Investigating tensile fracture mechanisms in bituminous coal microstructure across strain rates using molecular dynamics simulation

Abstract

The fundamental cause of coal mine dynamic disasters lies in microstructural failure under dynamic loading. This study employed molecular dynamics simulations to investigate the uniaxial tensile mechanical behavior and failure mechanisms of bituminous coal macromolecules under different strain rates. The results indicate that when the strain rate is below 1 × 1010 s−1, coal exhibits ductile failure. The stress–strain curve comprises four stages: elastic, plastic, strain hardening, and unstable crack propagation. When the strain rate exceeds 5 × 1010 s−1, the strain hardening stage disappears, and the failure mode transitions to brittle, with a critical transition interval between 1 × 1010 and 5 × 1010 s−1. Porosity shows a positive correlation with strain. At low strain rates, pores with a diameter less than 0.5 nm reversibly close, which is conducive to the progressive damage of toughness failure. Pores with a diameter greater than 0.5 nm irreversibly expand at high strain rates, directly causing brittle failure. The combined influence of hydrogen bonding and π–π stacking contributes to strain hardening during ductile deformation. After fracture, the planes of aromatic ring align parallel to the tensile direction, and aliphatic chains are straightened along it. The overall change in the potential energy of the system is primarily governed by van der Waals energy. At low strain rates, energy is gradually dissipated through molecular structural adjustments, manifesting as plasticity. At high strain rates, energy accumulates rapidly and cannot be dissipated in a timely manner, resulting in sudden brittle failure. This study provides a theoretical microscopic basis for understanding the disaster mechanisms induced by dynamic disturbances in coal mines.