Investigation of dynamic fracture properties of multi-crack tunnel samples under impact loads

Abstract

Cracks and joints generally exist in various underground structures, and affect the overall mechanical behaviour (strength, elastic modulus, fracture, etc.) of wall rocks significantly. Studying the crack failure behaviour in cracked tunnel samples containing single or multiple cracks can improve our understanding of the dynamic fracture mechanism of cracked tunnels. To investigate the dynamic fracture parameters of cracks in surrounding rock mass, several cracked tunnel model samples having single or multiple cracks were prepared from green sandstone and polymethyl methacrylate. Four groups of impact experiments were conducted using the drop hammer impact testing equipment to determine dynamic fracture properties. Corresponding numerical simulations were performed using a modified version of the finite difference method software and a traditional version of the finite element method software. The effect of single or multiple cracks on crack propagation speed, dynamic fracture initiation time, and fracture initiation toughness under impact loads was analysed and discussed in this study. By comparing the experimental and simulation results, we concluded that multiple cracks have a significant influence on crack propagation speed because the crack propagation speeds of multi-crack samples were larger than those of single-crack samples. Furthermore, the dynamic fracture initiation time decreases with the number of cracks. The dynamic fracture initiation toughness KId and KIId of green sandstone at the crack tip of single-crack samples was 1.94 times greater than that of multi-crack samples.