Development of a new ultrasonic actuator based on Langevin bending transducer

A new ultrasonic actuator based on a bending-type Langevin piezoelectric transducer has been developed. An actuator consists of a half-wavelength Langevin transducer and an aluminum plate that is mounted in the middle of the transducer. The driving tip is located at the bottom of the aluminum plate. Piezoceramic rings with opposite polarity within each half of the ring are divided into two groups. Two harmonic signals with shifted phases by π/2 are used to excite the first bending mode of the Langevin transducer and aluminum plate. Elliptical trajectory of the contact point motion is obtained by combining these two bending modes. Numerical modelling was carried out to analyze harmonic response of the actuator and to investigate the trajectories of the contact point motion. Optimization of the aluminum plate design was performed to maximize the parameters of elliptical motion of the driving tip. A prototype actuator was fabricated, and output characteristics were measured. The results of numerical and experimental study are discussed.     

Four-Point Bending Test of Determining Stress-Strain Curves Asymmetric between Tension and Compression

A method of determining both uniaxial tension and compression stress-strain curves from the result of a single four-point bending test was demonstrated. Stress-strain curves of magnesium showing tension-compression asymmetry due to twinning deformation and those of an S45C steel due to the Bauschinger effect were calculated. The Mayville-Finnie equation was modified slightly for this calculation. The calculation is sensitive to small change in the slope of bending curve, revealing an aspect of inverse problem.     

Size effects and dislocation patterning in two-dimensional bending

We perform atomistic Monte Carlo simulations of bending a Lennard-Jones single crystal in two dimensions. Dislocations nucleate only at the free surface as there are no sources in the interior of the sample. When dislocations reach sufficient density, they spontaneously coalesce to nucleate grain boundaries, and the resulting microstructure depends strongly on the initial crystal orientation of the sample. In initial yield, we find a reverse size effect, in which larger samples show a higher scaled bending moment than smaller samples for a given strain and strain rate. This effect is associated with source-limited plasticity and high strain rate relative to dislocation mobility, and the size effect in initial yield disappears when we scale the data to account for strain rate effects. Once dislocations coalesce to form grain boundaries, the size effect reverses and we find that smaller crystals support a higher scaled bending moment than larger crystals. This finding is in qualitative agreement with experimental results. Finally, we observe an instability at the compressed crystal surface that suggests a novel mechanism for the formation of a hillock structure. The hillock is formed when a high angle grain boundary, after absorbing additional dislocations, becomes unstable and folds to form a new crystal grain that protrudes from the free surface.     

动载实验中压电晶体应力量计的设计与应用

各种压电器件广泛用于电子工业、信息传输、医学诊断等许多领域,以实现能量转换、传感、驱动、频率控制等功能. 特别地,压电晶体以其高频响的压电效应,在动载实验中用于动态应力的测试. 利用其特有的各向异性特性,通过设计特殊的切型方向,可以实现对不同应力的测试. 本文通过分析各向异性晶体的特性,从压电效应的基本原理出发,导出了利用各向异性晶体作为剪应力量计的设计思想,并给出了两种典型的动态剪应力计:17.705°Y 切石英和165.44°Y 切铌酸锂晶体.      

The evolution of crystalline stresses of a polycrystalline metal during cyclic loading

The presence of a positive average applied stress during cyclic uniaxial loading leads to a reduction in fatigue life of metallic parts. The metals are typically polycrystalline, with stresses varying from crystal to crystal due to differences in lattice orientation and slip system strength. Simulations enable us to better understand how polycrystals behave under cyclic loading and how the changing stress over many cycles influences fatigue life. Specifically, uniaxial cyclic simulations of pre-strained HY100 steel were conducted using an elastic viscoplastic continuum slip model employing a Taylor hypothesis. Stress-controlled loading conditions were employed to mimic fatigue tests on cold-bent bar specimens for three different load levels. The macroscopic axial strains and the crystal axial stresses were monitored during the cycles. The stress–strain response for the first cycle was used to determine the load input for the material point simulations. The peak values of crystal axial stress were found to evolve continuously with the number of loading cycles. It was found that the stress change in a crystal is influenced not only by its own orientation but also by the orientations of the other crystals in the aggregate. Furthermore, the distribution of crystal stresses after thousands of cycles at a lower stress amplitude closely resembled the distribution after tens of cycles at a larger stress amplitude.     

DESIGN AND APPLICATIONS OF PIEZOELECTRIC CRYSTALS TRANSDUCER IN DYNAMIC EXPERIMENTS

各种压电器件广泛用于电子工业、信息传输、医学诊断等许多领域,以实现能量转换、传感、驱动、频率控制等功能. 特别地,压电晶体以其高频响的压电效应,在动载实验中用于动态应力的测试. 利用其特有的各向异性特性,通过设计特殊的切型方向,可以实现对不同应力的测试. 本文通过分析各向异性晶体的特性,从压电效应的基本原理出发,导出了利用各向异性晶体作为剪应力量计的设计思想,并给出了两种典型的动态剪应力计:17.705°Y 切石英和165.44°Y 切铌酸锂晶体.     

Microscale deformation of a tempered martensite ferritic steel: Modelling and experimental study of grain and sub-grain interactions

In this paper, a finite-element modelling framework is presented with explicit representation of polycrystalline microstructure for a tempered martensite ferritic steel. A miniature notched specimen was manufactured from P91 steel with a 20,000 h service history and tested at room temperature under three point bending. Deformation at the microscale is quantified by electron back scattered diffraction (EBSD) before and after mechanical loading. A representative volume element was developed, based on the initial EBSD scan, and a crystal plasticity model used to account for slip-based inelastic deformation in the material. The model showed excellent correlation with the experimental data when the relevant comparisons were made.     

Ultrasound thermometry in transparent and opaque fluids

We have exploited the temperature dependence of sound velocity to measure the thermal fields in transparent and opaque fluids. A chamber containing glycerol undergoing Rayleigh–Bénard convection was probed with an ultrasound transducer operating in the pulse-echo mode. The times-of-flight for the ultrasound pulse to traverse the fluid at several transducer locations were converted into a temperature profile that is in qualitative agreement with simultaneous thermochromic liquid crystal visualization of the flow pattern. Temperature profiles in a mercury-filled stainless steel chamber have also been obtained, both for quiescent and turbulent flows, thereby validating the ultrasound thermometry concept for opaque fluids as well.     

固定结构下的两向力及弯矩传感器设计

为了刀板切削过程中的受力满足最合理的设计准则,专用传感器设计须基于固定总体结构,对局部结构优化,以达到设计合理的目的。刀板切削时受到土体的阻力可分解为水平力和垂直力,多维力传感器测量了两个力的大小和对传感器产生的弯矩。刀板工作中,弯矩引起的正应力比垂直力引起的拉应力及水平力引起的切应力要大得多,因此特别设计了弹性体的局部结构使得三个应力值接近于同一量级。传感器的标定结果显示,输入输出有良好的线性关系,并且很好地消除了耦合效果。最终,将该传感器应用于土体切削测试并验证了其可靠性。     

On the Accuracy of Elastic Strain Field Measurements by Laue Microdiffraction and High-Resolution EBSD: a Cross-Validation Experiment

Determining the accuracy of elastic strain measurements in plastically deformed alloys is an experimental challenge. To develop a novel cross-validation procedure, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation. The corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situ by High Resolution EBSD. Good agreement is found for the two independent measurements and the predictions of a mechanical model, at plastic strains below 0.5 %. The accuracy of the measurements is estimated at 3.2 × 10^{? 4}.     

Dynamic instabilities in {1 1 1} silicon

The phenomena occurring during rapid crack propagation in brittle single crystals was studied by cleaving strip-like silicon specimens along the {1 1 1} low-energy cleavage plane under bending. The experiments reveal phenomena associated with rapid crack propagation in brittle single crystals not previously reported, and new crack path instabilities in particular. In contrast to amorphous materials, the observed instabilities are generated at relatively low velocity, while at high velocity the crack path remains stable. The experiments demonstrate that crack velocity in single crystals can attain the theoretical limit. No evidence for mirror, mist, and hackle instabilities, typical in amorphous materials, was found. The important role played by the atomistic symmetry of the crystals on controlling and generating the surface instabilities is explained; the importance of the velocity and orientation-dependent cleavage energy is discussed. The surface instabilities are generated to satisfy minimum energy dissipation considerations. These findings necessitate a new approach to the fundamentals of dynamic crack propagation in brittle single crystals.     

PROPAGATION CHARACTERISTICS OF HIGH ORDER LONGITUDINAL MODES IN STEEL STRANDS AND THEIR APPLICATIONS

Propagation characteristics of high order longitudinal modes of ultrasonic guided waves in seven-wire steel strands are investigated theoretically and experimentally. According to these analysis results, proper longitudinal modes are selected for defect detection in steel strands. Dispersion curves for helical and central wires in a 17.80 mm nominal diameter seven-wire steel strand are numerically obtained firstly, and propagation characteristics of high-order longitudinal modes, such as wave structures, attenuation and dispersion, are analyzed. In experiments, the signals of ultrasonic guided wave at different high frequencies are excited and received at one end of a steel strand by using the same single piezoelectric transducer. The identification of longitudinal modes in the received signals is achieved based on short time Fourier transform. Furthermore, appropriate L（0, 5） mode at 2.54 MHz is chosen for detecting an artificial defect in a helical wire of the steel strand. Results show that high order longitudinal modes in a high frequency range with low dispersion and attenuation whose energy propagates mainly in the center of the wires can be used for defect detection in long range steel strands.     

混凝土框架结构内力测量传感器研制

研制了一种可用于混凝土结构试验中测量柱底截面轴向力、剪力和弯矩的内力测量传感器。该传感器主要由加载板、基座、4根竖向测力杆和2根水平测力杆组成。使用时,柱底截面的内力通过加载板传递给6根测力杆,根据6根测力杆的结果,通过力平衡方程可以计算实际加载的轴向力、剪力和弯矩的大小。对该传感器进行了4种工况下的标定试验。结果表明,传感器的测量误差满足框架结构内力测量精度要求。将该传感器应用到单层两跨的钢筋混凝土框架推覆试验中,获得了整个试验过程中构件的内力时程。     

Dispersive bending waves in uniform bars

The traveling bending waves in a long beam of rectangular cross section were measured and calculated. The bending waves were induced by impacting with a steel sphere and measured with strain gages at several distances from the point of impact. The impact force was calculated as a function of time by integrating the dynamic equations of the sphere and the beam. The force spectrum was then found using a fast-Fourier-transform (FFT) calculation and multiplied by the moment-frequency response of the beam to determine the moment spectrum. The moment-time function was calculated by an inverse FFT. The traveling wave is dispersive; its spectrum was found from that at the point of impact by phase shifting each component by an angle proportional to the distance and to the square root of the frequency. Again the time curve was determined by an inverse FFT. The indentation stiffness of the beam was found to be very much less than that of the elastic half space because of transverse bending. After the impact force was recalculated with this correction, the calculated moment-time traces agreed very well with the measured ones.     

Q345钢和H62黄铜声发射信号及力学行为的关系研究

本文利用声发射装置采集了Q345钢和H62黄铜两种材料拉伸过程中的动态声信号。结果发现两种材料在线弹性阶段末端都会产生大量声发射信号,但有明显屈服流动的Q345钢在硬化阶段的开始时段也会产生信号峰值。为了研究两种材料在拉伸变形过程中声发射的微观机制,本文采用扫描电镜下原位拉伸实验方式观察了两种材料在不同拉应力下晶粒内部滑移线的产生和演变过程。在此基础上总结出了两种材料声发射信号与力学行为的关系,指出应以第一次声发射信号的峰值对应的工作应力作为材料屈服应力及其合理性,并按此标准给出H62黄铜的屈服应力为193MPa。     

Microbuckle initiation from a patch of large amplitude fibre waviness in a composite under compression and bending

A finite element couple stress formulation is used to predict microbuckle initiation from a patch of fibre waviness in a unidirectional fibre composite under remote compression and bending. Attention is focused on the knock-down in strength due to large amplitude waviness, with the effects of the physical size of the imperfection included by incorporating the fibre bending resistance within the formulation. The predicted strengths deviate significantly from the simpler kinking theory which neglects the role of fibre bending. Initial imperfections in the form of an infinite band and a circular wavy patch are considered: when these imperfections are of large spatial extent and possess a large misalignment angle, the compressive strength approximates the steady state band broadening stress for an infinite band. The effect of an imposed spatial gradient of stress within the composite is explored by determining the compressive strength of beams of finite height B for the loading cases of pure bending and axial compression. It is found that the compressive strength is sensitive to the magnitude of the imposed stress gradient: the compressive strength of the outer fibres of the beam in bending increases with diminishing height of the beam. This size dependence is much reduced for the case of uniform compression.     

Velocity measurements in liquid sodium by means of ultrasound Doppler velocimetry

A successful application of ultrasound Doppler velocimetry in liquid sodium flows is described. To obtain sufficient Doppler signals, different problems had to be solved: the transmission of the ultrasonic beam through the channel wall made of stainless steel, the acoustic coupling between the transducer and the channel wall, and the wetting of the inner surface of the wall by the liquid metal, respectively. A sodium flow in a square duct exposed to a transverse magnetic field is investigated. In accordance with the existing knowledge about MHD channel flows, we found that the velocity profiles modified to a M-shape owing to the effect of an inhomogeneous magnetic field. Received: 12 June 2001/Accepted: 27 October 2001     

On the strain hardening and texture evolution in high manganese steels: Experiments and numerical investigation

We present a systematic investigation on the strain hardening and texture evolution in high manganese steels where twinning induced plasticity (TWIP) plays a significant role for the materials' plastic deformation. Motivated by the stress–strain behavior of typical TWIP steels with compositions of Fe, Mn, and C, we develop a mechanistic model to explain the strain-hardening in crystals where deformation twinning dominates the plastic deformation. The classical single crystal plasticity model accounting for both dislocation slip and deformation twinning are then employed to simulate the plastic deformation in polycrystalline TWIP steels. While only deformation twinning is activated for plasticity, the simulations with samples composed of voronoi grains cannot fully capture the texture evolution of the TWIP steel. By including both twinning deformation and dislocation slip, the model is able to capture both the stress–strain behaviors and the texture evolution in Fe–Mn–C TWIP steel in different boundary-value problems. Further analysis on the strain contributions by both mechanisms suggests that deformation twinning plays the dominant role at the initial stage of plasticity in TWIP steels, and dislocation slip becomes increasingly important at large strains.     

含能单晶微纳米力学性能试验研究及数值表征

利用微纳米压痕实验测定β-HMX 单晶(010) 晶面和α-RDX 单晶(210) 晶面的力学性能参数和微观破坏特征,并利用数值拟合确定了含能单晶的部分本构参数. 通过微纳米压痕实验连续刚度法(CSM) 得到HMX 单晶和RDX 单晶的弹性模量和硬度,RDX 单晶的硬度和模量都大于HMX 单晶,其硬度值均表现出一定的尺寸效应. 利用原子力显微镜(AFM) 分析了HMX 单晶和RDX 单晶的微观破坏机理,裂纹随着载荷的增大生成并扩展,裂纹面产生方向为晶体的最易解理破坏方向. 利用ABAQUS 有限元软件进行了纳米压痕数值模拟,结合微纳米压痕实验加卸载曲线,选取了合适的含能单晶塑性损伤本构模型的损伤本构参数.      

高强钢波纹夹芯结构的力学性能研究

本文利用ABAQUS仿真软件分析了高强钢波纹夹芯结构在三点弯曲以及面内压缩时的力学响应。将计算结果与实验值和理论值进行了对比,验证了数值模拟的准确性。分析了芯板材料属性（高强钢DP900和普通低碳钢DC01）以及波纹芯板排列方向(横向、纵向)对夹芯结构力学性能的影响。结果表明DP900高强钢波纹芯板夹芯结构抗弯强度是DC01低碳钢夹芯结构的2.39倍,抗压强度是低碳钢夹芯结构的1.40倍,纵向波纹夹芯结构比横向波纹夹芯结构弯曲刚度高11.63倍。