Does Toughened Glass Crack?

Toughened glass, also known as tempered glass, is up to five times stronger than regular annealed glass. It is made through a thermal tempering process that makes it more resistant to impact.

This makes it ideal for use in glass doors and windows, shower screens, tables and other everyday home items. But does toughened glass crack?

Stress Cracks

While toughened glass is an excellent safety choice for domestic and commercial buildings it is not without its risks. Occasionally, it has been known to spontaneously crack, seemingly for no apparent reason. This is referred to as thermal stress glass breakage and it can be very confusing.

Toughened glass is made by subjecting final size, edgeworked panes of annealed glass to a heat and rapid cooling process. This sets up high compressive stresses in the surface and a counter-balanced tensile stress in the interior of the glass. If damage to the glass can penetrate this tensile zone, then a'static fatigue mechanism' operates and complete fracture of the glass occurs.

This can be induced by a number of things, including damage to the edge of the glass which leads to small stress concentrations; or environmental factors such as extreme variations in temperature. These cracks will progressively grow and may cause the glass to fail eventually, although this can be months or even years away from when the damage was first caused.

Corrosion Cracks

Toughened glass (also known as tempered glass) is up to five times stronger than regular glass and when it breaks it is much safer than shards of normal glass. It also breaks into smaller pieces which make it less likely to cause injury than shards of normal glass.

During the toughening process, glass is heated and then cooled at different rates to create differential expansion and contraction of the glass panes. The difference in expansion and contraction causes stress which could cause cracking if there are impurities in the glass.

Nickel sulphide (NiS) inclusions can form in the glass during manufacturing. When a NiS inclusion is exposed to ambient temperatures and atmospheric pressure changes it can expand at a faster rate than the rest of the glass which can then induce stress in the glass pane.

Heat soaking is a process that can be added to the manufacturing process that heats the glass panels for longer than normal. This reduces the potential for NiS inclusion shatterage but cannot eliminate it entirely.

Impact Cracks

One of the most common types of cracks in toughened glass is an impact crack. These are caused by a projectile like a stone, a branch or even a flying golf ball. The damage typically shows up as a hole in the centre of a starburst pattern radiating out from it.

These cracks are caused by nickel sulphide impurities that sometimes are unavoidably mixed in with the float glass during manufacture. When the glass is reheated and quickly cooled in the toughening process these nickel inclusions remain at a high temperature and when they cool down again they tend to expand.

When they expand in a toughened glass panel that is already under tension this can cause the glass to shatter. The heat soaking process used in the manufacturing of some insulated glass reduces the risk of these nickel sulphide inclusion breakages but unfortunately it is not 100% effective. Strata Managers, facility managers and builders should be aware of the risks and ensure that panels are fitted correctly to minimise stress on the corners which are the pane’s weakest point.

Explosion Cracks

Most people have experienced a time when toughened glass spontaneously breaks without any obvious cause. Known as butterfly cracks, these are caused by nickel sulphide (NiS) inclusions and heterogeneous phase particles.

During the float glass manufacturing process, nickel sulphide inclusions are created. During the toughening process these are reformed to their low temperature b-state but they may not be completely eliminated. This is primarily because they cannot be detected by current inspection methods and also because the glass is rapidly cooled during the toughening process.

The b-state of the inclusions is able to expand at a faster rate than the glass, leading to stress concentration and crack initiation. The cracks will then grow as the glass cools. The radial stresses are compressive while the tangential stresses are tensile, giving the appearance of a butterfly crack. The presence of these cracks is indicative of the presence of NiS. Heat soaking is a test that aims to eliminate the inclusions by exposing them to a high temperature environment before they can cause glass failure on site.

Also Read: Is 12mm Toughened Glass Breakable?