Experimental and numerical investigations on tensile mechanical properties and fracture mechanism of granite after cyclic thermal shock

Abstract

It’s necessary to understand mechanical and thermal fracture characteristics of hot dry rock under cyclic thermal loading to exploit geothermal resources. In this paper, the P-wave velocity test, saturated water absorption test and tensile test were carried out for hollow cylindrical granite samples after cyclic thermal shock. The influences of granite type, heating temperature (100–650 °C) and the number of thermal shock cycles were analyzed in the experimental study. In order to deeply study granite’s fracture and degradation mechanism during cyclic thermal shock, a grain-breakable numerical model which could catch the initiation and propagation of both inter- and intra-granular fractures was established by using the Discrete Element Method. The experimental results demonstrated that granite showed a significant reduction in P-wave velocity and tensile strength, and increase in porosity after cyclic thermal shock, more evident at higher heating temperature. These variations mainly occurred in the first 5 cycles, particularly in the first cycle, at all test temperatures. Cyclic thermal shock treatments introduced both grain-boundary and intra-granular fractures in rock samples, which was the key reason for the varying values of granite’s physical and mechanical characteristics. Thermal fractures increased remarkably after one cycle of heating–cooling treatments. With the increase of thermal loading cycles, the fractures showed a noticeable increasing trend in the first 5 cycles, then changed slightly in the following cycles, which agreed well with the experimental data. Thermal fractures propagated and expanded, accompanied by the closure of a few fractures as the thermal shock cycle increased. Inter- and intra-granular fracture behaviors were determined by test temperature and thermal shock cycles.

Article highlights

• The P-wave velocity and porosity of granite vary significantly after cyclic heating and liquid nitrogen cooling treatments.

• Granite’s tensile mechanical properties are determined by granite type, heating temperature, and thermal shock cycles.

• Inter- and intra-granular fracture behaviors of granite during cyclic thermal shock are numerically obtained by using grain-breakable model.