基于高速数字图像相关法的疲劳裂纹尖端位移应变场变化规律研究

应用高速摄像数字图像相关法研究了谐振式疲劳裂纹扩展试验中紧凑拉伸(CT)试件在高频正弦交变载荷作用下,裂纹稳态扩展阶段裂纹尖端区域位移和应变场的变化规律。采用数字化高速摄影设备采集系列正弦交变载荷作用下CT试件数字散斑图像,应用数字图像相关(DIC)法计算每幅图像裂纹尖端区域位移和应变场,对裂纹尖端区域特征点的位移、应变值采用最小二乘正弦拟合方法进行拟合,求出振幅、相位、平均载荷等特征量,将拟合出的应变或位移正弦曲线和所对应的系列散斑图像进行匹配,找到一个应力循环内特征位置的图像。使用动态高精度应变仪测量了CT试件在一个应力循环内裂纹尖端点应变值,试验结果表明,DIC应变测量最大误差为4.12%,验证了所提出DIC测量方法的可行性。在此基础上,进行了基于高速DIC方法的谐振式疲劳裂纹扩展试验,研究了疲劳裂纹未扩展时裂纹尖端应变幅值和疲劳循环次数的关系及疲劳裂纹扩展到不同长度时裂纹尖端区域位移和应变场的变化规律。

Research on Variation Law of Fatigue Crack-tip Displacement and Strain Fields Based on High-speed Digital Image Correlation Method

In order to research the variation laws of displacement and strain fields on the fatigue crack tip area of the compact tension（CT）specimen in the stage of steady crack propagation under high frequency sinusoidal alternating load, a method based on the digital image correlation (DIC) and digital high-speed photography technology is proposed. A series of digital speckle images of CT specimen under sinusoidal alternating load are collected by digital high-speed photographic equipment, and the displacement and strain fields on the crack tip in each image are calculated by DIC. The sinusoidal changing strain curve of the feature point on the fatigue crack tip area is obtained by the least square sine wave fitting method, and the characteristic parameters of sinusoidal strain, such as amplitude, frequency, phase and mean load, are calculated. The images of characteristic position in a stress cycle are obtained by comparing the fitted sine curve of strain with the corresponding speckle images. Then the strain values at the tip of CT specimen during one stress cycle are measured by dynamic strain gauge. The results show that the maximum measurement error of strain by DIC method is 4.12% . On this basis, the resonant fatigue crack propagation test is carried out based on the high-speed photography and digital image correlation method, the relationship between strain amplitude values at crack tip and fatigue cycle number before the fatigue crack extension, and the variation laws of displacement and strain fields on crack tip area with the different lengths of fatigue crack are studied.