Damage characteristics of nanopores in coal under stepwise loading and unloading based on small angle X-ray scattering

LI Xiangchun; SHI Yaoyu; NIE Baisheng; PANG Yihui; SONG Haonan; ZHANG Shuhao; ZHANG Liang; GAO Jiaxing

doi:10.13225/j.cnki.jccs.2024.1090

Abstract：To explore the micro-mechanical and damage characteristics of coal under load,this study conducted stepwise loading and unloading small-angle X-ray scattering(SAXS)experiments using a self-developed micro-loading system and an in-situ synchrotron radiation SAXS experimental station.The SAXS experimental data were used to analyze the dam-age evolution of nanopore structures in coal during the stepwise loading and unloading process,and the damage mechan-isms of the microstructures of open and closed pores in coal were discussed.The research results indicate that the step-wise loading-unloading SAXS experiments of coal samples consistently exhibited Porod positive deviation.Different load-ing methods did not alter the intrinsic Porod deviation pattern of the coal,but external loading had a significant impact on the scattering intensity.The surface fractal dimension,pore fractal dimension,specific surface area,and porosity of nano-pore structures in coal samples exhibit dynamic changes,increasing initially,then decreasing,and finally increasing signi-ficantly with the increase of loading.Based on these staged changes,the damage process of nanopore structures under stepwise loading-unloading conditions is divided into three stages:minor damage stage,compression stage,and fracture stage.In addition,the influence of uniaxial loading and stepwise loading-unloading on the damage mechanism of nano-pores was analyzed based on the variation patterns of the microstructure.It was revealed that stepwise loading-unloading exacerbates pore closure,structural collapse,and through-breaking processes.Furthermore,the failure and disaster mech-anisms of nanopores under loading-unloading conditions were discussed for both gas-bearing and non-gas-bearing coal.These findings provide new insights into the micro-damage mechanisms and disaster-causing mechanisms of coal under cyclic loading and unloading.