Experimental investigation of a fissured rock with thermal storage potential subjected to descending amplitude low cycle impacts

Abstract

As the advancement of deep earth energy engineering in the 21st century, the challenges imposed by dynamic loads and high temperatures will be increasingly regular. This paper studied the dynamic mechanical behaviors and fracture characteristics of defective red sandstone (single fissure inclination of 0°, 30°, 60°, and 90°) subjected to thermal treatments (25 °C, 200 °C, 400 °C, 600 °C, and 800 °C) under descending amplitude low cycle (DALC) impact loading. The experiments were conducted using a modified split Hopkinson pressure bar (SHPB) system, and the fracture surfaces of the failed specimens were characterized through the use of high-resolution 3D laser scanning to obtain point cloud data. Subsequently, the fractal, roughness, texture, and elevation features of fracture surfaces were analyzed using the boxing-counting method, grayscale co-occurrence matrix (GLCM), and elevation-based statistics. The scanning electron microscopy (SEM) was utilized to study the fracture micromorphology as well. Additionally, the mechanisms of damage and fracture evolution were discussed based on the results of X-ray diffractometry (XRD), synchronous thermal analysis (STA), discrete element method (DEM), and strain energy density theory. The results demonstrate that the peak stress and dynamic elastic modulus deteriorate with rising temperatures and increase with larger fissure angles. While the peak strain, strain rate, and peak stress increase factor (PIF) exhibit the opposite trend. The threshold values for the intensification of these events are 400 °C and 60°. The damage is dominated by tensile cracks, which transition to shear at 30° and 60° due to temperature dependence. Meanwhile, the fractal dimension (DC), joint roughness coefficient (JRC), second-order statistics of GLCM, and elevation eigenvalues have been modeled using multiple regression analysis to investigate their interaction with temperature and fissure angle. The best-fit models illustrate significant linear or non-linear correlations between the aforementioned parameters and the mechanical parameters. Furthermore, the magnitude and distribution pattern of thermal cracks, as well as the growth of shear fractures, are governed by the dehydration, thermal expansion, phase transition, plasticization, thermal fracture, and thermal decomposition. Finally, strain energy density theory manifests promising prospects for understanding and predicting DALC impact fracture patterns of specimens.