数字散斑相关方法及其在工程测试中的应用研究

发布时间：2018-05-29 13:11

本文选题：数字散斑相关 + 位移与应变测量　；参考：《北京林业大学》2014年博士论文

【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。
[Abstract]:As a method of full field displacement and strain measurement, the digital speckle correlation method has been developed and perfected in theory, and a great deal of work has been carried out in engineering application. However, there are still some shortcomings in the current method in solving the efficiency, measurement accuracy and the universality of the use. In this paper, the existing digital speckle correlation method is further studied and improved. The main research contents and innovations are as follows.
In this paper, a neighborhood search algorithm based on continuous deformation hypothesis is proposed to determine the effect of the existing integer pixel search algorithm on the search efficiency of the digital speckle correlation method. First, the algorithm determines the position of the target subregion Q 'of the reference subregion Omega in the deformed image by the point by point search method. We only need to search for a small area near the target region Omega, so it is only necessary to search the first reference subregion by the whole field, and the remaining subregions can be searched in a small range. The results can greatly improve the search efficiency. The gray invariant hypothesis used in the traditional gray gradient based subimage displacement algorithm is used. The gray linear change is introduced to the gradient based subpixel displacement algorithm. The algorithm assumes that the gray level of the same point in the image before and after the deformation is linearly changed due to the intensity fluctuation. The gray change coefficient is introduced into the gray level, and the subpixel displacement is obtained by using the least square iteration. The traditional gradient method is used to solve the light intensity. At the same time, the measurement error of the gradient based digital speckle correlation measurement method is analyzed qualitatively from three aspects of image noise, sub-pixel interpolation error and shape function error, and the mathematical expression of the standard deviation of displacement error is obtained.
In order to improve the quality of speckle pattern and the optical distortion of the lens, in addition to the algorithm error, in order to improve the speckle pattern quality and improve the accuracy of the digital speckle correlation measurement, a new simulated two value speckle pattern is proposed in this paper, which can ensure the random distribution of speckle particles. At the same time, by judging the position and size of speckle particles, the traditional simulated speckle pattern speckle particles are not too thin or overdense. This method provides a new idea for the realization of the simulation of speckle patterns. In the field of artificial speckle making, this paper introduces the thermal transfer technology of carbon powder into speckle making, and the method will be designed by the principle of hot pressing. The printed random speckle pattern is printed on the surface of the specimen, which can control the size, distribution density and repeatability of the speckle particles. It avoids the error caused by the shortage of artificial experience in the traditional spray speckle operation, and provides a new way for the fabrication of artificial speckle. On the basis of the estimation algorithm of the lens distortion parameters, the Harris method is used to extract the feature points of the calibration template, and the radial and tangential distortion of the lens are considered at the same time. The nonlinear distortion coefficient of the CMOS lens used in the digital speckle correlation measurement test is estimated and corrected. The importance of lens distortion correction to digital speckle correlation displacement measurement is verified by experiments.
In solving the strain solution of the digital speckle correlation method, the traditional displacement differential process magnifies the noise contained in the displacement field. In order to improve the accuracy of the strain solution, this paper introduces the finite element method into the calculation of the digital speckle correlation strain field, and proposes a strain calculation method based on the quadrilateral element. First, the digital speckle phase is used. The displacement field obtained by the closed method is established by the 4 node and the 9 node quadrilateral element, and the corresponding displacement value is calculated by the corresponding state equation. Then the strain of each point in the corresponding unit is calculated by the inverse interpolation and then the strain field is obtained. Finally, it is proved that the algorithm proposed in this paper can be effective through the tensile test of the free and porous specimen. The accuracy of the strain calculation is improved.
In order to prove the effectiveness and adaptability of the improved digital speckle correlation method in engineering application, three typical engineering testing problems are discussed. An application example is used to establish a non contact variable temperature deformation measurement system combined with temperature control test box. The system has been used to measure the temperature range of No. 45 steel at 20 C to 200 c. The full field thermal deformation and thermal expansion coefficient in the surrounding area. Application example two is applied in fracture mechanics. Through the tensile test of single side crack of wood, the composite stress intensity factor and crack tip plastic zone size of larch are obtained. Application example three is applied in Creep Mechanics, and the mechanical creep test machine designed by itself is applied to No. 45 The room temperature creep of steel is measured and the full field strain and creep strain time curve under different stress levels are obtained. The above examples verify the feasibility of the improved digital speckle correlation method proposed in this paper for engineering testing, which shows the practicability and adaptability of the method.
【学位授予单位】：北京林业大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TP391.41;TB22

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