How does the failure occur?
Nickel Sulphide Inclusion is a time dependant failure, where anytime from 1 to 20 years, the NiS will slowly grow and in time cause enough localised stress in the surrounding glass to cause crack propagation. This will result in a spontaneous failure of the entire panel.
In order of likelihood, the causes of fragmentation are affected by many factors such as:
- Impact damage – A sudden, spontaneous point load, exceeding the elastic strength of the glass.
- Edge or face damage before installation
- Poor Installation – Contact with fitting and fixings, missing setting blocks
- Poor design – Design loads, clearances, design flaws
- Poor toughening
- Non-NiS inclusions – Bubbles, scratches etc
- Finally, inclusions within the glass I.E. Nickel Sulphide
From the images, a butterfly pattern occurs when the failure takes place. This is the case in all failure types, NiS or Non-NiS related.
Why does it occur?
The treatment of the glass causes residual stresses to build when heated and rapidly cooled, effectively ‘trapping’ the stress and inclusions in the material. This can be considered as a ‘ticking time bomb’, where a small impact to the glass can have both a spontaneous and catastrophic failure, the inclusion is only a few hundred microns in diameter, but is enough to cause a failure to occur.
A good example to consider is the Prince Rupert’s Drop, where liquid glass is poured directly into a cooling solution thus causing it to harden quickly. The head is the hardest part whereas the tail of the drop, being by far the weakest, is what causes failure to occur. The tail in this scenario can be seen as not only the propagation point, but the weakest part of the material, IE Nickel Sulphide, which is often unknown.
By following Standard EN14179-1, Approved Document K and BS6262-4, we effectively produce glass panes in which drastically reduces the likelihood of spontaneous failure due to NiS. This is done by toughening and heat soaking the material. Heat soaking is in some ways a test of the material, bringing the material, in this case glass, to the temperature in which the foreign body [Nickel Sulphide] is most likely to cause a spontaneous failure. Once the heat soaking is complete, the panes which are most likely to fracture will do so during heat soaking, leaving material which is considerably less likely to spontaneously combust.
This means that once the glass is heat soaked, the occurrence of an inclusion occurs once in every 26,500m2 of 6mm toughened glass, this is an extremely low chance of failure from occurring. As a company, we typically use 10mm glass and therefore the chances of an inclusion from occurring is one in every 16,000m2 of glass, this is still an extremely low statistic.
In the worst-case scenario due to using laminated glass panes, the glass has safe breaking characteristics and therefore, if a fragmentation of the glass were to occur, the fragments are held together by the laminating process.
“Despite all these benefits, toughened glass is at risk from one unique fatal flaw: an inclusion of foreign material”.
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