Why Does Glass Break?

We’ve recently been involved with a project where glass has mysteriously broken and immediately the finger is pointed at either the glass manufacturer or the installer. Whilst these two elements of the glass delivery can be at fault, end users frequently neglect to consider a number of environmental and chemical issues that can result in glass breakage over time.


To try and establish the true cause of a breakage, our recommendations are as follows:

1) look at the glass make-up i.e. is it annealed/clear float, laminated or toughened.

2) consider how the glass break has manefested i.e. is it a clean line, does the break meander and curve around the glass, has the glass shattered, is there any evidence of an impact or projectile, is the breakage at the side of the glass, is there any damage to the surrounding area i.e. scratches on framework that would suggest the damage may have arisen as a result of a hard/soft body impact?

3) Look at the glass situation/environment: What QA procedures were conducted when the glass was installed, what activities have taken place in close proximity to the glass, is there a chance of significant temperature differential across a glass pane, have any manifestations stickers been stuck to the glass, is there external brise soleil, is there a radiator or heating grill in close proximity, are there blinds, if there are blind can hot air escape?

So as you can see, there are a whole host of considerations that must be assessed before allocating responsibilities.

www.cwct.co.uk have some great guidance note which we would recommend you read if you want to know more.

To focus on a few common breakage issues, we’ll start with toughened glass and an old favourite, Nickel Sulphide breakages:

Glass may contain nickel sulphide impurities, in the form of small crystals.  As glass is heated during the toughening process these impurities change state.  The high temperature a-state of the impurities may be frozen when the glass is quenched, and recovery to the low temperature b-state may then take several years.  Spontaneous breakage of the glass may follow, as the low temperature of the nickel sulphide impurities occupies a slightly greater volume and so generates a local stress concentration.  However, only the largest impurities will cause failure, and only then if they are in the core tensile zone of the glass.
Assuming that the fractured pane remains in place, at the origin or epicentre of the fracture there should be located two fragments which are larger than the rest and which form a ‘figure of 8’ or ‘butterfly’, image.  In the centre would be located the inclusion which is a small, round, shiny, yellow-black particle which may be as small as 0.05mm in diameter.
The risk of nickel sulphide failure can be reduced by heat soaking (HST) which in essence, it consists of reheating tempered glass panels for a third time, maintaining it at an elevated temperature that is still below the phase transfer temperature of NiS(Nickel Sulphide) for a set time, and then allowing it to cool. Recommendations from research from accredited bodies indicate that less than 1 break in 10,000 panes of glass was expected to occur after 2 hours. Maintaining an elevated temperature facilitates faster conversion of the alpha-phase NiS to beta-phase, and therefore the idea is that any panels which will fail from NiS fail in the HST oven rather than on the building.

 

Posted in Curtain Walling, Glass

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