Statistical interference of crack healing on the strength of thermally-treated glass. Experiments and modelling

The macroscopic strength of thermally-treated glass is determined by the critical propagation of surface microscopic cracks, occurring only after that the surface compression, induced by the heating and successive cooling, has been overcome. A large experimental campaign has been conducted for the statistical characterization of the bending strength of annealed (untreated) and thermally-toughened glass, after having preliminarily measured the surface compressions with non-destructive methods. A deviation is found between the measured strengths for thermally-toughened glass and the corresponding theoretical values, derived from the statistical interference between the populations of untreated-glass strengths and heat-induced surface compressions. This discrepancy is attributed to the healing of surface microcracks. A micro-mechanically motivated model of fracture mechanics is proposed, which allows to statistically interpret the experimental data by correlating the effects of healing with the size of the cracks. This study confirms, in general, that crack healing can alter the macroscopic strength of tempered glass, but it also evidences, in particular, that this phenomenon is of importance only for the highest quantiles of the population of strengths, which are not relevant in structural design. Therefore, neglecting this effect is certainly on the safe side, but not excessively conservative.