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Received Jul 31, 2017; Accepted Oct 9, 2017
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1. Introduction
In a variety of rock engineering and geophysical applications, such as earthquakes, mining, civil engineering, petroleum engineering, and physical geography, rock materials are in a dynamic loading state, where the strain rate is in the range of 10−4 to 103 [1, 2]. The mechanical characteristics of rock materials show strong strain rate dependency and, therefore, it is very meaningful and important to accurately determine the dynamic constitutive relation of rock materials in a specific range of strain rate.
The Split-Hopkinson pressure bar (SHPB), originally developed by Kolsky, is a commonly used apparatus to obtain the dynamic properties of materials at high strain rates [3, 4]. To accurately obtain dynamic constitutive relations of rate-dependent rock from SHPB tests, dynamic stress equilibrium and constant strain rate conditions in samples are required [5]. However, in a common SHPB test, the incident pulse is a square waveform characterized by a short rising time, which is not ideal in rocks for approaching dynamic equilibrium and a constant strain rate. Usually, most results from SHPB tests are presented with an average strain rate, which is not proper for most rock materials, as they show strong strain rate effects [6]. A longer rising time in the incident wave pulse is an indispensable condition for achieving dynamic force equilibrium in rock samples before failure. Meanwhile, the incident wave pulse must mimic the stress-strain behavior of rock materials to achieve a constant strain-rate state.
In the past, many studies have been performed into strategies for shaping the incident pulse. Among them, there are two main methods, involving a pulse shaper and a specially shaped striker. To shape the incident pulse to guarantee a constant strain rate condition in a rock specimen, Ellwood et al. [7] have presented...