Presenting in-situ AFM investigations for the evolution of micro-surface topography and elastic modulus of rock under variable loads

Abstract

Understanding the micro and nano-scale surface deformation and mechanical properties of rock under load possess enormous significance for underground engineering research and provides strong basis for comprehending rock failure as well. According to the documented research, the in-situ observation of micro-surface under dynamic load has not been yet reported. With this motive, a novel approach is presented in this work that takes advantages of combined Atomic Force Microscope (AFM) and customized micro-loading device for in-situ characterization of the the micro-surface topography and elastic modulus of rocks under different load strength. The in-situ comparison of micro-surface properties is carried out using interactive tools of AFM image post-processing software including crop and split, and section analysis. Taking the typical observation results of igneous rock and shale as an example, the results show that the micro-surface topography undulations of rock can reach up to tens of nanometers. Although no apparent fracture occurred, the local fluctuation of the rock samples’ micro-surface changed with the increasing load strength at nanometer level, and in the measurement area of 5 × 5 μm2, the topographies of different pixel points are various, showing significant local difference. With the increasing load strength, the elastic modulus distribution of the rock’s micro-surface shows heterogenity, and the local elastic modulus also increases or decreases, while the variation difference at different pixel points reaches hundreds of MPa. Additionally, the macroscopic fracture characteristics of rock under load, especially the non-uniform crack propagation and its mechanism are discussed. The analysis indicates that the heterogeneous distribution of the rock’s micro-surface elastic modulus and its local complex change with load are accounting for the complexity of the macroscopic crack path and the numerous crack branches. This study provides a novel insight into micro-surface behavior in rock and presents a new method for the investigation of the fracture mechanism of rock.