试样尺寸对钢板塑性应变比r的影响

选用三种尺寸的拉伸试样测试塑性应变比（r值）。通过试验和数据分析,得出当试验标距接近试样平行长度时,r值将下降。其原因是当标距接近试样平行长度时,试样标距内的应变是不均匀的。     

基于三维数字图像相关法测量复合材料泊松比

采用力学测试与模拟(MTS)拉伸试验机和三维数字图像相关(DIC-3D)准静态三维应变光学测量系统采集到拉伸试样的散斑图像,然后通过Origin/excel等计算软件对原始数据进行处理,逐步得到应力-应变曲线、全过程全场位移云图、应变场E1分布及E1、E2方向、矩形区域内E1、E2平均值时序曲线和全过程|E2/E1|时序曲线,再根据特定的数据段选择原则,将指定拉伸时间内的数据进行Origin拟合处理,最后得到30%玻纤增强聚醚酰亚胺(PEI)复合材料的横向-纵向应变比的绝对值,即泊松比,并为实验测量复合材料泊松比等提供了一种数据处理和计算方法。     

光测技术及其在钢材拉伸试验中的应用

介绍了拉伸视频引伸计、数字图像相关(DIC)技术等光测方案的技术原理,以及其在虚标实测、局域化强塑性能分析、拉伸应变痕(吕德斯效应)的直接观测等领域的实际应用。此外,通过分析传统电子式宽度引伸计在塑性应变比r值测试方面的技术缺陷,提出了通过跟踪试样宽度轮廓变化,或通过二维DIC技术定义多对初始宽向标距,利用宽度变化的均值计算r值,可提高r值测试的可重复性与可靠性。     

动态断裂韧性实验中DIC技术应用研究

高速冲击动态断裂韧性的加载和测试技术一直是近年关注的热点,随着计算机和光学传感器的发展,采用数字图像相关方法测量材料的动态断裂韧性已成为重要选择。该文基于分离式Hopkinson压杆原理的加载技术,通过高速摄影机拍摄高速冲击下三点弯曲试样裂纹的起裂和扩展,运用数字图像相关技术分析裂尖场的散斑图像,计算得到相应的应变场变化,试样外表面处于平面应力状态,其裂尖应变场呈现"0"型,而非试样对称面所处于的平面应力状态下呈"8"型。结果表明,DIC技术可以应用于动态断裂韧性实验,也证明裂尖场粘贴的应变片测量试样起裂的有效性。     

标距和应变率对Kevlar49单束拉伸力学性能的影响

为探究Kevlar 49单束的尺寸效应及应变率敏感性, 首先, 利用MTS万能试验机对不同标距(25、50、100、150、200和300 mm)的Kevlar 49单束进行了准静态(应变率为1/600 s-1)拉伸测试; 然后, 利用Instron落锤冲击系统对标距为25 mm的试样进行了动态(应变率为40~160 s-1)拉伸测试; 最后, 利用Weibull模型进行统计分析, 量化了不同标距和应变率下Kevlar 49单束拉伸强度的随机变化程度。结果表明: Kevlar 49单束的拉伸力学性能与标距和应变率有相关性; 拉伸强度随标距的增加而减小, 但随应变率的增加而增大; 峰值应变和韧性均随标距和应变率的增加而减小; 提取的Weibull参数可用于数值模拟及工程应用。      

金属圆棒试样室温下高应变速率拉伸试验浅析

在不同应变速率下对铸铁和铸铝圆棒试样进行了单轴高速拉伸试验,研究了它们的动态力学性能及断裂情况,分析了相关因素对试验的影响。结果表明:测试应变、应力的方法,试样标距长度及夹持端长度等对试验准确性和曲线振荡程度有较大影响;使用比刚度和比强度高的夹具、短标距试样、应变片测试应力、两台相机测试应变、适当增加夹持端长度可以提高试验结果的准确性。     

非接触数字图像相关应变测量的仿真与实验研究

数字图像相关法测量全场位移和应变是一种新的实验力学方法.该测量算法中相关函数和子区大小的选择是影响最终应变测量精度的重要因素.本文采用已知应变变形的仿真散斑图,研究了测量算法中主要相关函数和子区大小在正常光照与高斯不均匀光照条件下,对应变测量精度的影响.得出了不同测量要求下,可选择的相关函数和最佳计算窗口.通过实验验证与分析了此算法的测量精度.研究结果表明,数字图像处理技术测量应变的测试精度满足基本要求.     

用拉伸试验机横梁位移值计算断后伸长率

介绍利用拉伸试验机横梁位移数据计算拉伸试样断后伸长率的方法。由试样长度与标距不一致产生的测量偏差，可通过补偿值加以修正，其测量精度与常规测量方法基本相同。     

取样方向对含单边裂纹牛卡纸拉伸断裂行为的影响

目的 探究取样方向对含单边裂纹牛卡纸拉伸断裂行为的影响,以期提高牛卡纸为原料制作的包装盒等成品的强度。方法 首先在不同取样方向上制备含单边裂纹的牛卡纸试样,然后基于数字图像相关(Digital Image Correlation, DIC)方法,结合显微张力测试台测得不同取样方向与拉伸断裂载荷的关系图,分析不同取样方向下裂纹尖端微观应变场,最后结合电镜扫描测试进行对比分析。结果 结果表明,通常情况下,含单边裂纹的牛卡纸依然符合纸张纵向拉伸载荷大于横向拉伸载荷的规律,取样方向越靠近横向,裂尖区域应变值越大,发生断裂时的拉伸载荷越小。结论 牛卡纸取样方向和纤维与纤维的结合强度是影响牛卡纸拉伸断裂行为的重要因素。     

灰底白板纸与白卡纸的弹性常数对比

目的 为了更加精确地测量灰底白板纸和白卡纸的泊松比和弹性模量。方法 将数字图像相关方法与单向拉伸实验相结合。通过对灰底白板纸和白卡纸试样拉伸变形前后的图像进行分析, 测量其泊松比; 通过对灰底白板纸和白卡纸的力-位移曲线进行分析, 测量其弹性模量。结果 测得了定量为300, 400 g/m2的灰底白板纸的泊松比分别为0.281和0.245, 其弹性模量分别为154.47, 226.06 MPa;300, 400 g/m2的白卡纸的泊松比分别为 0.312 和 0.276, 其弹性模量分别为 209.09, 297.79 MPa。结论同种类别的纸张中, 定量越高, 其弹性模量越大, 而泊松比越小; 在定量相同的情况下, 白卡纸的泊松比和弹性模量都比灰底白板纸的大。     

基于数字图像相关技术测量刨花板材料力学参数

刨花板作为一种人造板材,其表面多孔性致使应变片的粘贴过程十分困难,且涂胶量的多少会直接影响测量的精度。基于对样品变形前后图像分析的数字图像相关(Digital Image Correlation,DIC)技术被用于测量刨花板的力学参数。相比于传统的贴应变片法,该技术具有高精度、非接触性及全场测量等优势。“横观各向同性”模型被考虑用来模拟刨花板的力学行为。于是,表征材料力学性能的弹性张量取决于5个独立的弹性参数:纵向、横向弹性模量EL、ET,纵向、横向泊松比νL、νT及纵向剪切模量GL。为了实现这一测量过程,刨花板被切割成一批梁样品,随后被应用于三点弯曲试验。通过比较感兴趣区域(Region Of Interest,ROI)内网格节点位移的测量值与铁木辛柯梁理论解析解,及有限元模型修正(Finite Element Model Updating,FEMU)方法的应用,4个弹性参数ET、GL、EL和νL被成功测量。对比相关文献,该文的测量方法简单易行,测量结果准确,可应用于刨花板材料并推广至各向异性材料的弹性参数测量之中。     

Displacement Measurement Through Digital Image Correlation and Digital Speckle Pattern Interferometry Techniques in Cold‐Expanded Holes

Abstract: In this paper, the displacement field induced by the split‐sleeve cold expansion of holes was measured using both digital image correlation (DIC) and digital speckle pattern interferometry (DSPI) techniques. Thus, the experimental results, which were evaluated on the inlet surface of a 6082‐T6 aluminium plate, were compared with those from theoretical prediction. DIC provided accurate measurements up to the elastic–plastic boundary, whereas the DSPI technique highlighted the changes of displacement in the elastic domain. Prediction of the displacement based on the existing analytical model agreed with the experimental results achieved with both techniques. Possible explanations for the differences are discussed.     

数字图像相关测量钢绞线弹性模量的应用研究

该文利用非接触式数字图像相关方法测量了预应力钢绞线的弹性模量。测量过程中用CCD相机记录不同载荷下钢绞线表面的数字图像,再利用数字图像相关方法对所采集的序列数字图像进行分析从中精确提取不同载荷下钢绞线表面的纵向平均正应变。根据试验机的载荷信息、钢绞线的参考截面积和由数字图像相关方法测量的平均正应变绘制出钢绞线的应力-应...     

Brazilian disk test and digital image correlation: a methodology for the mechanical characterization of brittle materials

In this paper, an optimized and reliable approach for the evaluation of the mechanical properties of brittle materials is proposed and applied to the characterization of geopolymer mortars. In particular, the Young’s modulus, the Poisson’s ratio and the tensile strength are obtained by means of a Brazilian disk test combined with the digital image correlation (DIC) technique. The mechanical elastic properties are evaluated by a special routine, based on an over-deterministic method and the least square regression, that allows to fit the displacement fields experienced by the samples during the experiment. Error sources, like center of the disk location and rigid-body motion components, were analyzed and estimated automatically with the proposed procedure in order to perform an accurate evaluation of the elastic constants. The strain field measured by DIC and the computed elastic properties were then used to perform a local stresses analysis. This latter was exploited to investigate the failure mechanisms and to evaluate the tensile strength of the investigated material and the obtained data were compared with those predicted by the ASTM and ISMR standards. Three different loading platens (flat, rod and curved) were adopted for the Brazilian test in order to evaluate their effect on the elastic properties calculation, on the failure mechanisms and tensile strength evaluation. Results reveal that the curve platens are the most suitable for the tensile strength calculation, whereas the elastic properties did not show any influence from the loading configuration. Furthermore, the proposed procedure, of easy implementation, allows to accurately calculate Young’s modulus, Poisson’s ratio and the tensile strength of brittle materials in a single experiment.     

Use of digital speckle pattern correlation for strain measurements in a CuAlBe shape memory alloy

A Cu–Al 11.2 wt.%-Be 0.6 wt.% shape memory alloy was subjected to a uniaxial tension test using an MTS load frame with an attached optical microscope. Digital images of the sample's surface were acquired using white light and He–Ne laser illumination. The obtained images were associated to the engineering stress–strain behavior, which was calculated from the measured displacement, strain and force. From the images, displacement vector fields were calculated for white light and laser illumination by digital image correlation (DIC) and digital speckle pattern correlation (DSPC) techniques respectively. Using white light it was possible to observe the grains and the martensitic phase transformation of the material more clearly than using DSPC; nevertheless, better quantitative results of displacement, in-plane strain and elastic moduli were obtained using DSPC than using DIC when they were compared to the reference values measured by electrical extensometry. Furthermore DIC and DSPC work as complementary techniques to determined the micro and macromechanical behavior of the CuAlBe shape memory alloy.     

基于声-超声技术的木材弹性模量测定方法研究

提取声信号基频是声-超声方法测定木材弹性模量的关键。由于噪声和传感器谐振频率等因素的影响,直接应用快速傅里叶变换（FFT）方法提取信号基频计算弹性模量通常比标准的力学方法大20%左右。基于此,提出应用最大公约数算法提取信号基频,构建一种测量木材弹性模量的改进方法,并给出此算法的详细步骤。应用该方法和FFT方法分别对杨树木芯样本进行测试,弹性模量Eu和Ef的计算值范围分别为8.23~40.32Gpa和7.94~51.87Gpa,对比标准力学方法（弹性模量Es测量值为6.72~36.35Gpa）,误差下降了约10%。进一步分析Eu-Es和Ef-Es的相关性,相关系数分别为0.94和0.86,都呈显著相关。实测数据表明,应用本文方法计算所得的木材弹性模量与力学方法测试的数值更加吻合,相关性更好。     

Speckle reduction in optical coherence tomography by adaptive total variation method

An adaptive total variation method based on the combination of speckle statistics and total variation restoration is proposed and developed for reducing speckle noise in optical coherence tomography (OCT) images. The statistical distribution of the speckle noise in OCT image is investigated and measured. With the measured parameters such as the mean value and variance of the speckle noise, the OCT image is restored by the adaptive total variation restoration method. The adaptive total variation restoration algorithm was applied to the OCT images of a volunteer’s hand skin, which showed effective speckle noise reduction and image quality improvement. For image quality comparison, the commonly used median filtering method was also applied to the same images to reduce the speckle noise. The measured results demonstrate the superior performance of the adaptive total variation restoration method in terms of image signal-to-noise ratio, equivalent number of looks, contrast-to-noise ratio, and mean square error.     

Measurement of nanometric displacements by correlating two speckle interferograms

This paper presents a method to measure nanometric displacement fields using digital speckle pattern interferometry, which can be applied when the generated correlation fringes show less than one complete fringe. The method is based on the evaluation of the correlation between the two speckle interferograms generated by both deformation states of the object. The performance of the proposed method is analyzed using computer-simulated speckle interferograms. A comparison with the performance given by a phase-shifting technique is also presented, and the advantages and limitations of the proposed method are discussed. Finally, the performance of the proposed method to process real data is illustrated.     

Image quality improvement of digital holography by superposition of reconstructed images obtained by multiple wavelengths

A method to improve the image quality of a digital holographic reconstructed image by means of speckle reduction is proposed. The size and position of the speckles are changed according to the wavelengths to record a digital hologram. By superposing reconstructed images with different wavelengths, the effect of speckle is reduced so that the image quality is improved. Optical experiments are given to confirm the proposed method.