Microstructure of damage in thermally activated fracture of Lennard-Jones systems

We investigate the effect of thermal fluctuations on the critical stress and the microstructure of damage preceding macroscopic fracture of Lennard-Jones solids under a constant external load. Based on molecular dynamics simulations of notched specimens at finite temperatures, we show that the crystalline structure gets distorted ahead of the crack in the secondary creep regime. The damage profile characterizing the spatial distribution of lattice distortions is well described by an exponential form. The characteristic length of the exponential form provides the scale of damage, which is found to be an increasing function of the temperature: At low temperatures, damage is strongly localized to the crack tip, while at high temperatures, damage extends to a broader range, leading to more efficient relaxation of overloads. As a consequence, the stress intensity factor decreases with increasing temperature. The final macroscopic failure of the system occurs suddenly and is initiated by the creation of vacancies and voids. The creep strength exhibits inverse square root scaling with the notch size corrected by the extension of the process zone.