SHPB tests and dynamic constitutive model of artificial frozen sandy clay under confining pressure and temperature state

Highlights

• We conducted dynamic experiments on frozen sandy clay under confining pressure state.

• We analyzed dynamic behavior of frozen sandy clay under different confining pressures.

• We proposed a dynamic constitutive model.

• The given dynamic constitutive model is feasible.

Abstract

The dynamic stress-strain curves of frozen sandy clay are obtained with the spilt Hopkinson pressure bar (SHPB) under different confining pressures (i.e., 0.5, 1.0 and 1.5 MPa) at the freezing temperature of − 5 °C and − 15 °C with the strain rate from 160 s^− 1 to 265 s^− 1. The experimental results show that the frozen sandy clay is a rate-dependent and temperature sensitive and confining pressure sensitive material and its dynamic peak stress increases with the increase of the strain rate and the confining pressure and the decrease of the freezing temperature. A dynamic constitutive model is proposed based on the test results. The theoretical and the experimental results are very close and the given dynamic constitutive model is feasible. The effect of confining pressure, strain rate and freezing temperature on the dynamic mechanical behavior of frozen sandy clay are well considered in the model.

Introduction

About 24% vast regions of the world's land is permafrost (Xu et al., 1997). In recent years, with the development of science and technology, a number of construction projects, such as railways, highways, and tunnels show an increasing trend in the cold regions of the earth. Due to the existence of ice particles, the mechanical behavior of frozen soil is complex and different from ordinary soil (Ma and Wang, 2012). Many studies are focused on the static or quasi-static behavior of frozen soil and acquire abundant accomplishment (Ma, 2009). However, the frozen soil will not only subject to the static loading, but also the impact loading, such as blasting and excavation (Ma, 2007a). A series of experiments are conducted to investigate the dynamic mechanical behavior of frozen soil under impact loading by many researchers (Chen et al., 2005, Ma, 2005, Ma, 2010). The main representative study results are as followed: the Sandia National Laboratory studied the effect of ambient temperature and strain rate on the dynamic mechanical behavior of Alaska nature frozen soil by the split Hopkinson pressure bar (SHPB), and a constitutive model is proposed (Lee et al., 2002). Liu et al. (2014) analyzed the dynamic mechanical behavior of frozen clay in various freezing temperatures, water contents, and strain rates by the SHPB device. Zhang et al. (2014), Zhang et al. (2013), Xie et al. (2016) and Xie et al. (2014) carried out a series of experiments to investigate the dynamic mechanical behavior of artificial frozen clay under uniaxial impact loading condition, and several constitutive models are established based on different theoretical bases to describe the dynamic mechanical behavior of artificial frozen clay. For example, Xie et al. (2016) established a dynamic micromechanical constitutive model by referring to the debonding damage theory of composite materials. Zhang et al. (2013) proposed a phenomenological model with thermal sensitivity to describe the dynamic behavior of frozen soil. Xie et al. (2014) constructed an energy-based dynamic constitutive model to simulate the dynamic stress-strain behavior of the frozen soil. Considering the stress state of frozen soil in underground engineering, Ma et al. (2014) studied the deformation behaviors and axial dynamic stress-strain relationship of artificial frozen clay under different stress states with the SHPB device, and the differences between uniaxial impact loading and passive confining pressure state are studied. Fang et al. (2012) designed a triaxial static confining pressure and temperature split Hopkinson pressure bar (TSCPT-SHPB), and the dynamic mechanical performance of salt rock with the confining pressures ranging from 5 to 25 MPa are studied experimentally and numerically, and the results show that the effect of confining pressure on the ductility of salt rock is tremendous under dynamic loading.

However, the theoretical and experimental study on dynamic mechanical behavior of frozen soil cited above are primarily concentrated on the conditions of uniaxial or passive confined pressure state but in shortage of confining pressure state. In many frozen soil engineering, the frozen soil sustained a certain stress before impact loading (Ma, 2007b), so it is significant to investigate the dynamic mechanical behavior of frozen soil in confining pressure state. The dynamic constitutive model proposed before could not used to describe the dynamic behavior of frozen clay under confining pressure state, since they did not consider the effect of the confining pressure.

In this work, artificial frozen sandy clay collected from a coalmine in Jining, Shandong province is used as the study sample, and SHPB test is conducted to investigate the dynamic mechanical behavior of the artificial frozen sandy clay under confining pressure state. The SHPB tests are conducted at different confining pressures (i.e., 0.5, 1.0 and 1.5 MPa) at the freezing temperature of − 5 °C and − 15 °C with the strain rate from 160 s^− 1 to 265 s^− 1. A constitutive model is established to describe the dynamic stress-strain behavior of artificial frozen sandy clay under impact loading in consideration of the effects of the confining pressure, the freezing temperature, and the strain rate. The constitutive model is verified through comparing the model results and the corresponding dynamic strain-stress results obtained from SHPB tests. It is concluded that the model can well predict the dynamic stress-strain curves of the artificial frozen sandy clay under confining pressure state.