Study on weakening of mechanical properties and ultrasonic law of sandstone under high temperature cycle loading and unloading

Abstract

In order to explore the weakening law of mechanical properties of rocks on the top of coalbed methane under high temperature and cyclic loading and unloading, a DDL-200 uniaxial electronic testing machine was used to conduct graded cyclic loading and unloading tests on the rocks, and the weakening laws of mechanical parameters, sound velocity propagation were analyzed. The results show that: at 25-400 ℃, the strain increment is small, the morphological failure is splitting failure, and the internal damage is uneven, and the longitudinal wave velocity propagates quickly; after 400 ℃, the strain increment is large, and the main failure mode is shear failure. The internal debris increases and the pores become larger, the outer layer of the rock has many and irregular fracture surfaces, and the longitudinal wave velocity is slow; during the loading period, the softening coefficient decreases with increasing temperature under the same cycle, and increases with increasing cycle under the same temperature. The degree of deterioration decreases with increasing temperature under the same cycle, and first increases and then decreases with increasing cycle under the same temperature; during the unloading period, the softening coefficient and degradation degree both increase with the increase of the same cycle (or temperature). The research results will provide reference for the stability of the upper rock mass of coal seams during gas mining and the exploration of rocks under complex stress conditions.

Keywords

• high temperature cycling
• strain
• wave velocity
• broken form
• softening coefficient
• deterioration degree