液氮低温环境下电阻应变片测试性能的试验研究

分别采用低温电阻应变片及拉线式位移传感器的电测技术,该文开展了液氮浸泡下的悬臂梁结构在静载条件下的应变测量。对两种实验测试结果和理论分析结果进行了对比和精度分析,给出了相应的实验标定曲线,并探讨了测量过程中应变片粘接、电桥连接方式、液氮冷却和数据采集对实验结果精度的影响因素等。结果表明：在采用温度补偿和应变片的正确粘贴和良好固化情形下,低温应变片能够在液氮低温区给出较高精度的应变测量;拉线式位移传感器几乎不受低温的影响,测量简单易行。相关技术和结果将为中科院近代物理研究所自主研制的兰州潘宁离子阱7T超导磁体的低温下应变测量提供方法和指导。     

A Method to Improve Experimental Validation of Finite‐Element Models of Long Bones

Abstract: Finite‐element (FE) simulations are extremely sensitive to boundary conditions, including the position of applied forces. This is particularly critical for FE models of bones, where the lack of a univocal reference system makes identifying the boundary conditions difficult. The aims of this work were to design a transducer (FP‐transducer) to accurately determine the position of the resultant joint force during in vitro tests, and to assess its accuracy for future application in validating numerical models of long bones. A strain gauge‐based transducer was designed to indirectly measure the position of the force applied to the long bones during in vitro tests, by measuring the reaction moments about two perpendicular axes, generated by the force applied. Validation tests were performed to quantify the intrinsic precision of the FP‐transducer (by applying calibrated forces at known locations), and the overall accuracy when the FP‐transducer was included in a typical set‐up for long bone testing. The intrinsic accuracy of the FP‐transducer when used to calculate the position of an offset force was satisfactory (0.66 mm). The overall accuracy of the FP‐transducer in measuring the position of the applied force, when included in a typical set‐up for long bone testing was 0.85 mm. In vitro validation of FE models of long bones may, therefore, be improved, thanks to a more accurate determination of the force application point.     

Vibration analyses of a portal frame under the action of a moving distributed mass using moving mass element

Four kinds of moving mass elements, 1st‐node, 2nd‐node, full and short‐range mass elements, are presented, where the 1st‐node (or 2nd‐node) mass element refers to that with mass distributed from the first node (or second node) to the arbitrary position of a two‐node beam element, the full mass element is the special case of the 1st‐node (or 2nd‐node) mass element with mass distributed over the full length of the beam element, while the short‐range mass element is the case with its location arbitrary on a beam element. If the total range of a distributed mass is denoted by R and the length of each beam element is denoted by ??, then, for the case of R???, one may model the distributed mass on the beam using the combination of the 1st‐node, 2nd‐node and full mass elements, while for the case of R<??, one may model the distributed mass using the short‐range mass element. It has been found that the effects of the vertical (?) and horizontal (x?) inertia forces, Coriolis force and centrifugal force induced by the moving distributed mass can be easily taken into the formulations by means of the last concept. To illustrate the application of the presented theory, the dynamic analysis of a pinned–pinned beam and that of a portal frame under the action of a moving uniformly distributed mass are performed by means of the finite element method and the Newmark integration method. Numerical results show that some pertinent factors, such as Coriolis force, centrifugal force, acceleration, velocity and total range of the moving distributed mass, have significant influences on the vertical (?) and horizontal (x?) response of a structure. Copyright © 2005 John Wiley & Sons, Ltd.     

Development and validation of a technique to measure and compare the opening characteristics of tamper‐evident bottle closures

Large sections of the population encounter difficulty in opening consumer packaging of many kinds. Screw‐caps in particular can cause problems for a range of people with a variety of impairments. This paper describes the development and initial testing of a novel multi‐axial force and torque transducer, designed for the study of loading conditions when tamper‐evident bottle closures are opened manually. The transducer, which comprises seven beams that are sensitive to direct forces and torques in each of three axes, can provide comprehensive information on the loading conditions that occur when an instrumented bottle is opened. Importantly, the transducer has been designed to fit inside a typical 500 ml capacity plastic soft drinks bottle so that it does not interfere with the way in which the subject grips the bottle and cap, or applies forces and torques, in order to open the bottle. The method to obtain load data from the calibration matrix, along with initial opening force and torque test data from two user groups, elderly and young, is described. It is clear from the results of these tests that the elderly and young groups exhibit significantly different torque and force profiles to open bottles. It is anticipated that the transducer will be a valuable tool in future studies of opening strategies. Copyright © 2006 John Wiley & Sons, Ltd.     

Further study on the refined theory of rectangle deep beams

Based on the refined theory for narrow rectangular deep beams, two different displacement boundary conditions of the fixed end of a cantilever beam are used to study the deformation of the beam. One is the conventional simplified displacement boundary condition, and the other is a new boundary condition determined by the least squares method. Three load cases are investigated, which are a transverse shear force at the free end of the beam, a uniformly distributed load at the top surface, and a linearly distributed load at the top surface, respectively. Solutions are given for both the refined theory and the Timoshenko beam theory and are compared with the known solutions from the elastic theory and results by the finite element method. It is shown that the solutions of the refined theory coincide with those of the elastic theory; the solutions from the Timoshenko theory by using the two different displacement boundary conditions are the same; the refined theory by using the new boundary condition provides better results than using the conventional boundary condition and also better than those of the Timoshenko beam theory.     

Post-buckling analysis of composite beams: A simple intuitive formulation

Post-buckling analysis of composite beams with axially immovable ends is investigated using an Intuitive formulation. Intuitive formulation uses two parameters namely critical buckling load and axial stretching force developed in the post-buckled domain of composite beam. Geometric nonlinearity of von-Karman type is taken into consideration which accounts for membrane stretching action of the beam. Axial stretching force developed in post-buckled domain of composite beam is evaluated by using an axial governing equation and is expressed either in terms of lateral displacement function as an integrated value, or as a function of both axial and lateral displacement functions at any discrete location of the beam. The available expressions of critical buckling load and derived expressions of axial stretching force developed in the beam are used for obtaining an approximate closed-form expressions for the post-buckling loads of various beam boundary conditions. Numerical accuracy of the proposed analytical closed-form expressions obtained from the intuitive formulation are compared to the available finite element solutions for symmetric and asymmetric lay-up schemes of laminated composite beam. Effect of central amplitude ratio and lay-up orientation on post-buckling load variation is briefly discussed for various beam boundary conditions considered in this study.     

抵肩力测量中动态修正问题探讨

本文简要介绍了采用双剪切梁式应变测力传感器测量抵肩力的系统组成,对测得的原始抵肩力信号作了深入的分析和讨论,提出了消除加速度影响的动态修正方法,并给出了经加速度修正后抵肩力信号滤波的截止频率的选择依据。     

应变式力传感器动态特性研究及动态补偿

通过测试传感器的固有频率和冲击响应,对不同结构的应变式力传感器的动态特性进行了研究。当力传感器动态特性不理想时,根据零极点配置法设计了动态补偿数字滤波器,对力传感器进行动态补偿,使之适用于动态测量。实验表明:柱式力传感器的动态特性优于S形力传感器。通过对S形传感器进行动态补偿,拓宽了力传感器的工作频带,提高了力传感器的动态性能,减小了动态误差。     

高温灵敏系数标定影响因素分析

高温应变计是高温结构强度测试、疲劳测试和寿命评估的主要测量传感器,其灵敏系数测试的准确性影响高温应变测试中应变修正数据的真实性。对应变计灵敏系数原理进行分析,明确灵敏系数是应变计输出与应变之间的关系,对砝码加载四点式简支梁结构和纯弯矩钢架梁结构灵敏系数测量模型进行分析,确定产生标准应变的影响因素。常温灵敏系数标定的影响因素有试验梁尺寸、试验梁轴向力,实际灵敏系数测试结果表明简支梁灵敏系数略大,分散明显较大。高温下应变计的输出与载荷应变、应变计热输出、试验梁尺寸变化、弹性模量变化和摩擦力变化有关,对热输出消除方法进行探讨,理论上明确温度下简支梁尺寸变化、弹性模量变化误差高达22%,钢架梁的刚性封闭静态力学平衡系统的温度影响误差较小。利用自研框架式高温裸栅应变计、高温陶瓷胶,进一步用简支梁、钢架梁实际测试灵敏系数随温度变化验证了理论分析的正确性。     

Recent Developments in Measuring Creep Strain in High Temperature Plant Components

Accurate measurements of creep strain are necessary to evaluate the condition and predict the remaining life of power plant constituent materials. Optical techniques are appropriate for this purpose as they are a non‐contact method and can therefore be used to measure strain without requiring direct access to the surface. Within this class of techniques, the Auto‐Reference Creep Management And Control (ARCMAC) camera system can be used to calculate the strain between two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two iterations in system design, the Conventional ARCMAC and Digital Single‐Lens Reflex (DSLR) ARCMAC. Experiments are conducted to determine the absolute limit of accuracy of the systems in comparison to a strain gauge, and the relative accuracy across several orders of magnitude until specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated temperatures to evaluate the effect of temperature on the gauges and to investigate whether its accuracy diminishes in creep conditions. It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 με or less. In accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable displacement transducer when used to measure creep strain. Strain variations (under 500 με) were noted on a steel plate subjected only to operational temperature and no stress. This error is very reasonable compared to a critical strain value of 93 000 με in a given high temperature‐service material. Digital image correlation (DIC) results using the DSLR ARCMAC system show approximately 4% error in measurement for plastic strains in the specimen. The two measures of strain measurement (using ARCMAC and DIC) can serve to complement each other.     

Analysis of Acoustic Emissions from a Steel Bridge Hanger

A steel bridge hanger with three fatigue cracks was monitored for acoustic emission (AE) using combined source location, strain gauge monitoring, and waveform analysis. AE activities from all three cracks were clearly identified and classified as crack growth or noise signals using location, strain magnitude, position on strain cycle, and uniqueness of waveforms as the primary criteria. A vast majority of AE from the cracks was found to be due to crack face rubbing and the crushing of corrosion products between the crack faces while limited crack growth emissions were detected. Results from laboratory tests on A588 compact tension specimens under variable-amplitude tension-tension fatigue loading were used to aid in interpreting AE data from the hanger. Crack growth AEs from these tests were detected only on overload cycles mostly above 92% of the maximum load while AE due to crack face rubbing occurred throughout the load cycle.     

Nanofriction visualized in space and time by 4D electron microscopy

In this letter, we report a novel method of visualizing nanoscale friction in space and time using ultrafast electron microscopy (UEM). The methodology is demonstrated for a nanoscale movement of a single crystal beam on a thin amorphous membrane of silicon nitride. The movement results from the elongation of the crystal beam, which is initiated by a laser (clocking) pulse, and we examined two types of beams: those that are free of friction and the others which are fixed on the substrate. From observations of image change with time we are able to decipher the nature of microscopic friction at the solid-solid interface: smooth-sliding and periodic slip-stick friction. At the molecular and nanoscale level, and when a force parallel to the surface (expansion of the beam) is applied, the force of gravity as a (perpendicular) load cannot explain the observed friction. An additional effective load being 6 orders of magnitude larger than that due to gravity is attributed to Coulombic/van der Waals adhesion at the interface. For the case under study, metal-organic crystals, the gravitational force is on the order of piconewtons whereas the static friction force is 0.5 μN and dynamic friction is 0.4 μN; typical beam expansions are 50 nm/nJ for the free beam and 10 nm/nJ for the fixed beam. The method reported here should have applications for other materials, and for elucidating the origin of periodic and chaotic friction and their relevance to the efficacy of nano(micro)-scale devices.     

A steadily moving load on an elastic strip resting on a rigid foundation

An infinite elastic strip resting on a flat rigid foundation is loaded by a downward directed concentrated force, which moves at constant speed. The resulting non-symmetric mixed boundary value problem is reduced on the basis of the plane strain theory of elasticity to singular integral equations over the regions of non-contact. With force magnitude and speed as parameters, the location of the non-contact regions, the lower boundary displacements, and foundation contact pressure are computed and illustrated graphically.     

Development and Characterization of a Diaphragm-Shaped Force Transducer for Static Force Measurement

There are different types of force transducers, used over the years, comprising of different principles and have different grades of uncertainty. Force transducers may be analog or digital and may be strain gauged, tuning fork type, piezoresistive type, Hall Effect based etc. Though different types of force transducers have been developed in last few decades, still strain gauged force transducers are used over the larger scale due to practical viability and reliability for force measurement. The paper describes low-cost indigenous development of a force transducer, which has been developed for a nominal capacity of 5 kN with ease of design and manufacturing. It has strain gauges applied at suitable locations to form a Wheatstone bridge. The force transducer has been calibrated according to the standard calibration procedures based on ISO 376 and IS 4169 standards. Measurement results and an uncertainty analysis have been presented. The uncertainty of measurement of the force transducer is found up to 0.06%. The paper also discusses the implications of applying multiple strain gauges at a single location in series to enhance the sensitivity of the force transducer. Two strain gauges are arranged in series for the given location and the sensitivity of the force transducer is enhanced. It also indicates that the uncertainty of measurement of the force transducer in such case is enhanced up to 0.05% or better to make force transducer suitable for most of metrological and industrial applications.     

弹性地基上受切向力作用梁稳定性研究

用广义微分求积法(GDQR)分析了弹性地基上复杂弹性支承条件下受切向力作用梁的稳定性问题。基于弹性支承梁的运动微分方程及边界条件,采用GDQR进行离散化,获得由动力方程组及边界条件合成的特征值矩阵方程。通过对相应特征值方程的具体分析,讨论了弹性地基模量、剪切系数、复杂边界条件对临界载荷的影响,研究了一端固定约束、另一端弹性约束梁弹性失稳区域随弹性地基模量和支承弹簧刚度变化的情况,得到了一些有益的结论。结果表明：GDQR能很好地解决此类系统的稳定性问题。     

Verification of the duplex gauge using variable strains and temperatures

Four steel specimens were equipped with "duplex nances", a recently proposed transducer based on two different electrical resistance strain gauges, capable of correcting the effects induced by temperature. The examined innovative transducer, in comparison with traditional strain measuring systems, has the advantage of utilising two strain gauge channels of the same type, which significantly simplifies automatic measuring systems. The specimens were subjected to temperature and strain field variations in the ranges of=30–150°Cand=100–700μm/m respectively The temperature and strain values obtained from the duplex gauges were compared with those measured by means of traditional methodology, i.e. a strain gauge and a thermocouple.     

冷弯薄壁型钢梁-OSB板组合楼盖静力挠度及振动性能试验研究

对冷弯薄壁型钢梁-OSB板组合楼盖足尺模型进行改变边界条件的振动试验, 研究组合楼盖在四边支撑和梁端简支、四边支撑和梁端固结、两边支撑和梁端简支、两边支撑和梁端固结四种不同边界条件时静力荷载下的挠度及人沿平行和垂直楼盖梁方向行走、脚跟冲击、沙袋从空中落下、激励锤冲击不同动力荷载工况下的自振频率。研究表明：边界条件的变化会引起组合楼盖挠度和自振频率的变化, 可以通过改变边界条件来提高组合楼盖的振动性能;边界条件不同时组合楼盖在1kN集中荷载作用下跨中挠度均小于2mm, 在动力荷载作用下除个别情况外自振频率均高于15Hz, 建议组合楼盖在1kN集中荷载作用下跨中挠度限值设为2mm, 振动舒适度要求较高时自振频率限值设为15Hz。另外, 采用等效刚度法提出的理论公式能较好地预测不同边界条件时组合楼盖在静力荷载下的挠度及动力荷载下的自振频率。     

移动荷载作用下梁裂缝识别的小波方法研究

利用小波分析对简支梁的裂缝进行识别。通过对带裂缝简支梁在移动荷载作用下的跨中响应用Mexicanhat小波进行连续小波变换，从小波系数的模极大值点有效地得到荷载经过裂缝的时间，从而识别裂缝位置，从模极大值处的Lipschitz指数判断裂缝深度，Lipschitz指数随裂缝深度的增加而减小。同时讨论了荷载移动速度和裂缝位置对Lipschitz指数的影响。通过分析和仿真计算获得满意结果，在梁结构损伤诊断中具有较高的应用价值。     

Calibration of a Speckle Interferometry Full-Field Strain Measurement System

Abstract:  This study describes the calibration of a full-field speckle interferometry strain measurement system using the calibration specimen and protocol defined in the Standardisation Project for Optical Techniques of Strain measurement (SPOTS) standard. The specimen was based on the monolithic embodiment of a four-point bending test and was manufactured from aluminium following the SPOTS design. Strain-gauge rosettes attached to the upper and lower faces of the beam were used to derive two correction factors of an analytical expression that predicted the strains generated in the gauge section of the beam. Following the SPOTS protocol, the comparison of measured and predicted strains yielded two fit parameters and their associated uncertainties for each of three displacement-load steps which indicated the closeness of the data sets. An acceptable calibration was achieved for the single normal component of in-plane strain considered in this study, for each load step employed. For the highest load range, which generated a maximum strain of approximately 810  μ strain in the gauge section, the overall calibration uncertainty was found to be 35.3  μ strain, which in relative terms can be expressed as 2.2% of the strain measurement range for which the instrument was calibrated.     

轴向载荷条件下弹性边界约束梁结构振动特性分析

采用改进傅里叶级数展开建立了轴向载荷条件下弹性边界约束梁结构振动分析模型。通过在梁结构两端引入平动和旋转位移约束弹簧,相应设置约束弹簧刚度系数可以实现对任意边界条件及其组合的模拟。梁结构振动系统位移场采用傅里叶级数附加边界光滑函数进行构建,利用能量原理建立轴向载荷作用下梁结构总动能、总势能和外力做功项,并结合瑞利-里兹步骤获得系统特征矩阵方程。通过数值算例,验证了该模型对不同边界条件、轴向载荷作用下梁结构振动特性分析的正确性与可靠性。在此基础上,研究了边界约束弹簧横向刚度、旋转刚度、轴向载荷等系统参数及激振力对梁结构振动特性的影响。该模型具有高效、高精度等特点,为研究轴向载荷作用下复杂边界条件梁结构振动行为提供了有效分析手段。