The multi-stage damage of sandstone under different stress conditions: implications for the behavior of minerals

Abstract

Stress is the key factor leading to rock deterioration by formation of rocks/cracks and changing meso-structures inside rocks. To explore the evolution law of rocks/cracks and meso-structures in the rock under different loads, sandstone specimens were damaged by 0, 15, 30, and 40MPa to be studied here. According to the macro-mechanical characteristics (e.g., stress-lateral strain curve, peak strength), the damaged specimens can be divided into two types: Low Damaged Specimens (LDS) (loading by 0MPa and 15MPa) and High Damaged Specimens (HDS) (loading by 30MPa and 40MPa). For the LDS, specimens damaged by 15MPa have more medium-pores and large-pores compared with initial specimens (0MPa), implying that pore/crack propagation is responsible for the low-degree damage. In addition, pore/crack propagation mainly occurs between skeleton minerals and filler materials illustrated by petrographic characteristics. Compared with the LDS, the HDS has significantly lower peak strength and higher total porosity, indicating high-degree damage. Besides the pore/crack propagation, micro-pores initiation is discovered inside skeleton minerals for the HDS, which illustrates that formation of transgranular cracks is the main factor in the high-degree deterioration of sandstones owing to transformation of skeleton structure. According to the meso-mechanics model, under low-stress conditions, particle rotation could lead to pore/crack propagation between skeleton minerals and filler materials for the LDS, but modification of skeleton structure is limited. As the stress level increases gradually, rotation of particles will gradually weaken, and particles will be under higher stress, leading to formation of transgranular cracks, resulting in transformation of skeleton structure for the HDS. Therefore, under different stresses, the meso-evolution behavior of minerals controls the evolution of pores/cracks and changes the meso-structure of rocks.