Spontaneous cracking of thermally toughened safety glass. Part one: Properties of nickel sulphide inclusions: Fid-Bau Portal

Spontaneous cracking of thermally toughened safety glass. Part one: Properties of nickel sulphide inclusions

Kasper, Andreas

Abstract Spontaneous breakage of glass in facades is under control today, due to application of a very effective prevention method, namely the Heat Soak Test (HST) following EN 14179-1 (2006/2016). Nevertheless, details of the latter are still subject to discussion, mainly due to the fact that some years ago, it was discovered in an R&D project that it’s holding temperature is too high, and it was reduced to $260 \pm 10^{\circ} C$ at the recent review. In the present paper we investigate the properties of nickel sulphide inclusions in order to show that there’s a huge difference in their comportment, and therewith their “criticality”, in the HST or on the façade. Namely, not only the expansivity difference between nickel sulphide and the glass plays a role. Nickel sulphide inclusions show a spectrum of possible compositions, and we approach this fact systematically, showing how the breakage probability under both conditions changes depending on the detailed composition of the inclusions. The result of this comparison is that, out of all nickel sulphide inclusions leading to breakage in HST, only 40% also lead to breakage at ambient. Another aspect is the time-to-breakage curve in the HST. By the example of a dataset where nearly only SiO $_{2}$ stones cause breakages therein, we show that not only the $α$ to $β$ transformation of NiSx causes these breakages. Also the HST process itself, through the fact that it’s impossible to heat up the glass panes therein in an absolutely homogeneous way, adds significant thermo-mechanic forces leading to boosting the breakages more than possible on façades. Besides this, also the potential impact of sub-critical crack growth at HST temperature is discussed. We conclude that the actual estimation of the residual breakage probability of Heat-Soak Tested Thermally Toughened Glass (according to EN 14179-1) is much too high, needs major revision, and until then, can only be looked at to be a lower limit with high safety margin of the real safety of this product. The present paper will be completed by at least two additional papers dealing with the detection of nickel sulphide inclusions in annealed glass and, on the other hand, a new way to evaluate statistically the data from nickel sulphide inclusion caused breakages.