【作者】 赵健；

【导师】 赵东；

【作者基本信息】 北京林业大学 ， 机械设计及理论， 2014， 博士

【摘要】 数字散斑相关方法作为一种全场位移和应变测量方法,在理论方面得到不断发展和完善的同时,在工程应用方面也开展了大量的工作。但目前的方法在求解效率和测量精度以及使用的通用性方面还存在一些不足。为了更好地满足工程测试要求,本文对现有的数字散斑相关方法进行了进一步的研究与改进,论文的主要研究内容和创新点如下。本文针对现有的整像素逐点搜索算法对数字散斑相关方法搜索效率的影响,提出了一种基于连续性变形假设的邻近域搜索算法。该算法首先通过逐点搜索法确定参考子区Ω在变形后图像中的目标子区Q’的位置后,根据连续性变形假设,只需要在目标子区Ω’附近的一个较小区域搜索即可,故只需要对第一个参考子区进行全场逐点搜索,其余子区都可以小范围搜索,结果可大幅提高搜索效率。针对传统的基于灰度梯度的亚像素位移求解算法中采用的灰度不变假设对位移求解精度的影响,将灰度线性变化引入基于梯度的亚像素位移算法。该算法假设变形前后图像内同一点的灰度由于光强波动而发生线性变化,引入灰度变化系数λ,应用最小二乘迭代得到亚像素位移,解决了传统梯度法由于光强变化引起的测量误差问题。同时,定性地从图像噪声、亚像素插值误差和形函数误差三个方面分析了基于梯度的数字散斑相关测量方法的测量误差,得到位移误差标准差的数学表达式。在影响数字散斑相关位移求解精度的因素中,除了算法误差外,还包括散斑图质量、镜头光学失真等外界因素。为了提高散斑图质量进而提高数字散斑相关测量精度,本文提出一种新的模拟二值散斑图,其可以在保证散斑颗粒随机分布的同时通过判别散斑颗粒的位置与尺寸,避免出现传统模拟散斑图散斑颗粒过稀或过密的情况。该方法为模拟散斑图的实现提供了一条新思路。在人工散斑制作方面,本文将碳粉热转印技术引入散斑制作,该方法通过热压原理将设计打印好的随机散斑图样转印在试件表面,具有能够控制散斑颗粒的大小及分布密度和可重复性等优点,避免了传统喷涂散斑操作中由于人工经验不足造成的误差,为人工散斑的制作提供了一条新的途径。针对镜头光学失真对数字散斑相关测量精度的影响,本文在已有镜头畸变参数估计算法的基础上,采用Harris法对标定模板的特征点进行亚像素提取,并同时考虑镜头的径向与切向畸变,对数字散斑相关测量试验中所使用的CMOS镜头进行了非线性畸变系数估计与校正。通过试验验证了镜头畸变校正对数字散斑相关位移测量的重要性。在数字散斑相关方法的应变求解中,传统的位移微分过程会放大位移场中所包含的噪声。为了提高应变求解精度,本文将有限元思想引入数字散斑相关应变场的计算,提出了一种基于四边形单元的应变计算方法。首先,利用数字散斑相关方法获得的位移场建立4节点和9节点四边形单元,通过对应的状态方程计算出对应节点的位移值,然后反插值计算出对应单元内每个点的应变进而求得应变场。最后,通过无疵和含孔试件的拉伸试验,验证了本文提出的算法能够有效地提高应变计算精度。为了证明本文提出的改进数字散斑相关方法在工程应用中的有效性和适应性,以三个典型的工程测试问题为例进行了讨论。应用实例一是与温控试验箱结合建立了非接触变温变形测量系统,用该系统测量了45号钢在20℃～200℃温度范围内的全场热变形和热膨胀系数。应用实例二是在断裂力学中的应用,通过木材单边裂纹拉伸试验,得到了落叶松的复合型应力强度因子和裂尖塑性区尺寸。应用实例三是在蠕变力学中的应用,结合自行设计的机械式蠕变试验机对45号钢的室温蠕变进行测量,得到了不同应力水平下的全场应变和蠕变应变-时间曲线。以上实例验证了本文所提出的改进的数字散斑相关方法应用于工程测试的可行性,表明了该方法的实用性和适应性。更多还原

【Abstract】 Two-dimensional digital speckle correlation mothod is an effective optical measurement technique for full-field displacement and strain estimation, which has been widely used in the fields of experimental mechanics and engineering measure and test. However, deficiency existed in terms of the calculation accuracy, calculation efficiency and the versatility. An improved algorithm here aims at making this method a widely applicatory measuring method. It means that ideally this method can be applied in various conditions. The key results of this dissertation are:In view of the inefficiency of point-by-point search algorithm, an adjacent domain search algorithm is proposed. This algorithm can greatly improve the efficiency of the integer pixel search. A grayscal linear change model λ used in the gray gradient-based method for subpixel-accuracy displacement measurement is proposed. This proposed method could deal with the gray change or illumination variation on the specimen surface during the experiment. A formula of displacement measurement errors by the gradient-based DSCM method was derived. The errors were found to explicitly relate to the image grayscale errors consisting of sub-pixel interpolation algorithm errors, image noise, and subset deformation mismatch at each point of the subset.A new algorithm of simulated random speckle pattern is proposed. In this way, the distribution of spots with respect to speckle diameter and location can be controlled and optimized. Toner transfer technique is exploited to provide an efficient method to realize speckle patterns on the specimen surface for DSCM. This technique provides us with a novel approach for artificial speckle pattern realization. Lens distortion practically presents in optical imaging system using in DSCM system, and gives rise to additional errors in the displacement and strain measurement. Camera calibration procedure based on Harris method is performed to obtain the coefficients of radial distortion and tangential distortion. The three-point bending test results show that the camera lens calibration method can effectively eliminate the effect of lens distortion and improve displacement and strain measuremen accuracy.This dissertation proposes a digital speckle correlation strain measurement method based on a finite element (FE) algorithm. Both simulation and experiment processing, including tensile strain deformation, show that the proposed method can achieve nearly the same accuracy as the cubic spline interpolation method in most cases and higher accuracy in some cases, such as the simulations of uniaxial tension with and without noise. The results show that it also has a good noise-robustness. Finally, this method is used in the uniaxial tensile testing for Dahurian Larch wood specimens with or without a hole, and the obtained strain values are close to the results which were obtained from the strain gauge and the cubic spline interpolation method.The other important part of my work is the application of DSCM in engineering measure and test. Some key issues are discussed in the prosess of testing. The key results of this part of work includes:(1) The thermal deformation fields and coefficient of thermal expansion of45steel at different temperatures are measured and compared with the existing literature value. The repeated experimental results confirm the effectiveness and accuracy of the noncontact varying temperature deformation measuring system.(2) The mixed mode stress intensity factors are calculated using displacements from digital speckle correlation method. With the crack angle increasing, the SIF of mode I in unilateral crack increase all along, but the SIF of mode II in unilateral crack shows an initial increase, followed by a decrease. Crack-tip plastic zones were determined using an anisotropic yield criterion.(3) A mechanical creep testing machine is developed by author and used in combination with DSCM. Creep strain-time curves of45steel at room temperature were measured using digital speckle correlation method.Actual measurement results demonstrate that the proposed DSCM algorithm is a high accurate measurement method and possesses preferable practicability and adaptability.更多还原