Photogrammetric determination of 3D crack opening vectors from 3D displacement fields

Abstract

This publication presents a procedure for the determination of all three components of crack opening vectors from stereoscopic image sequences of a specimen under load in civil engineering material testing. The method is based on analyzing stereoscopic image sequences of a concrete specimen with a surface texture, which is suitable for applying image matching techniques. Spatio-temporal correspondences are established by applying sub-pixel accuracy area based image matching techniques to a grid of surface points. Data acquisition starts at zero load. The load is stepwise or continuously increased during the experiment. The surface points are matched between the stereo images and tracked through each camera image sequence. As an intermediate result, we obtain a set of 3D object surface points for each epoch by spatial intersection. These 3D object points are triangulated into a mesh. Then, the mesh triangles are tested for deformations by transforming the triangles into 2D space and computing the norm of the 2D relative translation vector. Connected components of deformed triangles are determined and crack normals are computed. In the next step, the 3D relative translation vector can be derived for each deformed triangle. Defining local crack opening coordinate systems for the deformed triangles, the three components of the crack opening vectors can be computed. The method has been tested and validated in practical experiments. The technique is capable of quantitatively analyzing cracks with a width of less than one pixel in image space.