ABSTRACT

Due to difficulties associated with sample gripping in direct tension, indirect methods are commonly used to quantify the tensile strength of rocks. In this work, an indirect tensile test method - semi-circular bend (SCB) is used to investigate the tensile (flexural) strength of Laurentian granite (LG). The static tests are conducted with a servo-controlled material testing machine and the dynamic experiments are carried out using a split Hopkinson pressure bar (SHPB) system. For dynamic tests, pulse shaping technique is adopted to achieve dynamic force balance on both ends of the sample. The dynamic force balance eliminates loading inertial effect in the sample and thus enables quasi-static stress analysis. Momentum-trap technique is also employed in SHPB to ensure single-pulse loading. Finite element method is used to relate the failure load to the strength of the sample. Rate dependence of the strength is observed. The value of the flexural strength is higher than the tensile strength measured using Brazilian disc method. We rationalize this difference using the non-local failure theory. Furthermore, a coupled Discrete element - Finite element method (Fem/Dem) in conjunction with the smeared crack model is utilized to simulate the fracture process of a dynamic SCB test. The fracture pattern obtained from the simulation agrees with that of the recovered sample.

1 INTRODUCTION

Rocks are considerably weaker in tension than in compression. Understanding of tensile strength of rocks thus bears important engineering and geophysical applications. Due to the difficulties of experimentation in direct tensile test, various of indirect methods have been proposed and developed as convenient alternatives to measure the tensile strength of rocks; some examples are Brazilian disc test (Bieniawski and Hawkes, 1978; Coviello et al., 2005; Hudson et al., 1972; Mellor and Hawkes, 1971), ring test (Coviello et al., 2005; Hudson et al., 1972; Mellor and Hawkes, 1971), and bending test (Coviello et al., 2005). These methods aim at generating tensile stress in the sample by far-field compression, which is much easier in instrumentation than direct tensile tests.

Existing attempts to measure rock tensile strength are mostly limited to quasi-static loading, primarily due to the difficulties in the dynamic experimentation and subsequent data interpretation. However, in many mining and civil engineering applications, such as quarrying, rock cutting, tunneling, rock blasts, and rock bursts, rocks are stressed dynamically. Accurate characterizations of tensile strength over a relative wide range of loading rates are crucial. Direct dynamic tensile testing is rare (Goldsmith et al., 1976), and research efforts have rather concentrated on extending the indirect methods from quasi-static to dynamic loading. Zhao and Li (2000) measured the dynamic tensile properties of granite with the Brazilian disk and three point bending techniques, and the loading was driven by air and oil. Most researchers used the standard dynamic testing facility, split Hopkinson pressure bar (SHPB), to characterize dynamic tensile strength. For examples, conventional SHPB tests were conducted on Brazilian disk and flattened Brazilian disk specimens of marble (Wang et al., 2006) and on Brazilian disk specimens of argillite (Cai et al., 2007).

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