ABSTRACT:

Experiments of granite under dynamic compression at strain rate 42/s, 65/s and 84/s with a Split Hopkinson Pressure Bar (SHPB) system are conducted in the present paper. Quasi-static test at strain rate 3 × 10−5/s is performed with Material Testing System (MTS) for reference of specimen at low strain rate. A high speed camera and a Charge Coupled Device (CCD) are employed in dynamic and quasi-static tests respectively for images acquirement of specimen surfaces. Digital Image Correlation (DIC) method is applied to the images shot in the compression process for the quantitative full-field transverse normal strain measurement on surface of specimen. Color maps of strain field show the strain localized on regions where cracks concentrate. Statistics on strain filed exhibit a typical Weibull distribution. According to different features in strain distribution, the compression process is divided into three stages which coincide with the stages of cracks nucleation, interaction and breaking through specimen leading to failure, respectively. Evolution of Weibull parameters for strain field indicates the strain rate effects on the crack propagation and failure mechanism of granite.

1 INTRODUCTION

Brittle rocks such as granite, marble and sandstone consist of mineral grains with multiple species and sizes, and contain microscopic pores and cracks. Therefore, great heterogeneity of internal structure exists in brittle rocks, and the deformation is heterogeneous for brittle rocks. Tensile microscopic cracks nucleation, interaction and coalesce to macroscopic cracks is the main failure mechanism for brittle rocks subjected to compression (Tapponier & Brace 1976). The mechanical behaviors of brittle rocks have been substantially investigated at different strain rates (Frew et al. 2001, Zhang et al. 2000), varying from quasi-static to dynamic conditions. Significant strain rate effects are shown in brittle rocks. However, deformation of specimen is acquired indirectly from traditional methods such as strain gauges and mechanical extensometers. These measurements are not sufficiently accurate for the slight strain of brittle rocks. Moreover, the mesoscopic local strain is impossible to observe since only global deformation can be measured by traditional methods. Thus, investigations on strain rate effects and failure mechanism of brittle rocks are far from plenty.

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