Elastic constants measurement of anisotropic Olivier wood plates using air-coupled transducers generated Lamb wave and ultrasonic bulk wave

A hybrid elastic wave method is applied to determine the anisotropic constants of Olive wood specimen considered as an orthotropic solid. The method is based on the measurements of the Lamb wave velocities as well as the bulk ultrasonic wave velocities. Electrostatic, air-coupled, ultrasonic transducers are used to generate and receive Lamb waves which are sensitive to material properties. The variation of phase velocity with frequency is measured for several modes propagating parallel and normal to the fiber direction along a thin Olivier wood plates. A numerical model based mainly on an optimization method is developed; it permits to recover seven out of nine elastic constants with an uncertainty of about 15%. The remaining two elastic constants are then obtained from bulk wave measurements. The experimental Lamb phase velocities are in good agreement with the calculated dispersion curves. The evaluation of Olive wood elastic properties has been performed in the low frequency range where the Lamb length wave is large in comparison with the heterogeneity extent. Within the interval errors, the obtained elastic tensor doesn’t reveal a large deviation from a uniaxial symmetry.     

Ultrasonic SH wave velocity in textured aluminium plates

Neutron diffraction has been used to measure the crystallite orientation distribution function for a sample of rolled aluminium plate. Based on this study an expression is given for the ultrasonic velocity of the fundamental horizontally polarized shear (SH) wave propagating in the plane of the plate at an angle θ to the rolling direction. This expression is correct to second order in the leading texture component and is compared with the exact numerical result and with measurements of the SH wave velocity for this plate. In textured materials the group and phase velocities are not, in general, parallel and an expression for the angle between them is derived. Inclusion of this effect leads to better agreement between experiment and theory. The results have application to the texture-independent determination of stress with ultrasonics recently proposed by MacDonald and Thompson et al.     

Effects of texture,temperature and strain on the deformation modes of zirconium

Clock-rolled, high-purity, textured polycrystalline zirconium exhibits significant plastic anisotropy for compression along the through-thickness and in-plane directions and strong temperature dependence of flow stress for both orientations. Orientation imaging microscopy in a scanning electron microscope and defect analysis via transmission electron microscopy are used to characterize the defect microstructures as a function of initial texture, deformation temperature and plastic strain. The observed deformation mechanisms are correlated with the measured mechanical response.     

Identification and analysis of multimode guided waves in tibia cortical bone

The hollow cylinder model filled with viscous-liquid is performed to simulate tibial bone shape in this paper. Short time Fourier transform (STFT) was applied to identify and analyze guided wave modes of a series of ultrasonic signals acquired at various transmitter-receiver positions for a fixed transmitter. Experimental results were compared with theoretical simulation. The high correlation values for group velocity were obtained between results of experiment and theory for L(0,2) (r=0.89, p<10(-5)), and L(0,3) (r=0.87, p<10(-5)). The experimental group velocities correlated strongly with cortical thickness (cortTh) for L(0,2) (r(2)=0.79, p<10(-5)) and L(0,3) (r(2)=0.74, p<10(-5)) at 0.5 MHz center frequency. The results showed that the STFT is an effective method to identify the propagating modes and derive dispersion information. The experimental results were in good agreement with theoretical predictions. This study suggests that the guided wave L(0,2) mode is a promising mode to assess change in cortical thickness.     

Structural mechanisms of plastic deformation in nanocrystalline materials

A model of the initial stage of plastic deformation in nanomaterials is proposed. Within this model, the plastic deformation occurs through grain boundary microsliding (GBM). The accommodation processes accompanying the formation of GBM regions are considered. The relationships describing the regularities in the deformation behavior of nanomaterials and the dependence of the flow stress on the grain size are derived, and the temperature dependence of the GBM resistance stress is calculated. It is demonstrated that the results obtained are in good agreement with the experimental data.     

Acoustic wave transmission through piezoelectric structured materials

This paper deals with the transmission of acoustic waves through multilayered piezoelectric materials. It is modeled in an octet formalism via the hybrid matrix of the structure. The theoretical evolution with the angle and frequency of the transmission coefficients of ultrasonic plane waves propagating through a partially depoled PZT plate is compared to finite element calculations showing that both methods are in very good agreement. The model is then used to study a periodic stack of 0.65PMN-0.35PT/0.90PMN-0.10PT layers. The transmission spectra are interpreted in terms of a dispersive behavior of the critical angles of longitudinal and transverse waves, and band gap structures are analysed. Transmission measurements confirm the theoretical calculations and deliver an experimental validation of the model.     

Shape effect of elongated grains on ultrasonic attenuation in polycrystalline materials

Longitudinal and transverse wave attenuation coefficients are obtained in a simple integral form for ultrasonic waves in cubic polycrystalline materials with elongated grains. Dependences of attenuation on frequency and grain shape are described in detail. The explicit analytical solutions for ellipsoidal grains in the Rayleigh and stochastic frequency limits are given for a wave propagating in an arbitrary direction relative to ellipsoid axes. The attenuation exhibits classic frequency dependence in those frequency limits. However, the dependence on the grain shape in the stochastic limits is unexpected: it is independent of the cross-section of the ellipsoidal grains and depends only on the grain dimension in the propagation direction. In the Rayleigh region attenuation is proportional to effective volume of the ellipsoidal grain and is independent of its shape. A complex behavior of attenuation on the grain shape/size and frequency is exhibited in the transition region. The results obtained reduce to the classic dependences of attenuation on parameters for polycrystals with equiaxed grains.     

考虑晶粒分布的多晶体材料超声散射统一理论

现有超声散射统一理论可通过多晶体材料的微观结构和力学特性,实现全频域范围内衰减和相速度的正演建模,但其忽略晶粒尺寸分布的影响,进而降低了正演模型的计算精度.本文对不均匀介质的波动方程进行二阶Keller近似,用全频域格林函数推导介质中的平均波;以截断对数正态分布描述晶粒分布,构建加权的空间相关函数;结合材料的弹性模量协方差,建立含晶粒分布的超声散射统一理论,揭示晶粒分布对超声散射的影响规律;制备304不锈钢试块并开展超声散射实验.结果表明考虑晶粒分布特性后,纵波衰减谱和相速度谱相对于实验结果的相异性降低约49%和64%,横波衰减谱和相速度谱相对于实验结果的相异性降低约12%和4%.可见,本文的统一理论模型能有效修正晶粒分布导致的衰减谱和相速度谱偏差,为晶粒分布反演评价提供理论基础.     

Nonlinear dynamics of a domain wall in the field of a sound wave propagating in the plane of the wall

A theoretical model is proposed for describing the drift of a 180° domain wall in a weak ferromagnet in the field of elastic stresses produced by a strong sound wave propagating in the plane of the wall. The dependence of the drift velocity on the frequency, amplitude, and polarization of the sound wave is found. It is predicted that drift of a striped domain structure is possible.     

Temperature-dependent susceptibility in ALON

Herein, we propose a stochastic model of the complex susceptibility in aluminum nitride (ALON), a polycrystalline transparent ceramic. The proposed model yields an inverse power-law dependence of the dielectric loss tangent on frequency, in remarkably close agreement with data. In addition, the phenomenological parameters are found to be strongly temperature-dependent. This temperature dependence is determined, in the theoretical model, to be a consequence of the thermodynamic properties of the molecular dipoles in the material.     

无缝钢轨中温度对应力非线性超声检测的影响

研究了温度与应力共同作用下的非线性超声响应规律。构建了纵波在无缝钢轨内传播的数学模型,将传统原子间势能引入非线性波动方程,得到了无缝钢轨在自由状态及受约束状态两种情况下超声非线性系数随温度变化的函数关系。在理论分析的基础上,进行了无缝钢轨受约束时内部温度应力变化的实物模拟实验,获得的非线性系数变化趋势与理论预期相一致。研究结果表明:温度和应力都对固体介质非线性影响明显,介质非线性系数随着温度的升高而增大,随着内部压应力的增加而减小。因此,当利用非线性系数检测钢轨纵向应力时,需要对温度的影响进行补偿。      

激光等离子体及点爆炸空气冲击波波前运动方程的研究

根据激光等离子体空气冲击波在自由空间中传播的约束条件导出的空气冲击波前运动方程与点爆炸波泰勒解进行比较,证明泰勒解是该冲击波波前传播方程在中场的近似形式.并将该冲击波运动方程与强爆炸空气冲击波的实验进行比较,计算结果与实验数据符合得非常好 关键词： 激光 核爆炸 等离子体冲击波     

固支方板对水中爆炸作用的动态响应研究

 分析研究了在装药水中爆炸产生的强冲击载荷作用下固支方板的塑性动力响应过程。通过对水中爆炸理论和板的大变形理论分析，应用能量原理和Lagrangian函数，推导出反映固支方板在冲击载荷作用下变形的最终挠度的解析解。水中爆炸对固边方板破坏的实验在自制的水箱中进行。对不同的装药重量和炸距情况下方板的变形情况进行了理论计算，并对实验结果和误差来源进行了分析。实验结果和理论结果具有较好的一致性。     

Experimental and numerical studies of nonlinear ultrasonic responses on plastic deformation in weld joints

The experimental measurements and numerical simulations are performed to study ultrasonic nonlinear responses from the plastic deformation in weld joints. The ultrasonic nonlinear signals are measured in the plastic deformed30Cr2Ni4 Mo V specimens, and the results show that the nonlinear parameter monotonically increases with the plastic strain, and that the variation of nonlinear parameter in the weld region is maximal compared with those in the heat-affected zone and base regions. Microscopic images relating to the microstructure evolution of the weld region are studied to reveal that the change of nonlinear parameter is mainly attributed to dislocation evolutions in the process of plastic deformation loading. Meanwhile, the finite element model is developed to investigate nonlinear behaviors of ultrasonic waves propagating in a plastic deformed material based on the nonlinear stress–strain constitutive relationship in a medium. Moreover, a pinned string model is adopted to simulate dislocation evolution during plastic damages. The simulation and experimental results show that they are in good consistency with each other, and reveal a rising acoustic nonlinearity due to the variations of dislocation length and density and the resulting stress concentration.     

Relaxation and resonance ultrasound attenuation by Jahn-Teller centers in a GaAs:Cu crystal

The interaction of ultrasound with Cu_Ga4As in a GaAs:Cu crystal has been experimentally studied. The temperature dependences of the attenuation of all normal ultrasonic modes propagating in the ??110?? direction both in doped copper and in nominally pure gallium arsenide crystals have been measured. In the GaAs:Cu crystal, the attenuation peak has been revealed for a transverse wave polarized along the ??110?? axis whose elastic shifts correspond to the symmetry of the tetragonal mode of the Jahn-Teller effect. The temperature dependence of the attenuation of this wave indicates that two types of attenuation??relaxation and resonance??occur. The constructed temperature dependence of the relaxation time indicates that tunneling through the potential barrier between the minima of the adiabatic potential energy is the main relaxation mechanism at temperatures below 10 K. Tunneling splitting estimated from experimental data is in good agreement with the theoretical estimate.     

Nanocrack nucleation in polycrystalline silicon during grain-boundary sliding

A theoretical model is proposed to describe nanocrack nucleation in polycrystalline silicon. In terms of this model, nanocrack nucleation is stimulated by grain-boundary sliding, which creates sources of local stresses in triple junctions of grain boundaries. The relaxation of these local stresses is the main driving force of nanocrack nucleation near triple junctions in polycrystalline silicon, in which grain-boundary sliding contributes substantially to plastic deformation under cyclic loading at room temperature. The model is used to calculate the critical external stress required for nanocrack nucleation in polycrystalline silicon.     

Development of an advanced multimode automatic ultrasonic texture measurement system for laboratory and production line application

We present work on the development of an ultrasonic texture measurement system for sheet metals using non-contact transducers, suitable for use both in the laboratory and on the production line. Variation of the velocity of the zero-order symmetric (S0) Lamb wave is used to determine the crystallographic texture of polycrystalline metal sheets ranging in thickness from 0.1 to 3 mm. This system features improvements on previous state-of-the-art ultrasonic technology in that it probes velocity over a continuous range of angles using only two electromagnetic acoustic transducers (EMATs). This is demonstrated to offer a significant improvement in accuracy and allows the detection and investigation of asymmetric anisotropies in the sheets. Another advantage of the system is its potential for combining several different measurements using a single pair of transducers. The capability is demonstrated for through-thickness shear wave measurements as well as the zero-order symmetric Lamb wave measurements which are the primary means of determining the texture. The change between generating Lamb and through-thickness bulk waves can be made entirely by changing the electrical circuit connected to the EMATs without modifying the transducer assembly in any way. Measurement of all of the above waves can provide information on the sheet thickness and other physical properties of the sheet in addition to texture. Certain texture parameters can be calculated from both Lamb and shear wave velocities, allowing self-calibration of the system.     

Analysis of laser-generated ultrasonic force source at specimen surface and display of bulk wave in transversely isotropic plate by numerical method

Taking into account the effects of thermal diffusion and optical penetration, as well as the finite width and duration of the laser source, the laser-generated ultrasonic force source at surface vicinity is presented. The full acoustic fields of laser-generated ultrasonic bulk wave are obtained and displayed in transversely isotropic plate. The features of laser-generated ultrasound bulk waves are analyzed. The features of laser-generated ultrasonic bulk wave are in good agreement with the theoretical results (the phase velocity surfaces), demonstrating the validity of this simulation. The numerical results indicate that the features of laser-generated ultrasound waveforms in anisotropic specimen, different from the case in isotropic materials, have a close relation with the propagating plane and propagation direction. This method can provide insight to the generation and propagation of laser-generated ultrasonic bulk wave in transversely isotropic material.     

Modeling the strain rate-dependent constitutive behavior in nanotwinned polycrystalline metals

Experimental studies have demonstrated that both strain rate and temperature influence the mechanical behavior of nanostructured metals significantly. In this work, a theoretical model is developed to describe the strain-rate-dependent constitutive behavior of nanotwinned polycrystalline metals. The athermal flow stress and thermal-activated flow stress are both considered in modeling the plastic deformation of a nanotwinned metal. Numerical results are consistent with the experimental results, showing that the present model can well describe the strain rate-dependent deformation behavior of nanotwinned polycrystalline copper. Henceforth, the constitutive behaviors of nanotwinned copper at different strain rates and temperatures can be predicted, which will be useful for optimizing the dynamic mechanical properties at various temperatures for nanotwinned metals.