Stress intensity factors of the ISRM suggested CCNBD specimen used for mode-I fracture toughness determination

Abstract

Rock fracture toughness is an important parameter for the engineering geological design. In this present study, chevron straight-notched semi-circular bend (CSNSCB) specimens with wide-flat liner chevron ligament (Type-A) and long-narrow liner chevron ligament (Type-B) were designed and prepared using wire cutting method. Mode-I fracture toughness KIC values and dimensionless stress intensity factor Y* were calculated. Moreover, these two types rock failure properties and KIC values variation features were both analyzed. The obtained results indicate that the width of precast artificial crack is small which can ensure the flatness and symmetry of cutting plane and then the tested Stress Intensity Factor (SIF) which has smaller deviation is more accurate and reliable. The Y* values for each type of configuration were calculated using Finite Element Method (FEM) and Slice Synthesis Method (SSM) and they firstly had a decreased trend and then an increased trend with the increasing of normalized crack length α, which represent the transition from stable crack propagation (α > αm) to unstable crack propagation (α < αm). Compared to Type-A specimens, Type-B specimens had the calculated minimum dimensionless stress intensity factor Y*min with smaller deviation and average KIC values with smaller variation coefficients. Moreover, Type-A specimen has the average fractal dimension D value of 1.2209 which is 4.79% larger than that of 1.1651 for Type-B specimen, indicating its more complex crack generation and rougher fracture surfaces. That is to say, Type-B specimen has the advantages of small crack FPZ and crack propagation without deviation as well as smooth fracture surface. Since it can offset the harmful influence of rock anisotropy and good crack guided effect, its KIC value is much satisfactory and closer to the true fracture toughness of rock-like materials.