The strength and crack behavior of the rock-like gypsum under high strain rate

Abstract

The strength of rock and rock-like brittle materials is loading-rate dependent in a wide range of strain rate. Besides, the crack initiation and propagation processes under the high strain-rate loading and quasi-static loading are different. In order to investigate the dynamic fracture performance under the high strain rate conditions, rock-like artificially moulded gypsum specimens with and without pre-existing flaw(s) are loaded under different strain rates. A quasi-static loading is applied to the specimens by a uniaxial compression machine. The dynamic loading is produced by the split Hopkinson pressure bar (SHPB). The strain-time history recorded by the strain gauges attached on the incident and transmitted bars is used to obtain the strain-stress curve in the specimen. At the same time, the entire fracturing process is recorded by a high speed video system at a frame rate of 40,000 frames per second. It is found that the strength of the gypsum specimens increases apparently as the strain rate increases from approximately 10 2 s-1 to103 s-1, while the strength is nearly constant under a quasi-static loading of a strain rate from 10 -6 s-1 to 10-3 s-1. With regard to the fracturing processes, for specimens containing a pre-existing flaw, the high speed camera images show that the first tensile wing cracks initiating under the dynamic condition are similar to those under the quasi-static condition. However, the dynamic secondary crack patterns are distinct from those of the quasi-static ones. The experimental findings provide insights into the relationship between the material strength and the dynamic fracture mechanism. Copyright 2012 ARMA, American Rock Mechanics Association.