Spontaneous cracking of thermally toughened safety glass. Part four: a case study of isothermal breakages in a building, and conclusions thereof for the heat soak test

Abstract

The present paper continues the publication of the R&D work already published by Saint-Gobain in 2018. The focus of the present publication is to overcome a widespread but, however, erroneous and momentous presumption concerning HS testing, namely that the transformation speed of the nickel sulphide inclusions would define the time-to-breakage curve devolution in the heat soak test (HST). According to our previous papers, on the one hand, refractory stones proven can lead to breakage in HST. On the other hand, also $$\text {Ni}_9\text {S}_8$$ inclusions, proven not to be subjected to a phase transformation, cause breakages on heating to HST temperature. Now, in the present paper, we publish and thermodynamically evaluate an example of an isothermal breakage series in a building’s lobby. By this, we produce evidence for that almost every critical nickel sulphide inclusion is transformed in the HST, but this transformation is, in comparison with the time-to-breakage curve of the test, much faster and does explain nor breakages before neither c. Three hours after completion of the transformation. Even if other obvious impact factors, such as thermal expansion on heating and the variability of the heating-up rate, are taken into calculation, a significant difference between the real (recorded) time-to-breakage curve and the simulation remains. We conclude that at least one significant impact factor is missing in this simulation, namely the mechanical load induced by the principally inhomogeneous heating-up of the panes in any HST. Hot air first heats up the glass rims, therewith inducing additional tensile stress into the whole pane’s surface and volume; this procedure always keeps the rims hotter than the inner of the stack. The breakages stop when the heating-up process is completed, but this is significantly later than formally reaching the holding step of the HST. This effect would explain both pre-tailing (i.e., some inclusions turn to be critical before maturity) and tailing (i.e., additional breakages due to less critical inclusions occur during the holding period) in comparison with the simulation. Another effect could be faster under-critical crack growth at increased temperature; however, this effect would only cause tailing of the real curve (or, contribute to its tailing), but no pre-tailing. In other words, the transformation of the nickel sulphide inclusions, be it on a facade or in HST, is a necessary condition for breakages caused by this kind of inclusions. However, transformation alone is not a sufficient condition for a breakage event. Just like the true sentence “A King is a Man”, the argumentation “Transformation is necessary” is not reversible. Additional explanations have to be quantified to substantiate the highly visible difference between transformation kinetics and the recorded time-to-breakage curve, and this is the main theme of the present paper. On the other hand, the interpretation of the present finding in terms of thermodynamics shows a way how to more correctly derive the safety of HS-tested glass from the transformation kinetics of the nickel sulphide and other known impact factors of the HST. It might become the basis of a sound “bottom up” simulation that does more than simply extrapolating the time-to-breakage curve recorded in the HST. In summary, also this finding reinforces our standpoint that the HST, if correctly carried out, produces glass panes that are extremely safe against spontaneous failure.