex V(z) curve. The elastic moduli for glass obtained from wave velocities by acoustic microscope agree fairly well with those by other methods. The present method is applied to aluminium alloy, and it is shown that this method is useful in measuring the microscopic characteristics in inhomogeneous materials.     

The atomistic modeling of wave propagation in nanocrystals

This paper presents non-equilibrium molecular dynamics simulations of wave propagation in nanocrystals. The width of the traveling wave front increases with grain size, d, as d^1/2. This width also decreases with the pressure behind the front. When the results are extrapolated to micro-crystals, reasonable agreement with experimental data is obtained. In addition, this extrapolation agrees with models that only take into account the various velocities of propagation along different crystalline orientations without including grain boundary effects. The results indicate that, even at the nanoscale, the role of grain boundaries as scattering centers or as sources of plasticity does not increase significantly the width of the traveling wave. For more information, contact E.M. Bringa, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore CA 94550, USA; (925) 423-5724; fax (925) 422-4665; e-mail ebringa@llnl.gov.     

带空气柱的旋流器内流场的数值和试验研究(英文)

针对d50 mm水力旋流器内流场,分别进行了基于CFD模拟的数值研究和基于PIV流场测试的试验研究。两种方法对空气柱的研究结果均表明,空气柱在0.7 s左右大致成形,且二者所得空气柱具有相似的特征。流场速度特性研究结果显示,在流场矢量图中,涡流被两种研究方法几乎在同一位置捕捉到;CFD模拟和PIV测试所得轴向速度分布结果也高度一致。用CFD数值研究的方法考察不同参数对流场切向速度和轴向速度的影响结果表明,旋流器内切向速度分布符合组合涡特征;给矿流量增大会使流速增大;增大锥角或者溢流口直径,会加快溢流的排出速度;而沉砂口的变化对流速的影响则很小。     

In situ synchrotron characterization of mechanically activated self-propagating high-temperature synthesis applied in Mo–Si system

An original experiment was designed to monitor structural and thermal evolutions during the MASHS (Mechanically Activated Self-propagating High-temperature Synthesis) process in the Mo–Si system. Time-Resolved X-Ray Diffraction (TRXRD) coupled with an infrared imaging technique was performed to study, in situ, the formation of the α-MoSi₂ phase in the combustion front. Despite a temporal resolution of 50 ms between two consecutive diffractograms, no intermediate phase was observed during the passage of the combustion front. The only reaction responsible for the self-sustentation is Mo+2Si→MoSi₂ in the primary zone inside the combustion wave. The mechanical activation was found to influence Self-propagation High-temperature Synthesis (SHS) parameters such as the propagation front velocity (>13 mm/s), the maximal combustion temperature and the local thermal gradient. After the MASHS process, the α-MoSi₂ compound is nanostructured (D_MoSi2=88 nm) and some explanations are expressed in order to understand why the nanostructure of the as-milled powders can be maintained during the combustion reaction.     

Ultrasonic velocity measurements for characterizing the annealing behaviour of cold worked austenitic stainless steel

Precise measurements of shear wave velocities have been made in 20% cold worked and annealed samples of alloy D9 (Ti-modified austenitic stainless steel) to characterize the microstructural changes during annealing. The variation in wave velocity with annealing time exhibited a three stage behaviour. In the first stage, a slight increase in the velocity during recovery regime has been observed. This is followed by a significant increase in the velocity in the recrystallization regime (second stage) and reaching saturation on completion of recrystallization (third stage). Microstructural observations using optical metallography confirmed these three stages. The maximum variation in velocity is observed only when the polarization or the propagation direction of the shear wave is alingned with the cold working direction. Variation in shear wave velocity during annealing process, in general, is just opposite to that of the variation in longitudinal wave velocity. A number of velocity measurements were made by changing the propagation and polarization directions of the shear waves with respect to the cold working direction. Based on these measurements, a suitable velocity ratio parameter is suggested for determining the degree of recrystallization during annealing of cold worked alloy D9.     

The role of environmental exposure in the fatigue behaviour of an aluminium alloy

This paper reports measurements of fatigue crack velocities for an aluminium alloy 7017-T651 in high purity gaseous environments with different values of water vapour pressure and frequency. Also a detailed fractographical analysis by SEM is presented and the fracture surface morphology is correlated with the testing parameters and with the crack stress intensity level. Within a range of environmental exposure (defined as the product between the pressure and the time available for surface reaction during one fatigue cycle) crack propagation rates depend linearly on the exposure. The experimental fatigue crack propagation data are satisfactorily explained in terms of a corrosion-fatigue model for gaseous environments.     

Application of the ultrasonic characterization methods for highly attenuating plastic materials

In this paper, prediction of the back surface reflection, which is based on attenuation and phase velocity dispersion estimation method, in highly attenuating plastic (polyvinylidene fluoride—PVDF) has been described. Estimation of the attenuation law is based on the inverse transfer function of the object approximation in the frequency domain. The oscillating character of the inverse transfer function of the highly attenuating plastic material gives rise to an ill-posed problem for approximation. It has been solved in two ways: application of the Tikhonov regularization process and iterative adjustment of the approximation parameters. The estimated attenuation coefficient α₀ and power n have been used for calculation of phase velocity dispersion using the Kramers–Kronig relations and having attenuation coefficient measured at single-frequency value.The reconstructed waveforms of the ultrasonic back surface reflections using the estimated attenuation and phase velocity dispersion curves have been presented and compared with the experimental one.     

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S₀ and anti-symmetric A₀ waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A₀ mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A₀ mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.     

Lamb and SH waves generated and detected by air-coupled ultrasonic transducers in composite material plates

Electrostatic, air-coupled, ultrasonic transducers are used to generate and detect guided waves in anisotropic solid plates. Waves considered in this study are Lamb-type and SH-type, guided modes. If the plane of propagation coincides with a plane of symmetry of the material, then Lamb modes only are launched and detected by the transducers. If the plane of propagation does not coincide with a plane of symmetry of the material, then Lamb modes are still generated and detected, but guided, SH-like modes are, too. The variation of phase velocity with frequency is measured for several modes propagating in different directions along a glass–epoxy composite plate.A numerical model that takes into account the anisotropy of composite materials is developed to predict the dispersion curves (phase velocity, group velocity or wave-number versus frequency) and the displacement fields of plate waves, the plane of propagation being either a plane of symmetry or not.The experimental phase velocities are in good agreement with the predicted dispersion curves, thus showing that the forward problem concerning the propagation of plate waves in anisotropic, homogeneous, composite material plates is properly solved. The dispersion curves associated with the predicted displacement fields show that guided modes in composite plates have different behaviors depending on their direction of propagation.     

Symmetry analysis of magnetic structures with a wave vector k<Subscript>6</Subscript> = μb<Subscript>3</Subscript> possible in the intermetallic compound Ce<Subscript>2</Subscript>Fe<Subscript>17</Subscript>

To estimate the reliability of literature data on the magnetic structure of Ce₂Fe₁₇, we performed a symmetry analysis of the possible magnetic structures characterized by the wave vector k ₆ = μb ₃ = 2π(0, 0, π/c). The previously developed procedure for calculating the basis functions of the irreducible representations of the space group D_3d ⁵ (R[`3]m)(R\bar 3m), which enter into the magnetic representation with the above wave vector, is described briefly and is employed for an analysis. The results of the calculations of these basis functions for Fe atoms located in 6c, 9d, 18f, and 18h positions are given and discussed. A conclusion is made that in this intermetallic compound there can exist magnetic structures of the type of longitudinal or transverse spin waves or of the type of a complicated spiral with the magnetic moment which varies according to a harmonic law as the coordinate z of the site occupied by the Fe atom changes.     

Measurement of residual stresses in austenitic stainless steel weld joints using ultrasonic technique

Abstract

A methodology has been developed using a non-destructive ultrasonic technique for measuring surface/subsurface residual stresses in 7 mm thick AISI type 316LN stainless steel weld joints made by activated tungsten inert gas and multipass tungsten inert gas welding processes. Measurement of residual stresses using an ultrasonic technique is based on the effect of stresses on the propagation velocity of elastic waves. Critically refracted longitudinal L _CR wave mode was employed and accurate transit time measurements were made across the weld joints. Quantitative values of the longitudinal residual stresses across the weld joints were estimated from the measured transit times and predetermined value of acoustoelastic constant for AISI type 316LN stainless steel. The nature of the residual stress profiles and their variations across the two types of weld joints were compared and interpreted.     

Reflection and refraction of acoustic waves at the insulator-ferromagnetic heusler alloy boundary

Reflection and refraction of longitudinal and transverse acoustic waves on a flat boundary between an insulator and a ferromagnetic crystal of a Heusler alloy Ni_{2 + x + y} Mn_{1 − x} Ga_{1 − y} have been considered in the region of premartensitic and martensitic phase transitions. The possibility is shown of using temperature to effectively controlling the angles of wave refraction and the coefficients of transformation of wave modes by changing strong acoustic crystalline anisotropy induced with approaching the point of the phase transition. The conditions of the appearance of critical angles and the angles of total internal reflection, as well as the effect of temperature on them, have been considered. The possibility has been analyzed of emission, in the vicinity of the phase transition, of a wave that propagates along the boundary into the bulk of the material. Based on the experimental data obtained by Trivisonno for the temperature dependences of the sound velocity in the Ni_{2 + x + y} Mn_{1 − x} Ga_{1 − y} crystal, the conclusions of the theory developed are illustrated by numerical calculations for a concrete structure, namely, Ni₂MnGa-quartz.     

铝型材出口速度分布的数值模拟

铝型材挤压过程中的出口速度,是模具优化设计的重要参数之一,型材出口速度不均的原因之一是模具设计不当。为了获得高质量的挤压制品,对模具出口区域速度分布的研究是必须的。本文采用DEFORM3D对铝型材进行了3D模拟,获得了在模具制造之间铝型材的出口速度。数值模拟的结果得到了实验的验证,这对优化模具过程是非常重要的。     

Numerical analysis of shear deformation localization using a double-variable damage model

A double-variable damage model was introduced into the constitutive equations to demonstrate the effect of the material damage for the isotropic elastic, hardening, and damage states, and for the isothermal process. The shear damage variable D _s and the bulk damage variable D _b may be, respectively, used to describe the effect of shear damage and bulk damage for material properties without the superfluous constraint, D _b=D _s, that is found in the single-variable damage model. The double-variable damage model was implemented to form the finite element code for analyzing the effect of shear damage and bulk damage. In this article, two numerical simulation examples were completed to model the whole process of initiation and propagation of shear bands in an aluminum alloy. The numerical computational results are coincident with the experimental results.     

Influence of phase miscibility and morphology on crack resistance behaviour and kinetics of crack propagation of nanostructured binary styrene–(styrene/butadiene)–styrene triblock copolymer blends

The crack toughness behaviour of nanostructured polymer blends based on two SB triblock copolymers with different molecular architectures were studied, where one component is a thermoplastic (LN3) and the other a thermoplastic elastomer (LN4), using the essential work of fracture (EWF) concept. The crack resistance behaviour was correlated to the morphological features from transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) and to the phase miscibility characteristics as revealed from the nature of the tan δ peak in dynamic mechanical analysis curves. Swelling of the polystyrene-layer thickness with increasing LN3 content could be observed from the fast Fourier transform analysis of the TEM images. An increase in the crack toughness behaviour between 60 and 80 wt.% LN3 was observed and is attributed to a change from cylindrical to lamellar morphology, as revealed from the magnitude of non-EWF ( βw_p) and the crack resistance (R-curves) curves. The kinetics of stable crack propagation is discussed with respect to deformation mechanisms, post-yield crack-tip blunting and stable crack propagation behaviour. It was observed that the crack-tip opening displacement rate was more sensitive to phase miscibility. R-curves analysis, in this study, fundamentally establishes that, while crack initiation (increases linearly with the increase in LN3 content) and crack propagation (increases as the morphology changes from cylindrical to lamellar) are dependent on composition and morphology, respectively, the crack propagation stability (dδ/da) is highly sensitive to phase miscibility. The time-resolved crack propagation studies offer new dimensions to understand the kinetic aspects of fracture behaviour, while the strain field analysis explains the time-dependent deformation behaviour to characterize the time dependence of the strain energy dissipation modes.     

An Improved Mechanical Material Model for Ballistic Soda-Lime Glass

In our recent work (Grujicic et al., Int. J. Impact Eng., 2008), various open-literature experimental findings pertaining to the ballistic behavior of soda-lime glass were used to construct a simple, physically based, high strain rate, high-pressure, large-strain mechanical model for this material. The model was structured in such a way that it is suitable for direct incorporation into standard commercial transient non-linear dynamics finite element-based software packages like ANSYS/Autodyn (Century Dynamics Inc., 2007) or ABAQUS/Explicit (Dessault Systems, 2007). To validate the material model, a set of finite element analyses of the edge-on-impact tests was conducted and the results compared with their experimental counterparts obtained in the recent work of Strassburger et al. (Proceedings of the 23rd International Symposium on Ballistics, Spain, April 2007; Proceedings of the 22nd International Symposium on Ballistics, November 2005, Vancouver, Canada). In general, a good agreement was found between the computational and the experimental results relative to: (a) the front shapes and the propagation velocities of the longitudinal and transverse waves generated in the target during impact and (b) the front shapes and propagation velocities of a coherent-damage zone (a zone surrounding the projectile/target contact surface which contains numerous micron and submicron-size cracks). However, substantial computational analysis/experiment disagreements were found relative to the formation of crack centers, i.e. relative to the presence and distribution of isolated millimeter-size cracks nucleated ahead of the advancing coherent-damage zone front. In the present work, it was shown that these disagreements can be substantially reduced if the glass model (Grujicic et al., Int. J. Impact Eng., 2008) is advanced to include a simple macrocracking algorithm based on the linear elastic fracture mechanics.     

基于泄漏瑞利波的等离子喷涂涂层质量评价

采用瑞利波的特性评价了类金刚石涂层的质量.制备了3种试样,无涂层试样,完好试样,缺陷试样.得到了3种试样的瑞利波群速度频散曲线.结果表明,由于无涂层试样的群速度在不同频率下不变,表明该测量结果是可靠的.由化学气相沉积制备的类金刚石涂层的频散曲线表明,涂层对群速度有一定的影响,完好试样的群速度与缺陷试样的群速度随着频率的变化而变化,因此,泄漏瑞利波可以很好地作为评价类金刚石涂层质量的一种无损检测方法.     

变极性方波电源的换向与控制

首先分析了变极性方波电源二次逆变换向过程的特点 ,即开关器件切换的电压低 ,电流大 ,回路电感引起的续流过程在换向中起着重要的作用 ,它能提供适宜的反向驱动时间 ,或者促进再引弧电压形成。指出了反向驱动延迟时间td 并不等于电弧电流间断时间tD(即熄弧时间 )的实际情况。归纳出td>0、td<0两种换向控制方法 ,深入分析了两种控制方式下的换向过程机理 ,得出了相应的换向控制策略 ,并给出了实现方法及试验结果。最后指出了两种控制方法的不同特点及适用范围。从而深化了方波电源的换向理论 ,为改善这种电源的工作性能和可靠性 ,为其合理的设计 ,提供了进一步的依据。     

Study on the structure and surface acoustic wave properties of SNGS and STGS piezoelectric crystals

The geometry structure, band structure and density of states of Sr₃NbGa₃Si₂O₁₄ (SNGS) and Sr₃TaGa₃Si₂O₁₄ (STGS) single crystals were investigated by using density functional theory (DFT) method. Parameters including the phase velocity, electromechanical coupling factor and power flow angle (PFA) were calculated for SNGS and STGS at X, Y, Z cuts, respectively. Our calculated data are in good agreement with the experimental results. It is found that the peaks of partial density of states of Sr, Ga, Si and O atoms of STGS have a tendency of shifting to the higher energy levels relative to those of SNGS. Compared with quartz, SNGS and STGS crystals are of lower phase velocities and higher electromechanical coupling factor. The propagation direction of 167.5° of X-cut SNGS, 0° of Y-cut SNGS and 170° of X-cut STGS are found to be optimum direction for SAW device applications.