Fracture behavior of heterogeneous layered sandstone under mixed loading after thermal treatment: A thermal–mechanical grain-based FDEM model

Abstract

In deep rock engineering, temperature and bedding plane properties have a critical influence on the mechanical behavior of layered sandstone. Therefore, this study develops a cracked straight through Brazilian disc (CSTBD) numerical model for layered sandstone using the combined finite-discrete element method (FDEM), incorporating material inhomogeneity and bedding plane characteristics. The fracture behavior of layered sandstone under splitting conditions after high-temperature treatment was examined, with model validation achieved through laboratory experiments. The results indicate that the microcrack density in layered sandstone increases and the crack distribution becomes more disordered as the temperature rises. During the heating phase, initial damage predominantly appears as tensile cracks, whereas in the cooling phase, the damage shifts to shear cracks, with the growth rates of microcracks generally slowing down. On the microscale, high thermal treatment temperatures cause thermal cracking of mineral grains, which damages the bedding planes. On the macroscale, thermal damage results in a reduction of peak load, with a notable transition from brittle to ductile fracture occurring between 600 °C and 800 °C. The crack propagation in layered sandstone is synergistically influenced by a combination of factors, including thermal treatment temperature, loading angle, bedding plane strength, and prefabricated crack length. The fracture toughness diminishes with rising temperatures or prefabricated crack length, and rises with increasing strength of the bedding plane. Furthermore, the effects of bedding plane strength ratio, bedding plane distribution pattern, and prefabricated crack length on the fracture toughness of layered sandstone were examined under diverse thermal treatment temperatures and loading modes. The results indicate that these factors exert significant control over the fracture mode and fracture resistance of layered sandstone.