Efficient Investigation of Rock Crack Propagation and Fracture Behaviors during Impact Fragmentation in Rockfalls Using Parallel DDA

The study of the rock crack propagation and fracture behaviors during impact fragmentation is important and necessary for disaster evaluation of rockfalls. Discontinuous Deformation Analysis (DDA) incorporating virtual joints can offer a powerful tool to solve such a problem. In the analysis process, the computational efficiency is critical because the mesh must be very dense to make crack propagation more realistic. Thus, parallel DDA using OpenMP is applied. The flattened and precrack Brazilian disc tests are first reproduced, respectively, to verify the accuracy and efficiency of the parallel DDA with virtual joints. Then, the impact fragmentation process is simulated and validated with corresponding laboratory experiments in terms of crack propagation results. Furthermore, the effects of joint-slope angle, joint connectivity rate, and impact velocity on rock fracture behaviors are investigated. It is concluded that the peak number of cracks occurs when the joint-slope angle ranges between 30° and 45°; the higher impact velocity and joint connectivity rate tend to cause more cracks and larger damages to the specimen.

EFFICIENT MULTI-VIEW 3D TRACKING OF ARBITRARY ROCK FRAGMENTS UPON IMPACT

This paper presents a new methodology to accurately obtain 3D rotational velocities of blocks and fragments. Four high speed cameras are used to capture the scene. An additional two tilted mirrors are used to multiply the number of views. Hence, a total of six different viewing perspectives can be used to track translational and rotational velocities in 3D. The focus in the current work is on the rotational velocities, as tracking of the translation is generally straightforward. A common outline tracking algorithm based on the visual hull is adapted. The visual hull is further meshed using triangular elements to approximate the shape of the object. This 3D reconstruction is then used to track the 3D motion of the object. However, the accuracy of the results strongly depends on the accuracy of the 3D reconstruction which is mainly influenced by the number and position of the available views. In any case, the 3D reconstruction from the visual hull is only an approximation and significant errors can be introduced which influence the tracking accuracy. Hence, an in-house post-processing algorithm based on the knowledge of the real geometry of the object, which can generally be accurately determined after a test, was developed. The improved performance of this new post-processing method is shown by controlled spinning tests. Finally, results of a real example of an impact fragmentation test are discussed.