固体力学研究的趋向和良机(Ⅳ)——美国机械工程师协会应用力学分会固体力学研究方向委员会的报告——8.实验力学

机器部件、构件和微电子元件的力学破坏,对人们的安全、可靠和生产有很不利的影响.防止这些破坏是固体力学的主要焦点,即通过分析、实验和计算来改进设计、制造和检验,以提供减少破坏所必要的根据.实验力学在这里起了关键作用,因为它提供了计算用的基本数据和检验所提出破坏理论模型的手段.实验力学当前的趋向表明,人们愈来愈多地采用光学方法来监测表面的位移、速度和应变.对于有害环境以及动态加载的物体来说,由于非接触方法的吸引力,上述趋向得以发展.激光技术的进展使这些方法便于实现.另一个趋向是由于采用了有计算机接口的伺服控制试验机而可以对更复杂载荷条件下的材料性能加以研究.还有另一个趋向是因为缺陷在破坏机理中很重要,人们加强了对缺陷如夹杂、裂纹、空洞等的注意.看来实验力学对未来作出贡献的机会是很大的,并且涉及范围广大的技术问题.未来研究的中心问题,看来是加强对微米尺度和小于微米尺度的测量,以进一步了解细观力学层次的材料的响应和破坏.对缺陷、变形和残余应力的内部测量也将给以更多注意,因为它们对于加深对破坏的基本了解是重要的.自动化的数据处理和实验控制,将大大增加实验所得的信息及其对建立数学模型的用途.另外一些重要的研究方向,包括改进在现场测量岩石应力的方法,改进生物系统中位移和生理参数的测量,长期监测结构完整性的能力,以及改进用于力学系统反馈控制的传感器.      

热障涂层高温失效行为的正射投影-单相机三维数字图像相关实验研究

热障涂层(Thermal Barrier Coating,TBC)作为一种隔热材料,可降低发动机高温部件的表面温度,提高涡轮发动机的整体性能。涂层服役过程中,TBC陶瓷层的失效将导致高温部件中的金属基底直接暴露于恶劣的高温环境中,严重影响发动机的安全性。其高温服役环境下的力学性能研究是备受材料和力学工作者关注的问题。本文结合热障涂层高温检测的需求,实现了涂层-基底异质表面的高对比度高温散斑制作;发展了正射投影-单相机三维数字图像相关方法(正射投影-单相机3DDIC),构建了跨界面形函数,实现了涂层与基底的三维形貌及位移、应变的同时测量;基于涂层结构的高温拉伸实验结果,建立了依据涂层-基底应变方差差值的涂层失效判断方法,得到了热障涂层在室温(27℃)至1000℃的断裂极限应变。实验结果表明,正射投影-单相机3DDIC方法作为一种可靠测试方法,可以应用于涂层结构高温下的力学失效行为研究。     

基于Harris角点检测的位移测量算法

在力学实验中,位移和应变的测量是最基本任务之一。本文提出了一种基于Harris角点检测的位移测量算法。通过检测变形前图像的角点,然后利用光流跟踪技术在变形图像中搜索其匹配点,最终计算得到位移值。算法对图像采用网格划分方式进行计算,达到了全场测量的目的。实验结果表明,该算法位移测量精度高、稳定性好,可作为基于图像处理的无损测量方法。     

关于极端力学

随着前沿科学和新技术不断发展,工程材料与结构的超常规尺度、密度、硬度、刚度等性能以及在超常规温度、速度、场强和恶劣天气等极端服役环境中的力学响应规律,需要力学提供更为有效的理论和方法. 本报告从极端力学的基本定义和科学内涵出发,结合重大工程问题和大科学问题,从极端性能、极端载荷、学科发展等三个方面系统介绍了极端力学的研究现状,并总结了极端力学的特点及其对力学理论、计算方法和实验技术的挑战,最后对极端力学未来的发展进行了展望.      

高温超声拉压疲劳试件设计与温度自动控制

本文较深入地开展了高温环境条件下的超声拉压疲劳实验系统研究,深入分析了试件温度高精测量与自动控制和高温试件设计的关键问题。实验系统可以实现高温环境条件下大于1010周次超高周次的疲劳加载。本实验系统利用传统超声疲劳试验机,使用非接触式红外测温仪对试件温度进行实时动态监测,采用间歇振动的方法保证温度波动小于±3℃。实验结果显示,采用本系统得到的实验结果与传统疲劳加载方式的结果一致性很好。本文工作为进一步开展高温环境条件下金属材料高温超高周(1010周次)疲劳损伤和疲劳破坏机理研究提供了新的有效的平台。     

高温花岗岩遇水快速冷却后力学性质实验研究

通过对高温加热–遇水快速冷却后的花岗岩试样进行单轴和三轴实验,研究了800℃内高温花岗岩遇水快速冷却后的力学性质随温度和围压的变化规律。实验结果表明:(1) 400℃为高温加热–遇水快速冷却对花岗岩力学性质影响的阈值;(2)同一温度条件下,峰值偏应力、峰值应变随围压的增大而增大;弹性模量随围压的增大先增大后减小;(3)单轴实验中,温度低于400℃时,岩样表现为复合破坏,随着温度的升高破坏形式转变为拉破坏;三轴实验中,岩样整体上表现为剪切破坏。     

高温花岗岩遇水快速冷却后力学性质实验研究1)

通过对高温加热-遇水快速冷却后的花岗岩试样进行单轴和三轴实验,研究了800°C内高温花岗岩遇水快速冷却后的力学性质随温度和围压的变化规律。实验结果表明：(1)400°C为高温加热-遇水快速冷却对花岗岩力学性质影响的阈值;(2)同一温度条件下,峰值偏应力、峰值应变随围压的增大而增大;弹性模量随围压的增大先增大后减小;(3)单轴实验中,温度低于400°C时,岩样表现为复合破坏,随着温度的升高破坏形式转变为拉破坏;三轴实验中,岩样整体上表现为剪切破坏。     

虚位移场方法在石墨材料力学参数测量中的应用

光测实验技术在现代力学研究中得到了广泛的应用。对于材料力学参数如杨氏模量和泊松比的测量,可利用典型加载试验如拉伸试验、弯曲试验并结合光测方法(如云纹和数字图像相关技术)得到位移值,利用载荷信息和应变场信息通过计算获得相关的力学参数。本文利用虚位移场方法测量石墨材料的力学参数。结合石墨材料的三点弯曲实验,由数字图像相关法测量得到试件表面的非均匀变形场。通过选择两组不同的虚位移场,可以反算出材料的力学参数:杨氏模量和泊松比。结果表明这种方法可以有效测量石墨材料的弹性参数。该方法可望在材料力学行为检测中得到推广应用。     

实验力学在未来国际上的地位

本文论述了测量和实验技术当前和未来在分析结构、结构单元以及技术系统的位移、应变和应力中所起的作用.今天比以往任何时候都有更多的原因在广泛的领域中采用实验力学的方法。现代先进的新技术,新的测量设备,自动的量测和评价过程已应运而生。先进的测量技术和计算机技术的结合——杂交技术已得到了发展,这些技术使我们能对结构和产品进行优化设计:节省能源和原材料,提高产品质量以及结构和技术系统的可靠性和安全度,管理生产的过程,监控运行系统的安全。这些方法能用于减少技术系统的危险性和对环境的破坏,对发展中国家传授先进的实验力学方法的知识,显得更为重要。     

用于高温和有害环境的结构系统

对有害环境中高温下工作的承载结构系统的需求,已经导致新的金属合金和非金属材料的发展.因为工作条件恶劣,材料常经受严峻的加载环境和腐蚀环境,所以只能设计有限寿命的结构系统.为了完成有效的设计,必须了解材料的物理和力学性质,以及这些性质与系统的运转性能有何关系.不但讨论了在寻求更先进和更经济的性能时必然提出的基础问题和计算问题,而且讨论了与此有关的一系列问题.      

用电阻应变片测量应力强度因子方法的研究

本文利用Ｗｉｌｉａｍｓ应力函数导出的应变场渐进表达式和电阻应变片测量技术，测量张开型（Ｉ型）裂纹尖端附近两点应变来确定应力强度因子ＫＩ。通过对不同裂纹体的测试结果表明，该技术具有较好的适用性。     

Strain gage method for determining stress intensities of sharp-notched strips

An efficient and simple strain gage method for determining the stress intensities of sharp-notched strips is proposed. The bisector of the notch angle is inclined to the edge so that the mixed-mode loading is created simultaneously at the notch tip. A theory of determining the stress intensities using strain gages is described on the basis of the two-dimensional theory of elasticity. Experiments on specimens with various notch shapes are carried out to verify the theoretical results. Experimental results are in good agreement with theoretical results.     

固体发动机保险机构在高温高压下的实验研究

针对固体发动机部件保险机构所出现破现象,对其进行了激光散斑和电阻应变法联合测试。由于保险机构在高温和高压环境下工作,材料有蠕变现象,出现较大的热变形,传统的光我法实施,发展了一种叠合散斑技术,使之能够对大变形进行非接触式测量,在实验室测试的基础上,对保险机构进行了热容器试验,测量了点火后应变随时间的变化规律。实验结构发现,在保险机构机体上存在着一个接近破环应力的高危应力区,这是保险机破坏的主要因素     

Strain gage evaluation with four-point bending at moderate temperatures

This paper describes the application of the traditional four-point bending technique to conduct strain gage performance tests at moderate temperatures. The tests measure the apparent strain, linearity, drift, creep and hysteresis behavior to ascertain the reliability of strain gages to be used in the testing of experimental cylindrical vessels subjected to thermal loading. The four-point bending rig was designed to take advantage of the advances in personal computer datalogging and graphical processing software. The test rig is designed to provide a thermal barrier to minimize the heat losses and thermal isolation of the test beam while maintaining structural continuity. A low wattage and lightweight electric foil heater incorporating a precise temperature controller minimizes weight and enhances temperature stability. Gage performance data on five candidate strain gages at temperatures ranging from ambient to 215°C (419°F) are presented, and the evaluation procedure is outlined. The strain gage with the most acceptable performance was identified within the validity limits from ambient to a temperature of 215°C and from zero to a maximum strain of 1000 .     

Embedded optical fiber Bragg grating sensor in a nonuniform strain field: Measurements and simulations

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.     

Sensitivity of the shear gage to normal strains

This paper documents an experimental study that was conducted to demonstrate the sensitivity of the shear gage to the presence of normal strains. The shear gage is a specially designed strain gage rosette that measures the average shear strain in the test section of notched specimens such as the losipescu, Arcan and compact shear specimens. These specimens can have complicated stress states with high shear and normal strain gradients. To evaluate the sensitivity of the shear gage to normal strains, shear gages were tested on an Arcan specimen. The Arcan specimen is a notched specimen that can be loaded in pure shear (90 deg), pure tension (0 deg) and at intermediate 15- deg increments. The shear modulus for an aluminum specimen was determined at each of these loading angles. It was found that the gages display nearly zero sensitivity to normal strains ( _x, _y). Moiré interferometry was used to document the shear and normal strain distributions in the test section and to provide an independent method for determining the average shear strain. These results reinforce the robust nature of testing with the shear gage.     

Hybrid stress analysis of perforated composites using strain gages

A strain gage hybrid method is described for determining individual stresses on the boundary and in the neighborhood of cutouts in orthotropic composites. Results agree with independent measurements and finite element analysis. Few measured strain data are needed, and the measured strains originate away from the hole. Ability to determine the stresses on the edge of a cutout from nonboundary measurements recognizes the difficulties in obtaining reliable measurements very near an edge while circumventing the challenge of attempting to bond gages to the transverse curved surface of a small hole or notch. The method also alleviates the problem of not knowing a priori where the most serious stress will occur on the geometric boundary and, hence, where to locate strain gages.     

回路J积分守恒性的测定与研究

本文从回路定义的Ｊ积分出发，采用激光全息干涉及散斑干涉法测出紧凑拉伸试件裂纹前端区域弹性情况下的应力场和位移场。采用两条不同回路的测点，计算出应力分量和形变分量，经计算得出的两个回路Ｊ积分值，相差仅为３．３３％，从而证明了Ｊ积分的守恒性。     

FJ662A公路客车骨架的脆漆和静态电测试验分析

本文介绍用脆漆法和电测法测定和分析车身骨架强度的方法及结果、并讨论使用这两种方法相辅相成的关系。     

Enhanced measurement of strain distributions

A theoretical approach is presented that uses multiple strain gages to accurately measure complicated strain distributions. The technique is based on the method of weighted residuals in conjunction with measured strain data and is applicable for arbitrary in-plane strain distributions. Conventional measurements using strain gages are shown to represent a particular case of the approach presented. The experimental characterization of unidimensional strain fields is discussed in detail. Two approaches are presented; these are based on linear and quadratic approximations of the strain field. The strain distribution for two important practical problems is evaluated assuming ideal conditions to assess the performance of the proposed approach. In both cases, the simulated results demonstrate that measurement error resulting from the finite size of a strain gage may be reduced. That is, a larger strain gage may be used for a given maximum admissible error. The method also allows a minimal error of measured nonlinear strains.