基于数字图像相关的激光修复镍基合金疲劳损伤表征方法研究

开展激光修复镍基合金的疲劳试验，对研究修复材料疲劳性能及分析其失效行为具有重要意义。疲劳过程中，试件产生局部损伤，会导致材料力学性能恶化。如何建立光学变形场与疲劳损伤参数的定量关系实现对疲劳损伤的有效表征，是实验力学领域备受关注的问题。从激光修复镍基合金疲劳失效行为研究的需求出发，本文建立了基于单相机三维数字图像相关的疲劳光力学测试系统，并设计了包含“修复基体-修复边界-修复区”的组合型拉伸疲劳试件，在建立的测试系统下可同时测量和表征试件三种区域的全场变形。利用所建立的测试系统，实现了激光修复材料在拉-拉疲劳过程中的全场变形在线测量，得到了不同疲劳周次下的试件基体、修复边界和修复区的位移场与应变场；根据测量得到的应变场提出了疲劳中以试件历经不同循环周次后的残余应变为损伤参量的表征方法，并获得了修复镍基合金试件疲劳过程中残余应变场的演化规律。试验结果表明，在疲劳过程中，试件残余应变场内存在应变集中现象，应变集中幅值随疲劳周次的提高而增加，试件残余应变集中区域内的应变集中值远高于非应变集中区域，因此根据该结果可有效地预测疲劳裂纹萌生的时机与位置。     

单个弹丸撞击316L不锈钢引起的变形场

运用金属材料表面纳米化试验机对单个弹丸撞击316L不锈钢表面进行了撞击实验;采用激光共聚焦显微镜观察了弹坑的三维形貌,测量不同振动频率下弹坑的直径及离面位移;采用云纹干涉法对弹坑周围的面内应变场进行测量,并分析振动频率及撞击方式对弹坑尺寸、塑性应变大小以及塑性应变区范围的影响;采用有限元方法对单个弹丸垂直撞击试件表面的应变场进行数值模拟,与实验结果进行比较,分析了弹坑周围残余应力的分布。结果表明:随振动频率的增加,弹坑直径和离面位移都增加,频率在50~55Hz,弹坑直径有突变,离面位移和振动频率呈线性关系;振动频率越大,塑性应变越大,塑性应变分布范围均大于弹坑直径的2倍;同一振动频率下弹丸垂直撞击比倾斜撞击的塑性应变大,而塑性应变分布范围相差不大;面内残余应变场的数值模拟结果和实验结果吻合较好,最大误差小于10%。     

数字图像相关分析法增量位移场测试技术

利用位移场的连续性,对亚像素位移场的算法进行了一些改进,设计了一套分步计算位移场、应变场的测量计算方法,较好地解决了数字图像相关分析法计算精度和效率.采用增量位移场叠加的方法计算大应变位移场,采用局部平面拟合的方法计算应变场.通过对高分子材料拉伸试验位移场的测量和结果标定,说明该方法具有较强的实用性和计算精度.同时,由于避免了对亚像素点的搜索,大大提高了计算效率.     

网格数字图像相关方法测量位移场的研究

本文将数字图像相关方法与古老的网格法相结合,给出了一种测量面内位移场的新方法——网格数字图像相关方法,实现了高精度的测量,精度可达0.02亚像素．在该方法中网格点采用圆标记点,使该方法特别适用于小线应变位移场的研测．本文运用Windows下的新的图像采集方法测量物体位移场,突破了Dos系统下对内存操作的限制,并使每一帧图像的像素数由512×512增至768×576,提高了图形的分辨率．     

图像相关法在高分子材料拉伸性能研究中的应用

本文对聚碳酸酯(PC)和丙烯腈丁二烯苯乙烯(ABS)的合金(PC/ABS)高分子材料不同环境温度下的拉伸性能进行了试验研究。根据图像相关分析法编制了图像法位移测量分析软件,并对该分析软件的适用性进行了分析。系统研究了PC/ABS高分子材料拉伸试验时三个方向的位移场和应变场。根据测得的位移场研究了该高分子材料拉伸过程中应变和应变率的变化以及应力应变变化规律,并对试验结果进行了详细分析。结果表明,文中采用的图像法位移测量系统具有较高的测试精度;拉伸过程中,试件厚度方向的收缩变形大于宽度方向的收缩变形;颈缩过程区具有非常高的应变率,颈缩后的平直颈缩区的应变率快速下降到一个很低的应变率继续缓慢变形;尽管载荷位移曲线出现了较大的载荷下降现象,PC/ABS拉伸时的真应力应变曲线没有明显的应力下降现象出现,因此,载荷下降现象主要由颈缩时的截面减小引起;高分子材料PC/ABS的屈服应力随环境温度的升高而降低。     

孔边循环应变与疲劳循环次数相关性实验研究

材料的应变疲劳寿命与裂端的塑性应变有关。研究孔边循环应变与疲劳循环次数的相关性,对于研究材料损伤及微裂纹的产生,预防宏观裂纹的产生及扩展具有十分重要的意义。所以本文对有限宽板中心带孔试件进行了拉伸疲劳试验,用以研究这种相关性。在疲劳试验前首先进行逐级的加、卸载缓慢循环实验,同时利用数字图像相关技术测量孔周的位移场,进而求出孔边应变。在疲劳试验过程中,每当疲劳循环1500次就停机,进行同样的缓慢循环实验,并测量出孔周的位移场及孔边应变。实验研究中获得了一些很重要的结果。     

基于双线性位移模式数字图像相关方法的误差分析及应用

探讨数字图像相关方法测试结果的误差分布规律对提高该方法的测试精度具有重要意义。本文从理论上分析了基于双线性位移模式数字图像相关方法的子区变形场测试误差分布规律,结果表明,当试件的真实变形可由双线性位移模式描述时,变形场最大测试误差通常出现在子区的边界或节点处。因此,若试件发生均匀拉伸等常应变变形,可利用相关系数选取一个最优子区,认为测得最优子区中心应变为试件真实应变。零变形实验验证了该测试方法的可靠性。最后对手机导光板试件在单轴拉伸载荷下的数字图像进行分析,并利用本文方法测试了其弹性常数。     

双频自动网格法研究

传统的自动网格法常采用均匀布点法，这对应变集中区域的变形测量极为不利。节点过多，检测精度会下降，节点少了，会降低应变集中区域的空间分辨率。本文提出了双频自动网格法，在应变集中区域增加节点。文中研究了双频网格点的制作技术及对双频网格图的自动处理与分析。并应用双频自动网格法测量了中心带孔铝试件在拉伸加载过程中的孔边位移场。     

基于主应变场的混凝土全表面开裂特征实时测量与分析

测量裂纹的扩展过程对于揭示混凝土结构的破坏机理和评价其力学性能十分重要. 本文提出了一种基于混凝土表面变形场的裂纹定位和宽度测量方法, 首先基于多相机数字图像相关方法得到混凝土试件表面的高分辨变形场, 发现开裂引起的位移梯度使裂纹附近的虚主应变场明显区别于未开裂处, 且主应变场在裂纹法线方向近似高斯分布. 借鉴在激光条纹中心线定位中广泛采用的Steger算法思想, 提出了基于主应变场的裂纹定位方法, 并将裂纹两侧位于法线上的面内位移向量做差沿裂纹法线方向上的投影为Ⅰ型裂纹宽度, 沿裂纹切线方向上的投影为Ⅱ型裂纹宽度, 最终得到了裂纹每一点的位置和宽度. 利用高精度平移台设计了模拟裂纹扩展的实验, 以验证Ⅰ型裂纹宽度的测量精度. 实验结果表明: 裂纹宽度的测量误差在0.010 ~ 0.017像素之间, 与理论预测相符; 测量误差的标准差在0.006 ~ 0.008像素之间, 测量结果比较稳定. 在同等分辨率下, 本文方法的测量精度优于基于图像的裂纹测量方法. 本文提出的方法可以全自动、实时地测量裂纹扩展, 为混凝土实验提供了一种可靠、精确的全场裂纹测量手段.      

应用固有应变理论计测对接焊钢管残余应力场的实验研究

本文采用改进后的固有应变法对对接焊管接头残余应力计测进行了理论分析,确定了测量方案,研究果结表明对于可处理成轴对称问题的对接焊管残余应力问题只需不太多的测量值就能获得残余应力全场。本文就一个对接焊钢管试件进行了实验,实验计测结果同其它文献较为一致。它显示了应用改进后的固有应变理论决定对接焊钢管残余场的有效性。     

弹性力学问题的虚边界元-配点法

本文提出虚边界方法,建立了离散化虚边界元-配点法,给出了离散化求系数的积分解析式。本文方法完全避免了边界奇异积分及其复杂耗时的运算,成功地提高了普通边界元法(以下简称边界元法)中边界附近区域内包括边界上解的精度,保留了边界元法的优点并扬弃了其弱点。边界元间接法是本文方法中的一个特例。数值算例表明,程序可靠,节省机时,计算精度较高。     

确定大跨径悬索桥主缆成桥线形的虚拟梁法

精确计算悬索桥主缆索形是悬索桥设计和实现施工开环控制的关键。本文通过柔计算的虚拟梁法,导出了确定大跨径悬索桥主缆成桥线形的非线性方程组,建立了索型迭代的计算流程。     

Evaluation analysis of prediction methods for two-phase flow pressure drop in mini-channels

Two-thousand and ninety-two data of two-phase flow pressure drop were collected from 18 published papers of which the working fluids include R123, R134a, R22, R236ea, R245fa, R404a, R407C, R410a, R507, CO₂, water and air. The hydraulic diameter ranges from 0.506 to 12 mm; Re_l from 10 to 37,000, and Re_g from 3 to 4 × 10⁵. Eleven correlations and models for calculating the two-phase frictional pressure drop were evaluated based upon these data. The results show that the accuracy of the Lockhart–Martinelli method, Mishima and Hibiki correlation, Zhang and Mishima correlation and Lee and Mudawar correlation in the laminar region is very close to each other, while the Muller-Steinhagen and Heck correlation is the best among the evaluated correlations in the turbulent region. A modified Chisholm correlation was proposed, which is better than all of the evaluated correlations in the turbulent region and its mean relative error is about 29%. For refrigerants only, the new correlation and Muller-Steinhagen and Heck correlation are very close to each other and give better agreement than the other evaluated correlations.     

The transient response of second order non-linear equations

A method is presented for calculating the transient response of second order non-linear differential equations. The increase in accuracy over such methods as the Kryloff-Bogoliuboff technique, physically speaking, is due to the inclusion of the phase-lag effects caused by the linear damping term in the equation. Mathematically the increase in accuracy is obtained by an appropriate choice of the zero-th order terms for the dependent variable and its first derivative. This inclusion requires little more calculation effort than that required in the Kryloff-Bogoliuoff technique. Additionally, this method may be iterated to obtain solutions of increased accuracy ; the next higher order solution being carried out in detail in the text. Examples are presented to show the increase in accuracy of the present method over the Kryloff-Bogoliuboff technique.     

INTERPOLATION PERTURBATION METHOD FOR SOLVING THE BOUNDARY LAYER TYPE PROBLEMS

ＹｕａｎＹｉｗｕ（袁镒吾）（ＲｅｃｅｉｖｅｄＯｃｔ．２，１９９４；ＣｏｍｍｕｎｉｃａｔｅｄｂｙＣｈｉｅｎＷｅｉｚａｎｇ）ＩＮＴＥＲＰＯＬＡＴＩＯＮＰＥＲＴＵＲＢＡＴＩＯＮＭＥＴＨＯＤＦＯＲＳＯＬＶＩＮＧＴＨＥＢＯＵＮＤＡＲＹＬＡＹＥＲＴＹＰＥＰＲＯＢ...     

Performance of digital image velocimetry processing techniques

Digital particle image velocimetry (DPIV)-processing techniques have become increasingly more sophisticated in recent years. However, much work is still done using standard traditional methods of analysis. This paper investigates several traditionally based techniques for cross-correlation image processing in terms of computational efficiency and measurement accuracy. Direct spatial domain correlation, standard fast Fourier transform (FFT) correlation, a dynamic FFT correlation technique, and a new hybrid correlation method are discussed and evaluated. In addition, a particle-tracking velocimetry scheme based on that of Cowen and Monismith (1997) is examined in the same context as the DPIV methods. A detailed examination of the behaviors of each correlation method reveals that direct spatial domain correlation is more accurate than FFT-based methods, with the standard FFT correlation showing the weakest performance. Using the more robust methods (dynamic FFT and hybrid correlation), accuracy can be improved significantly over the standard FFT method in many cases, while still remaining computationally efficient. The particle-tracking algorithm studied was found to yield comparable accuracy to the DPIV routines and can provide much higher spatial- resolution possibilities. Received: 3 September 1999 / Accepted: 21 June 2001 Published online: 29 November 2001     

A New Study on the Relative Kinematic Accuracy Reliability of a Novel Exoskeleton with Series-Parallel Topology

An excellent exoskeleton worn by the operator might be able to move with a high degree of accuracy and repeatability. Based on the previous kinematic accuracy reliability model, this article proposes two new definitions of the relative kinematic accuracy reliability to investigate the motion accuracy ability of a novel exoskeleton with series-parallel topology thoroughly. According to the new definitions, the calculating results show that x-, y-, and z-directions of the exoskeleton end-effector have different motion accuracy abilities, the motion accuracy ability of x is the best of all, and the motion accuracy ability of y is better than that of z. To study error sources, we illustrated the geometric tolerances and driving errors, which cause the uncertainties of the exoskeleton end-effector. The effects of geometric tolerances and driving errors on kinematic accuracy reliability and relative kinematic accuracy reliability, which are the sensitivity analyses, were numerically simulated and elucidated. The results reveal that the error sensitivity in the x-direction is the lowest and the error sensitivity in the y-direction is lower than that in the z-direction. Besides this, the effects of driving errors on the motion ability of all directions of the exoskeleton end-effector are larger than those of geometric tolerances. This approach for modeling and calculating kinematic accuracy reliability, relative kinematic accuracy reliability, and their sensitivity analysis provides a novel and unique reference standard for exoskeleton and other mechanism design.     

考虑畸变影响的箱梁横向弯矩计算方法

为研究偏心垂向荷载作用下梯形截面单室箱梁的横向弯矩,对框架分析法计算箱梁横向弯矩的方程进行优化,并在刚性支承法的基础上提出一种更加简单的横向弯矩计算方法;与框架分析法不同,横向弯矩可采用能量变分法求解,建立以箱梁顶板剪力差为未知量的四阶控制微分方程,采用比拟的弹性地基梁解法解出剪力差,得出梯形截面单室箱梁横向弯矩的能量变分法解。对几种箱梁横向弯矩计算方法用两个算例进行验证,结果表明,能量变分法解将箱梁底板上的弯矩误差绝对值由15.41%降到了9.68%;本文方法得出的横向弯矩结果和有限元结果吻合较好,弯矩误差绝对值最大不超过6.01%;本文方法和能量变分法可有效降低箱梁底板上的弯矩误差,计算精度得到提高。     

Fast nanoparticle sizing by image dynamic light scattering

In this paper,an image dynamic light scattering method for nanoparticle sizing is introduced.The spatial distribution of the scattered lights from nanoparticles undergoing Brownian motion was captured at a high frame rate by a digital camera within one second,which is considerably faster than the conventional photon correlation spectroscopy method.The captured series of photographs were meshed into thousands of small units for calculating the intensity autocorrelation functions in parallel.Experimental results from the measurements of three reference nanoparticle samples(27,80,and 352 nm in diameters)demonstrated the feasibility of this method.     

一种考虑旋转位移分量的数字图像相关法及其精度分析

提出了一种新的考虑旋转位移分量的数字图像相关法[CD2]旋转相关匹配法,其特点是先对图像子区进行预旋转,再进行相关匹配、相关系数、整像素和亚像素位移的计算。在物体产生旋转位移分量时,该方法可有效提高变形前后图像子区间的相关系数和位移计算精度。通过斜拉伸、剪切和悬臂梁弯曲实验,对旋转相关匹配法和直接相关匹配法进行了对比研究,并用灰度梯度法计算旋转后新图像子区和变形后图像子区间的亚像素位移。结果表明,在旋转位移分量小于10°的情况下,相比于直接相关匹配法,旋转相关匹配法可以得到更高的相关系数;在用灰度梯度法计算亚像素位移条件下,位移精度与试件位移不产生旋转时的直接相关匹配相当。