Brittle-to-ductile transitions in glasses Roles of soft defects and loading geometry

Understanding the fracture toughness of glasses is of prime importance for science and technology. We study it here using extensive atomistic simulations in which the interaction potential, glass transition cooling rate, and loading geometry are systematically varied, mimicking a broad range of experimentally accessible properties. Glasses’ non-equilibrium mechanical disorder is quantified through A_g, the dimensionless prefactor of the universal spectrum of non-phononic excitations, which measures the abundance of soft glassy defects that affect plastic deformability. We show that while a brittle-to-ductile transition might be induced by reducing the cooling rate, leading to a reduction in A_g, iso-A_g glasses are either brittle or ductile depending on the degree of Poisson contraction under unconstrained uniaxial tension. Eliminating Poisson contraction using constrained tension reveals that iso-A_g glasses feature similar toughness, and that varying A_g under these conditions results in significant toughness variation. Our results highlight the roles played by both soft defects and loading geometry (which affects the activation of defects) in the toughness of glasses.