ABSTRACT:

Combined effect of specimen size and loading rate on the rock tensile strength was investigated in this work. The dynamic rock fracture and disintegration was simulated numerically using the 3D hybrid finite-discrete element computer code CA3. The rock under the impact loading was studied using the Split Hopkinson Pressure Bar testing system. The Incident and Transmitted bars were modeled by the finite element method while the Brazilian rock specimen was simulated using a Bonded Particle Model (BPM). The bars were assumed to behave elastically while the simulated specimen could develop micro and macro cracks which eventually could end up to complete disintegration and failure. Brazilian specimens with different sizes were numerically modeled. Each specimen contained a notch so that fracture mechanics size effect under high strain loading rate could be studied. The specimens were subjected to different loading rates by adjusting the incoming wave in the incident bar. A micromechanical model in which the contact bond strength was allowed to vary in proportion to the relative velocity at the contact point of the involved particles was employed to capture the loading rate effect. The effect of specimen size on the dynamic tensile strength of rock was explored and compared with the static size effect. It is shown that the dynamic size effect on tensile strength is substantially different from what happens under static loading. While for small loading rates, the rock strength decreases as the specimen size increases, this is not the case when high loading rates are involved. The numerical results suggest that the loading rate strength enhancement is able to negate the fracture mechanics size effect and can cause a substantial increase in the strength as the specimen size is increased. This interesting observation is compared with published data and discussed.

1. Introduction

Measuring the mechanical properties of rock is a challenging task for engineers. In-situ testing is difficult and expensive, so these properties are usually measured on a small scale in the lab and then generalized to the actual scale (Palmström and Singh, 2001). Therefore, in most situations, the size difference between laboratory specimens and actual scale is inevitable. This difference is called "the size effect" and can affect the mechanical properties such as tensile strength of quasi-brittle materials.

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