Some brittle materials, like the strong glass SiO2, are very defect-sensitive. The fracture in these  materials,  which is due to the ever-present  micro defects, occurs abruptly and without warning. This type of failure is determined by the loss of stability of a single crack: the deter- mination of the conditions  leading to unstable  propagation of single crack is adressed  deter- ministically  in fracture   mechanics  by investigating   the  Griffith's  stability of the crack.  In strong glasses, which are nonequilibrium systems,the properties depend on cooling rate with which the sample has been quenched, so  the structure is not statistically homogeneous and the local stresses may differ from their average  values and the stress threshold  at which one microcrack starts propagating  is a random variable. So the  problem of the onset of  propation of a crack in  strong glasses is  intimately connected to the physics of the elastic objects pinned by random strength.  The  determination of the local fracture toughness in strong glasses, where the microscopic properties are strongly depending on the thermodynamical history (aging) and on the stress attribuable to the direct interaction of closely spaced defects,  depends almost entirely on the extreme value statistics  of the largest defects. Here we try to connect the theoretical strenght,the aging effects on mechanical properties and the local toughness fluctuations in this class of materials.

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