基于退相关DIC的疲劳裂纹全局动态测量方法

工程材料和结构在反复荷载长期作用下容易发生疲劳开裂, 疲劳裂纹测量对于开展科学试验研究和工程问题分析都至关重要, 但现有方法无法实现高精度的疲劳裂纹全局动态测量. 本文基于数字图像相关(digital image correlation, DIC)技术, 合理利用DIC的退相关效应, 提出一种疲劳裂纹全局动态测量及可视化方法. 该方法首先在相机采集得到的裂纹图像内, 建立具备拓扑关系的目标点云结构, 并运用DIC亚像素算法得到裂纹区域位移场, 再基于零均值归一化互相关(zero-mean normalized cross correlation, ZNCC)计算结果剔除退相关的DIC目标点(灭点). 进一步通过“三生点”算法提取得到裂纹离散边界, 并采用最小二乘法将离散边界拟合为连续裂纹边界, 实现裂纹形态的几何重构, 最终自动计算得到裂纹长度和宽度的动态变化过程. 该方法原理清晰、理论简单, 易于实现. 开展数值模拟和钢节点疲劳试验, 对相关算法和图像采集参数进行了验证, 结果表明本文方法对疲劳裂纹边界的数字化重构误差在0.5个像素内, 基于重构结果计算得到的裂纹长度和宽度误差分别为0.46像素和0.08像素(类同于0.06 mm和0.01 mm), 并成功实现了对疲劳试验裂纹扩展形态的精细化动态测量及可视化. 研究成果证明了DIC技术用于疲劳裂纹全局动态测量及可视化的有效性, 并在测量精度、效率、成本等方面具有显著优势, 可在实验室测量和工程现场测试中推广应用. 

FULL-FIELD DYNAMIC MEASUREMENT METHOD FOR FATIGUE CRACKS BASED ON DECORRELATION DIC1)

The decorrelation effect of digital image correlation (DIC) can cause DIC calculation failure, and it is always been regarded as a defect of DIC, the problem seriously hinder the promotion and application of DIC in the field of fracture mechanics. Meanwhile, structures (such as steel structures) are prone to fatigue cracking under repeated loads. Fatigue crack measurement is very important for carrying out model test research and engineering problem analysis. However, the existing methods are not suitable for full-field dynamic fatigue crack measurement with high-precision. This research proposed a novel full-field dynamic fatigue crack measurement approach and its visualization by using the principle of DIC. The approach first constructs point-cloud data structure with topological relations and calculates the crack displacement fields for the captured digital images of cracks. The zero-mean normalized cross-correlation (ZNCC) criterion is employed to eliminate the vanishing points within cracked regions, and the discrete birth and death boundaries of cracks are extracted and interpolated by a presented "three-living point" algorithm. The least square method is finally utilized to convert the discrete crack boundaries into continuous crack boundaries, and as a result the dynamic varying process of crack length and width are automatically calculated. Numerical simulation and fatigue tests are carried out to verify the accuracy of fatigue crack measurement algorithm. Results show that the digital reconstruction errors of fatigue crack boundaries are within 0.5 pixel. The calculated errors of crack length and width are respectively 0.46 pixel and 0.08 pixel. Furthermore, refined measurement for the dynamic propagation process of cracks are successfully achieved in fatigue tests of welded steel joints. This research proves that, due to the advantages in accuracy, efficiency, and cost, the presented full-field dynamic fatigue crack measurement approach and its visualization using DIC technology is highly effective, and thus is applicable to laboratory measurement and engineering field testing.