Effect of Slow Crack Growth on the Thermal‐Stress Resistance of an Na 2 O‐CaO‐SiO 2 Glass

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Abstract
The effect of slow crack growth on the thermal‐shock resistance of an Na 2 O‐CaO‐SiO 2 glass was studied. An analytic and numerical technique was developed to calculate the critical quenching temperature difference, Δ T c , of circular rods quenched in water from known crack velocity data. For rods of a specific radius and crack depth, Δ T c , was calculated to be 147°C, in favorable correspondence with experimentally observed values of 155° to 160°C. In the absence of crack growth, Δ T c was estimated to be 238°C, well in excess of the observed value and indicative of the significant effect of slow crack growth on thermal‐stress resistance. It is also shown that crack growth significantly extends the time‐to‐failure to a value much greater than the time of maximum thermal stress. A form of subcritical crack instability is predicted at stress intensity factors well below the critical stress intensity factor, at which catastrophic failure becomes inevitable over the duration of the transient thermal stress. It is suggested that, when all other factors are equal, the effect of slow crack growth on thermal‐stress resistance can be minimized by maximizing thermal diffusivity. It is also argued that surface‐compression strengthening will be more effective than reduction in flaw size in increasing thermal‐stress resistance. Recommendations are made for the design and selection of brittle materials subjected to thermal stress in stress‐corrosive environments.

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