Study on phase-Transition-Like characteristics and volume expansion model of weakly consolidated sandstone under triaxial cyclic loading and unloading

In order to study the phase transformation characteristics and dilatation-related changes of weakly consolidated sandstones with different cementation degrees, the cyclic loading and unloading experiments of weakly consolidated sandstones under triaxial conditions were carried out by using the MTS816 rock mechanics test system. Based on the test results, the phase-Transition-Like characteristics of weakly consolidated sandstone during the whole stress-strain process are identified, and the fracture mechanism of weakly consolidated sandstone during the phase-Transition-Like process is analyzed in combination with the meso-structural experiments. By introducing the response ratio of loading and unloading, the damage variable based on phase-Transition-Like is defined, and the percolation expansion model based on phase-Transition-Like index is established, which can describe the volume expansion of weakly consolidated sandstone after entering phase-Transition-Like. The results show that the volume strain expansion point and the dilatancy Angle mutation point can be used as the characteristics of the starting point and the ending point of the phase transition respectively, and the deformation and fracture forms of rock structures in different regions can be obtained. With the increase of cementation degree, the main failure factors changed from tension-weakening cementation structure to compression-shear failure cementation material. The damage development of weakly consolidated sandstone is more sensitive to confining pressure with the increase of cementation degree. It provides a theoretical basis for controlling the damage and volume expansion of weakly consolidated rock after entering the phase-Transition-Like like state. • According to the phase transformation characteristics of weakly consolidated sandstone, the crack propagation modes in different stages are divided • With the increase of cementation degree, the damage factors of weakly cementation sandstone change from tensile weakening cementation structure to compressive shear failure cementation material. • Based on the response ratio of loading and unloading, the in-situ state and disturbance state of weakly consolidated sandstone are described and the dilatancy deformation prediction model of weakly consolidated sandstone is established.