Spontaneous failure of toughened glass

Introduction

Recently, ATTAR has been asked to investigate an increasing number of spontaneous failures associated with toughened glass panels, or “lites”.

Toughened glass is widely used as windows, skylights and balcony balustrades in both commercial and domestic applications, and is often a requirement under Australian standards and Codes.

Failure mechanism

The failure of these panels usually occurs at a seemingly random point in time and not necessarily corresponding to the application of outside stresses (e.g. wind, thermal, impact).

The fragmentation of the glass is a deliberate feature and it is a result of the thermal treatment during manufacture. The resultant small fragments are significantly less likely to result in cuts and injuries versus plate glass which fractures into large, razor sharp shards.

Glass broken into small fragments
Glass broken into small fragments

After failure

When spontaneous failure occurs often the glass panel will remain in place through the mechanical interlocking of all the individual pieces. In instances where they do not hold, a significant volume of glass fragments will fall to the ground. In this event, there is a real risk of significant injury or even death to pedestrians underneath.

If the panel is still holding together, it is valuable to investigate the failure in-situ prior to at our lab. This enables the identification of the point of failure origin and an assessment of any nearby factors which may have influenced the failure.

Failure origin

At the glass failure origin shown in our most recent failures investigated, is the characteristic feature known as the butterfly pattern.

Fracture origin showing butterfly pattern
Fracture origin showing butterfly pattern

This pattern is usually typical of the presence of a microscopic inclusion, which, under the right lighting conditions, can sometimes be observed photographically.

Presence of an inclusion

Identification of a glass inclusion requires the segments be carefully separated and inspected using high-powered microscopes.  Optical microscopy can readily identify the presence of particles as shown below:

Optical microscopy of inclusions
Optical microscopy of inclusions

However for further analysis, the use of a scanning electron microscope (SEM) is required.  As one of the few commercial laboratories in Australia with an SEM, we are able to analyse the particles quickly and directly.  SEM microscopy of one particular particle is shown below at a magnification of x500.

Nickel Sulphide particle as viewed at x500 magnification
Nickel Sulphide particle as viewed at x500 magnification

A feature of our SEM is the ability to target specific areas on an image and scan for chemical composition.  With this ability, we were able to show the particle was composed of Nickel and Sulphur, or that it was a Nickel Sulphide (NiS) inclusion.

Nickel Sulphide

Nickel Sulphide is an impurity picked up in the manufacturing process of glass whereby a slow phase transformation results in a volumetric growth.  The growth is restricted by the surround glass, however as the particle tries to expand, it places the glass into tension, until the point where the glass begins the runaway of complete fragmentation.

The insidious nature of nickel sulphide inclusions is that they are almost undetectable to the eye, and may result in spontaneous failure months, or years after installation.  Inspection for these inclusions prior to failure and on post-installed panels can be conducted and any enquiries into this methodology can be made to any of the ATTAR engineers.