REAL TIME MEASUREMENT AND ANALYSIS OF FULL SURFACE CRACKING CHARACTERISTICS OF CONCRETE BASED ON PRINCIPAL STRAIN FIELD

The measurement of the crack propagation process is significant to reveal the failure mechanism and evaluate the mechanical properties of concrete structure. This paper presents a method of crack location and width measurement based on the deformation field of concrete surface. The high-resolution deformation field of the concrete specimens is obtained by using multi-camera digital image correlation method at first. It is found that the virtual principal strain field around the crack obviously differs from that in uncracked zone because of the gradient of displacement field caused by crack, and the principal strain field can be regarded as approximately Gaussian distributed in the normal direction of the crack. A new method of crack location based on the principle strain field is proposed based on the Steger algorithm which is widely applied in laser stripe center extracting, and the difference of in-plane displacement vectors on the normal direction between two sides of the crack is obtained. The projection along the normal direction is taken as the width of mode I crack, while the projection along the normal vertical direction is taken as the width of mode II crack. The experiment of simulating crack propagation is performed by using a high precision translation table to verify the measurement accuracy of crack width measurent. The results of the experiments show that the measurement error of crack width is between 0.010 pixel and 0.017 pixel, which is consistent with the theoretical prediction. The standard deviation is between 0.006 pixel and 0.008 pixel, which illustrates that the measurement accuracy of the proposed method is high. The accuracy of the proposed method is better than that of image-based crack measurement method at the same image resolution. The method proposed can automatically measure crack propagation in real time, which provides a reliable and accurate method for the concrete experiment.