The Influence of Multiple Dynamic Loading on Fragmentation Characteristics in Dynamic Compression Tests

Abstract

Dynamic fragmentation is one of the primary fracture types for rock mass in rock engineering. The dynamic fragmentation process and dynamic fragment size characteristics are important for the fragmentation optimization in field. Most researchers studied the fragmentation forming process via dynamic compression test using the traditional split Hopkinson pressure bar (SHPB) system, in which the multiple dynamic loading is generally applied on the rock specimen. However, the effect of the multiple loading on the dynamic rock fragmentation characteristics was rarely attempted. In this study, the momentum-trap system is used in SHPB to achieve the single dynamic compressive loading on the rock specimen. Westerly granite and Fangshan marble were chosen to investigate the effect of multiple dynamic compressive loading pulses on the fragmentation characteristics. During the dynamic loading, the fragmentation process was captured using the high-speed camera. The results show that under the specific loading rate, the multiple loading has no influence on measuring the dynamic compressive strength of rocks, but it was observed in the high-speed images that more fragments were produced in the multiple loading. Moreover, the failure mode of tested specimens under distinct compressive loading rates was examined. The results indicate that the fragmentized degree of rocks with single dynamic loading is less than that with multiple dynamic loading at a given loading rate. This trend is relatively obvious when the loading rate is lower. In addition, the D50 values of rocks with multiple dynamic loading are smaller than those with single dynamic loading. The fragment size distribution also indicates that the multiple loading pulses dominantly influence the produce and size distribution of the fine-grain fragment. Furthermore, the mean fragment size derived from Weibull distribution decreases as the loading rate increases. The multiple loading pulses could decrease the mean fragment size and generate more fine grains when comparing to the fragment size of rocks derived from the single loading pulse. Therefore, the multiple compressive loading may result in an inaccurate fragmentation size analysis, and the single dynamic compressive loading should be applied when evaluating the dynamic fragmentation efficiency of rocks.