Effect of the three-dimensional static pre-stress on the dynamic behaviours of conglomerate: True triaxial Hopkinson pressure bar tests

Abstract

As the main component of gravel layer, the dynamic mechanical properties of conglomerate are closely related to the efficient rock-breaking methods while drilling in the formation rich in gravels. The characteristics of high strength and strong heterogeneity of conglomerate lead to severe vibration of drill string, causing the bottom hole rock of gravel layer is in a complex stress state of impact loads and three-dimensional confining pressure. Due to the lack of relevant study previously, this kind of dynamic mechanical behaviour is still unclear. Using the three-dimensional split Hopkinson pressure bar (3D SHPB) system, the dynamic mechanical behaviours of conglomerate under the condition of various impact loads and different levels of static pre-stress state were studied. The CT scanning results indicate that the conglomerate is characterized by strong randomness in gravel size, shape, content, and distribution, which plays an important role in the heterogeneity. The dynamic mechanical behaviours of conglomerate show a significant strain rate correlation, and that is influenced by the impact loads and static pre-stress state. There will be the highest dynamic strength and the lowest failure strain in the impact direction when the specimen is in a hydrostatic pre-stress state. With the increase of hydrostatic pre-stress, the dynamic strength of conglomerate increase linearly, while the failure strain changes the opposite. The research makes a beneficial exploration of the dynamic behaviors of conglomerate, which is expected to provide a reference for the design of high-efficiency rock-breaking methods while drilling in the stratum rich in gravels.