A New Testing Machine for the Dynamic Characterization of High Strength Low Damping Fiber Materials

This work describes a novel method for measuring the damping, the elastic modulus and the non-linear behavior of high strength low damping fiber materials such as para-aramids, silicon carbide (SiC) and carbon. The method is based on resonant response characterization of a spring-mass system excited by a sine-wave forcing term which is applied as a vertical force to the suspended mass. The damping is obtained from the measured resonance quality factor Q, the elasticity modulus is calculated from the resonance frequency, and the non-linear coefficient is obtained with the backbone approach from resonance profile variations as a function of the forcing term amplitude. It is argued that the method is very sensitive, to the point that a maximum excitation amplitude of the order of a few percent of resistance is sufficient to obtain an estimate of the non-linear coefficient. This claim is supported by experimental results. A testing machine is also discussed, which provides the necessary sensitivity at such small excitation amplitudes and the capability of evaluating very small damping values, as expected in high strength low damping fiber materials. The sensitivity is guaranteed by an optical position sensor with sub-micron resolution. To evaluate small damping values, particular care has been taken to ensure that energy dispersions in the generator are much smaller than energy dispersions in the fibers themselves. Examples of dynamic characterization are shown for para-aramid, silicon carbide, and carbon fibers.     

A Kolsky Torsion Bar Technique for Characterization of Dynamic Shear Response of Soft Materials

A novel Kolsky torsion bar technique is developed and successfully utilized to characterize the high strain rate shear response of a rate-independent end-linked polydimethylsiloxane (PDMS) gel rubber with a shear modulus of about10 KPa. The results show that the specimen deforms uniformly under constant strain rate and the measured dynamic shear modulus follows reasonably well the trend determined by dynamic mechanical analysis (DMA) at lower strain rates. For comparison, Kolsky compression bar experiments are also performed on the same gel material with annular disk specimens. The dynamic moduli obtained from compression experiments, however, are an order of magnitude higher than those obtained by the torsional technique, due to the pressure caused by the radial inertia and end constraints.     

An Experimental Technique for the Dynamic Characterization of Soft Complex Materials

We describe an experimental technique to study the dynamic behavior of complex soft materials, based on high-speed microscopic imaging and direct measurements of dynamic forces and deformations. The setup includes high sensitivity dynamic displacement measurements based on geometric moiré interferometry and high-speed imaging for in-situ, full-field visualization of the complex micro-scale dynamic deformations. The method allows extracting dynamic stress-strain profiles both from the moiré interferometry and from the high-speed microscopic imaging. We discuss the advantages of using these two complementing components concurrently. We use this technique to study the dynamic response of vertically aligned carbon nanotube foams subjected to impact loadings at variable deformation rates. The same technique can be used to study other micro-structured materials and complex hierarchical structures.     

Comparison of Low Impedance Split-Hopkinson Pressure Bar Techniques in the Characterization of Polyurea

The split-Hopkinson pressure bar (SHPB) technique has been widely employed for over fifty years in characterizing the high strain-rate properties of many common engineering materials. Historically, however, this technique has had limited success in characterizing soft materials, since their low mechanical impedances can increase delays in attaining dynamic equilibrium and result in transmission pulses with extremely low signal-to-noise ratios. Due to interest in improving characterization of soft materials at high strain rates, numerous modifications to the traditional SHPB technique have been proposed. These include: using more sensitive piezoelectric gauges, employing hollow transmission bars, utilizing lower impedance polymeric pressure bars, and the use of pulse shaping techniques. To date, there has been no comparative studies or consensus within the SHPB community as to which approach is most advantageous. The goal of this investigation is to compare a number of these techniques, specifically the use of PMMA pressure bars and a hollow aluminum transmission bar (both with and without pulse shaping), alongside more traditional solid aluminum pressure bars in the characterization of polyurea, a common low impedance polymer. The advantages and disadvantages of each technique in generating high strain-rate stress-strain curves are discussed.     

一种新的高应变率复合压剪实验技术

提出了一种新的基于Hopkinson杆实验技术的在102~103s-1高应变率下实现压剪复合加载的实验装置,并给出了相应的理论分析和数值模拟。为了获取材料在复杂应力下的本构关系,借助斜飞片冲击实验的思想,对Hopkinson杆进行改造,将入射杆的末端改进为截锥形,以便在试样中同时产生压缩和剪切应力。利用有限元分析软件LS-DYNA对试样中的应力波传播进行模拟计算,并利用改进装置进行了初步实验。计算和分析结果表明,利用所设计的装置可以实现对试样的动态压剪复合加载,获得材料在高应变率复杂应力加载下的本构响应,进而建立材料在复杂应力状态下本构行为的描述。     

高温高应变率下材料拉伸性能测试的一种新方法

为了实现高温环境下材料高应变率动态拉伸实验技术,将分离式Hopkinson杆直接拉伸装置中试样与拉杆的螺纹连接形式变成楔形连接形式,并加装了气动同步装置系统。这样,在对试样加高温时,能使靠近试样的入射和透射杆端处于较低温度。当撞击管向传递法兰运动时,气动同步装置瞬间拖动透射杆和试样,使两者之间的间隙为零,此时沿入射杆传递的入射波同时对试样拉伸加载。经实验验证,此方法可以有效实现材料高温高应变率拉伸加载。     

建筑构件动态检测新技术探索

介绍用自行研制的单片微处理机控制的振动波时差采集显示仪进行动态无损检测建筑构件质量的新技术,该检测仪结构简单、袖珍轻便、价格低廉并满足辅助工程测试要求精度。     

船用金属材料声发射信号特性研究

利用声发射技术对6种船用金属材料拉伸试验过程中产生的声发射信号进行了详细研究。通过对材料从弹性、屈服、强化直至破坏过程中的声发射信号参数统计分析,总结了船用金属材料拉伸过程中声发射信号事件数量、幅度、频率随拉力变化的分布规律,为在船用金属材料与结构缺陷中进行声发射检测的设备参数的配置、传感器的选择和危险等级的划分提供了基础数据。     

一种结构振动系统移频方法

本文对修改结构局部刚度和质量参数，从而使其具有给在有频率的动力修改问题提出了一种求解方法。该方法将结构固有频率修改问题化为一个低阶实对称矩阵特征值问题求解。文中给出一个算例来说明方法的有效性。     

适用于SR-CT技术的新算法

SR-CT(Synchrotron Radiation Computed Tomography)技术中有多种重建算法,其典型算法有滤波反投影算法和迭代算法,这两类算法各有其特点。综合考虑重建结果的质量和重建运算的时间,本文提出了一种新的算法混合算法。通过不同算法重建图像的分析比较,结果表明混合算法包含了前两类算法的优点,而且避免了它们的缺点,是一种行之有效的算法。同时对混合算法中的一些重要参数如初始解系数、迭代步长也进行了详细讨论,并给出了这些参数对重建图像质量的影响关系。     

A Sequential Tensile Method for Rapid Characterization of Extreme-value Behavior in Microfabricated Materials

A high-throughput sequential tensile test method has been developed to characterize the fracture strength distribution of microfabricated polycrystalline silicon, the primary structural material used in microelectromechanical systems (MEMS). The resulting dataset of over 1,000 microtensile tests reveals subtle extreme-value behavior in the tails of the distribution, demonstrating that the common two-parameter Weibull distribution is inferior to a three-parameter Weibull model. The results suggest the existence of a cut-off or threshold stress (1.446 GPa for this particular material) below which tensile failure will not occur. The existence of a cut-off stress suggests that the material’s flaw size distribution and toughness distribution are both also bounded. From an application perspective, the cut-off stress provides a statistically-sound basis for reliable design. While the sequential method is demonstrated here for tensile strength distributions in polycrystalline silicon MEMS, the technique could be extended to a wide range of mechanical tests (bending strength, elastic modulus, fracture toughness, creep, etc.) for both microsystem and conventional materials.     

A New Apparatus for Large Scale Dynamic Tests on Materials

The development of an innovative apparatus, based on Hopkinson bar techniques, for performing large scale dynamic tests is presented and discussed. The activity is centered at the recently upgraded HOPLAB facility, which is basically a split Hopkinson bar with a total length of approximately 200 m, with bar diameters of 72 mm and where force pulses up to 2 MN and 40 ms duration can be generated and strain rates up to 50 s^?1 can be achieved. Several modifications in the basic configuration have been introduced: twin incident and transmitter bars have been installed with strong steel plates at their ends where large specimens can be placed. A series of calibration and quantification tests has been conducted in order to prove the reliability of the experimental technique proposed. Moreover, real tests on concrete cylindrical samples of 200 mm diameter and of up to 400 mm length have been performed. Analyses of recorded signals indicate proper Hopkinson bar testing conditions and reliable functioning of the facility.     

金属橡胶材料阻尼测试的一个新方法

金属橡胶材料是一种新型减振缓冲材料。本文提出了一个金属橡胶材料阻尼性能的测试方法。基于粘弹性阻尼耗能的基本原理,在给定应变激励函数的条件下,构造了一个等效应力响应函数,由滞环面积相等建立了一组包含实测应力响应和等效应力响应的等效耗能方程,将应变激励函数离散得到了损耗因子的计算表达式,由此可以通过实测应力方便地计算出损耗因子。该方法不仅便于对大阻尼材料的阻尼进行测试,而且具有精度高的优点。依据这一方法对金属橡胶材料进行了测试,测试结果表明我们开发的金属橡胶材料有很好的阻尼减振性能。     

基于神经网络技术的管道机电阻抗健康状况定量评估研究

将机电阻抗法用于管道法兰和主干结构健康监测,并利用BP神经网络对结构损伤进行定量评估.首先实验研究了管道法兰与主干结构健康状况对阻抗谱的影响,不同的结构损伤可通过阻抗分析仪测量的阻抗实部谱变化反映出来;然后利用BP神经网络技术对管道不同工况下得到的阻抗实部谱进行定量分析.采用阻抗值实部作为输入样本对神经网络进行训练,并使用受训的神经网络实现了对管道中不同结构损伤状况的定量评估.研究结果表明,将机电阻抗法与神经网络数据处理技术结合起来用于复杂管道的结构健康监测,不仅可实现对不同类型损伤的定量评估,同时还具有较高的稳定性.     

Methodology for the Stress Measurement of Ferromagnetic Materials by Using Magneto Acoustic Emission

Magneto acoustic emission (MAE) is a magnetic nondestructive testing (NDT) method that has been used in different fields for 30 years. MAE is a promising method for the early characterization of damage and the evaluation of the residual stress of ferromagnetic materials. However, this technique is still in its early stage and requires further development. The mechanism and influence factors of MAE are still under investigation. Quantitative NDT is difficult because of the lack of robust theoretical bases and models. In this study, we investigated the influence factors of MAE signals systematically and established a mathematical model to describe these influences. The special design of the specimen and the precise control of experimental conditions are the key points for obtaining reasonable experimental results and for developing the model. A methodology for stress assessment was developed on the basis of the proposed model and was verified by using the pure bending test. Results show that stresses within a measureable depth of 4 mm can be evaluated and that the maximum testing error is 30 %.     

基于压电阻抗技术的奥氏体不锈钢力学损伤定量监测

采用压电阻抗(Electro-mechanical impedance,EMI)技术对奥氏体不锈钢在拉伸过程中的力学损伤进行了定量监测实验研究.选择不同损伤状态下阻抗谱谐振峰频率偏移量△f及均方根偏差(Root-mean-square deviation,RMSD)值作为损伤识别指数,结果表明:在试样弹性变形阶段,△f和RMSD值很小,并且在加载过程中基本保持不变或小有波动;试样发生塑性变形以后,二者均明显增加,其中,△f值由0.05kHz随加载应力逐渐增加至1.65kHz时试样发生断裂,而RMSD由1.3％增大至17.6％后逐渐减小.文中对这种差别出现的原因及两参数各自的特点进行了讨论.显微分析结果证实材料弹塑性转变过程中两参数的变化与试样微观损伤具有很好的对应性,表明利用压电阻抗技术监测和评价核电管道用奥氏体不锈钢的力学损伤具有可行性.     

应用电磁超声技术实验分析自由表平面凹坑上P—SV波的转换

本文对正入射条件下自由表平面半球形凹坑上Ｐ－ＳＶ波的转换进行了实验分析，实验中使用了电磁超声技术，确保在各个方向上等声程接收散射ＳＶ波和良好的测量重复性．实验结果表明，随着凹坑直径（Ｄ）与入射波波长（λ）之比的改变，转型ＳＶ波的指向特征有显著改变；当Ｄ／λ＝１时，ＳＶ波则集中于近表平面，并与散射Ｐ波耦合为强烈的瑞利波．本文为自由表平面半球形凹坑上弹性波的散射方向提供了一些实验解，并为应用波型转换方法检测金属孔蚀提供了实验基础．     

A Long Split Hopkinson Pressure Bar (LSHPB) for Intermediate-rate Characterization of Soft Materials

In this study, we developed a long split Hopkinson pressure bar (LSHPB) for mechanically characterizing soft materials at intermediate strain rates. Using a proper pulse shaper, a loading pulse over 3 ms was produced for compression experiments on a PMDI foam material at the strain rates in the order of 10/s. The pulse shaping technique minimized the dispersion effects of stress wave when propagating through such a long bar system. Consistency of stress–strain curves obtained from the LSHPB and an MTS in the same strain rate range shows that a gap currently existing in intermediate strain-rate range is closed by the introduction of the LSHPB.     

Noncontact Acoustical Techniques for Nondestructive Characterization of Materials and Structures

The Johns Hopkins University Center for Nondestructive Evaluation (CNDE) is an interdisciplinary cooperative center between the university, government, and industrial organizations dedicated to the development of more accurate and innovative methods for the nondestructive evaluation of materials and to the education of talented students. This paper will describe several innovative noncontact nondestructive materials characterization methods developed in the CNDE. Lasers, optical interferometers, electro-magnetic transducers, and air(gas)-coupled systems are used for noncontact generation and detection of ultrasonic waves to inspect art paintings, metals, wood, and composites. Optical detection of acoustic emission signals permits detection of surface displacements caused by acoustic emission events without any modification of the detected waveforms or frequency spectra.