Spontaneous breakage in toughened glass

When you find a butterfly shaped pattern at the origin of a broken glass pane, it might be the clue that you are in front of a spontaneous breakage.

The butterfly shape is noted at the point of origin of the break.

The inclusions are impurities introduced in the mass glass prior to the production of float glass. The tempering process is based upon to heat up the glass firstly and afterwards is cooled down to compress the surface of the glass pane, obtaining improved mechanical properties.

If one of those inclusions within the mass glass is Nickel Sulphide (NiS), this remains in a high temperature form because it did not get enough time to cool down as the rest of the mass glass. Subsequently, the NiS will return to its low-temperature after a period of time. During the process, the NiS volume will increase its volume, causing the sudden breakage of the glass pane. This is known as spontaneous breakage.

NiS will only cause the breakage of the tempered glass providing they are situated in its tensile zone. As the inclusion increases the volume, it releases all the energy embedded in the compressed zone.

Stresses in toughened glass 

Nowadays, the production of float glass totally free of nickel sulphide does not appear to be possible.

The spontaneous breakage follows a Gaussian function; such graph is a characteristic symmetric bell curve shape. This shows that the number of breakages is initially reduced, and then it goes up to a maximum value and finally decreases following a similar curve. This function is formulated collecting data of the number of breakages over a period of time.

The spontaneous breakage follows a Gaussian function, at the vertical axis includes the number of breakages and at the horizontal includes the time

One of most effective way to prevent this phenomenon is to carry out the Heat Soak Test (HST) after the tempered fabrication process. This is a destructive method that consists of introducing the tempered glass panes into an oven during a reduced period of time to accelerate the expansion of the NiS inclusions. In such way, the contaminated glass panes will fail inside the factory prior to be delivered to the Client.

Tempered glass panes within a Heat Soak Test oven

Façade engineers should always undertake such failure investigations to determine the most likely root of the glass failure and if the NiS inclusions are the cause of the failure.