Lawrence Berkeley National Laboratory

Understanding the fracture toughness (resistance) of glasses is a fundamental problem of prime theoretical and practical importance. Here we theoretically study its dependence on the loading rate, the age (history) of the glass, and the notch radius ρ. Reduced-dimensionality analysis suggests that the notch fracture toughness results from a competition between the initial, age- and history-dependent, plastic relaxation time scale τ0pl and an effective loading time scale τext(KI,ρ), where KI is the tensile stress-intensity-factor rate. The toughness is predicted to scale with ρ independently of ξ≡τext/τ0pl for ξ1, to scale as Tρlog(ξ) for ξ1 (related to thermal activation, where T is the temperature), and to feature a nonmonotonic behavior in the crossover region ξ∼O(1) (related to plastic yielding dynamics). These predictions are verified using 2D computations, providing a unified picture of the notch fracture toughness of glasses. The theory highlights the importance of time-scale competition and far-from-steady-state elasto-viscoplastic dynamics for understanding the toughness and shows that the latter varies quite significantly with the glass age (history) and applied loading rate. Experimental support for bulk metallic glasses is presented, and possible implications for applications are discussed.