非连续数字图像相关方法在裂纹重构中的应用

数字图像相关方法作为一种新的非接触式位移测量方法,在力学工程中有广泛的应用前景,然而受限于标准方法对图像变形的连续性要求,这种高效的测量方法在断裂力学领域的推广受到了限制. 为解决这一问题,提出采用引入子区分离数学模型,代替标准方法的连续模型,来对非连续区域进行精确识别和匹配的非连续数字图像相关方法. 研究子区被裂纹等非连续分割后原始像素点的位移情况,并引入裂纹张开向量用以表征被分割子区的主区和副区的位移关系;从而建立子区分离模型的数学表达式,并且为所提出的模型设计相应的图像相关算法;然后将所提出的非连续数字图像相关方法应用于重构平板拉伸试验开裂过程中图像的位移. 研究结果表明,相比于标准的数字图像相关方法,所提出的非连续数字图像相关方法解决了图像相关法在非连续区域失效的问题,提高了数字图像相关方法对位移测量的正确率,特别是能够准确重构裂纹面及附近的位移场,其测量精度能够达到亚像素级别. 

APPLICATION OF DISCONTINUOUS DIGITAL IMAGE CORRELATION IN CRACK RECONSTRUCTION 1)

The digital image correlation (DIC) method is a new non-destructive, contactless displacement measurement approach and it can be broadly applied in mechanical engineering with wide application prospect. However, this efficient and convenient displacement measurement method is difficult to apply in fracture mechanics due to the limitation that continuous deformation is required when processing the deformed images with standard digital image correlation methods. Aiming at solving this problem, this paper proposes a novel discontinuous digital image correlation (DDIC) method by introducing the splitted subset model to take the place of continuous model and use it to analyze the discontinuous areas where standard digital image correlation method is not valid in. The displacement of the original pixel points is studied when discontinuities such as cracks occurs, and the crack opening vector is introduced to represent the displacement relationship between the master zone and the slave zone after subset splitting into two parts. Thus the mathematical model of the splitted subset can be established by using the crack opening vector to correlate the master zone and the slave zone, and the corresponding image correlation algorithm can be designed based on this model. Afterwards, the proposed discontinuous digital image correlation method is used to measure the displacements of the deformed images obtained from the cracking process when a tensile test is applied to the notched plate. The research results show that the proposed DDIC method works well in both continuous and discontinuous areas, and when compared with the standard DIC method, the DDIC method is capable to solve the validation problem of image correlation method in discontinuous region, and improves the accuracy for displacement measurement, moreover, the proposed method is able to reconstruct the crack faces and the displacement fields in the vicinity region crack, and the accuracy of displacement measurement can be controlled within the range of subpixel level.