  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 $\rho$. Reduced-dimensionality analysis suggests that the
notch fracture toughness results from a competition between the initial, age-
and history-dependent, plastic relaxation timescale $\tau^{pl}_0$ and an
effective loading timescale $\tau^{ext}(\dot{K}_I,\rho)$, where $\dot{K}_I$ is
the tensile stress-intensity-factor rate. The toughness is predicted to scale
with $\sqrt{\rho}$ independently of $\xi\!\equiv\!\tau^{ext}\!/\tau^{pl}_0$ for
$\xi\!\ll\! 1$, to scale as $T\sqrt{\rho}\,\log(\xi)$ for $\xi\!\gg\!1$
(related to thermal activation, where $T$ is the temperature) and to feature a
non-monotonic behavior in the crossover region $\xi\!\sim\!{\cal O}(1)$
(related to plastic yielding dynamics). These predictions are verified using
novel 2D computations, providing a unified picture of the notch fracture
toughness of glasses. The theory highlights the importance of timescales
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.
