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Nickel Sulfide (NiS) inclusions can provoke the rupture of thermally treated glass due to a phase transformation with volume increase that stresses the surrounding glass. Starting from a Pareto statistics for the population of inclusion sizes, from an assumed kinetics of the phase transformation, a micro-mechanically motivated model provides the statistical characterization of the probability of spontaneous failure of glass during lifetime. A distinction based upon the composition of NiS is used to discuss the effects of the heat soak test (HST), where glass remains at high temperature for a certain time to speed-up the phase transformation and destroy those elements with critical inclusions. Three functions à la Weibull for the probability of spontaneous rupture during lifetime are theoretically derived for the case of no HST, short HST, and long HST. In particular, the probability of collapse for long HSTs depends upon the holding time in the oven. An explanatory example shows the potentiality of the model for optimizing the HST parameters toward a target probability of failure, but experimental campaigns are needed for a proper calibration. K E Y W O R D Sfracture mechanics/

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Because the strength of glass is governed by randomly distributed surface flaws that can propagate catastrophically when the applied stress reaches a critical value, the weakest-link-in-the-chain rationale is the universally accepted interpretation of its significant variability. The two-parameter Weibull extreme value distribution is currently the most commonly used model for structural design, although it is recognized that it fails to capture the experimental data within the region of small failure probabilities, associated with the lowest strengths. However, the precise characterization of this left-hand-side tail of the distribution is crucial for structural applications, for which only very low probabilities of failure are accepted. Experiments have provided evidence of the existence of a lower bound for the strength of glass, a finding that, if proved, could revolutionize the approach to the safety of glass structures. Referring to the large-scale experimental program of the Technical Committee 129-Working Group 8 of the European Committee for Standardization (CEN/TC129/WG8), various generalized statistical distributions like Weibull, either prescribing a strength limit or not, are compared in their ability to interpolate the experimental data using the chi-square goodness-of-fit test. Arguments are presented that support the existence of a minimal strength, which can be reduced, but not annihilated, by the inevitable degradation of the glass surface produced by aging and in-service-related damage.