Spontaneous cracking of thermally toughened safety glass part three: statistic evaluation of field breakage records and consequences for residual breakage probability

The present paper is the third part of a small series of publications dealing with the problem of spontaneous breakage of thermally toughened glass. The HST standard EN 14179-1:2005 (revised 2017) made this glass significantly safer; the present small series aims primarily to bring new arguments into the discussion on its real safety. Another aim is to rectify obsolete (pseudo-)facts repeated again and again in respective publications although they are disproven since a long time. The present third part of the series deals with statistic evaluation of datasets from field breakages. In these, every inclusion has caused a breakage, contrariwise to those in the 2nd part. The data have already been partly (and as one mixed dataset) published in 1997; since then, the author’s working group continued accumulating breakage data. Today, finally, this data collection is sufficiently numerous to be split into the two parts, namely breakages recorded on buildings and in Heat Soak Test, and to be evaluated separately by means of statistical analysis. Like in the previous papers, also this evaluation results in a clear difference in breakage probability, very much higher in HST. As a matter of fact, the latter’s breakages’ devolution cannot simply be extrapolated in order to obtain the safety of the tested glass on buildings; the result of this extrapolation is only a minimum limit, but factually far away from reality. Finally, the experience of the author’s employer, Saint-Gobain in Germany, is described for the time since the introduction of said standard. Also this pragmatic approach reveals that the safety of Heat-Soak Tested Thermally Toughened Safety glass against spontaneous breakages is much better than forecasted from the HST breakages’ evaluation.