Diagnosing Glass Breakage

Mark Meshulam — Sat, 20 Jun 2009 16:27:31 +0000

Mark Meshulam is a consultant for glass breakage and
expert witness for personal injury due to glass breakage.

Sometimes it’s easy to know why glass breaks.

Diagnosing the cause of glass breakage can be a no-brainer

For example, a ball on the floor combined with neighborhood kids running away generally can be considered symptoms of impact breakage.

Today we will look at four types of breakage:

Impact breakage
Stress cracks
Edge damage
Spontaneous breakage

Impact breakage

Typical impact break without puncture

If you know anything about glass, you know that it can break, and when it breaks, it’s not a good thing.

Back to the ball-through-the-window example, the breakage pattern will vary depending on the speed and mass of the ball, and the size, thickness and post-annealing treatments that were performed on the glass prior to the ballgame.

A very well-hit hardball, or a well thrown rock squarely hitting a piece of annealed glass will produce a circular puncture with cracks emanating outward from the point of impact.

Glass shards from broken annealed or heat strengthened glass are dangerous and must be handled with care.

The resulting shards between these cracks are dangerous! If they fall out on your arm (as often happens during clean-up) you will soon be in the emergency room. Experienced glaziers often tape the shards together with duct tape, then remove the entire panel. If you must remove these shards, remove the upper ones first, then the lower ones. Use heavy rubber gloves, protect your arms, eyes and feet, and place the shards in a cardboard box, not a garbage bag.

Blunt or Distributed Impact on Long, Narrow Lite of Annealed Glass
In this example, we see a horizontal crack at the center of the blunt impact, with cracks radiating away from the impact. Due to the aspect ratio (relationship between width and height), shards are long and narrow.

Blunt impact on long lite of annealed glass

Small Rock High Velocity
Congratulations to this vandal who was able to select just the right sized small rock, and hurl it at a speed righteous enough to completely puncture this glass. The combination of size and speed resulted in a localized pattern of damage.

This vandal chose quality over quantity. He selected a smaller stone, which he was able to accelerate to a greater velocity, sufficient to completely puncture the glass.

Larger Rock Less Velocity
This vandal’s eyes were bigger than his throwing arm. Although he scored a large area of glass damage, he failed to achieve the goal of full glass penetration. The impact was large enough, however, to break the interior lite of the insulating glass unit. In the picture below you can see two sets of impact breakage patterns. You can also see the rich source of projectiles: railroad tracks. One week after we finished installing windows in a new high school, local kids had a field day with the rocks and our new windows.

Vandal with big ideas hurled a sizeable rock and achieved a big breakage pattern, but failed to breach the fortress

Cracks in tempered glass

Tempered Glass Breakage
When tempered glass breaks, the energy retained in the glass due to internal tension/compression releases explosively and produces a breakage pattern sometimes called “cubes’.

Seeing a cubist breakage pattern does not tell you why the glass broke, it only tells you that the glass was tempered. Generally, there are three reasons tempered glass will break: impact, edge damage or nickel sulfide inclusions.

Normally, when tempered glass breaks, it falls down into a pile of little cubes. Only the most patient window consultants with the most generous client would ever consider piecing the cubes together to determine the cause of breakage. However, occasionally the pieces of broken tempered glass will stay in the opening, locked to each other like blocks in a masonry arch. And just like in a masonry arch, if you remove the keystone, the arch – or glass in this case – comes tumbling down.

If you look closely, you can see the point of impact on this tempered glass. A laminated interlayer holds the cubes in place.

This picture shows broken glass which was a part of a laminated unit. The PVB (polyvinyl butyrate) interlayer held the pieces in place, giving us an opportunity to observe that impact damage is visible, even in tempered glass.

Spontaneous Breakage in Tempered Glass
Glass, and especially tempered glass, sometimes breaks all by itself. This can be quite disconcerting when, as has happened in a public place which will go unnamed here in our great city, large, thick panes of tempered glass basically blew up fairly frequently. The unusual cause in this rare instance: the glazing contractor attempted to grind the edges of the glass after it was tempered, creating a series of time bombs. It is a very bad idea to modify glass after it is tempered!

A more well-known, but also quite rare cause of spontaneous breakage is nickel sulfide inclusions. If you read the previous post, you will already be down with the fact that glass is made from melted powders. A nickel sulfide inclusion is a tiny rock of unmelted material that remains in the glass. Below see an artists graphic representation of a nickel sulfide inclusion.

Artists representation of nickel sulfide inclusion.

You can well imagine that a little rock embedded in a slab of glass which is under high tension/compression forces, could weaken the glass and eventually cause breakage.

Stress Cracks
A “stress crack” will usually only happen in annealed or heat strengthened glass. Stress cracks emanate from the edge of the glass and meander about apparently without purpose. But there is a purpose: to relieve stress. However, the term “stress crack” can be misleading.

If annealed glass is subjected to thermal fluctuations beyond its capabilities, it will break in a way that will relieve the stresses induced by thermal changes. This type of failure is a design issue. Heat strengthened glass should have been specified for the application.

However, there can be a near-identical breakage pattern which emanates from damage in the glass edge that fails as stresses, such as thermal, are applied.

Classic meandering pattern of heat-related stress crack, accompanied by evidence of edge damage. In this case, the building had both: edge damage and underspecified glass. Reflective blinds and a South exposure combined to create a high frequency of this type of breakage.

To tell the difference between a true stress crack and a crack due to edge damage, look at the edge of the glass for a chip, which we window linguists call an “oyster”. You might have to look hard because the oyster could be buried in the primary seal on the #2 or #3 surface.

Another clue would be the distribution of glass breakage in the building. It would be normal to find stress-like cracks on elevations with greater temperature swings. But does the breakage also coincide with the use of reflective interior blinds, especially in a partially opened position? That would be indicative of a true stress crack, rather than a crack induced by edge damage.

Great additional resource:
Viracon Technical Information: Thermal Stress Breakage

Mark Meshulam observing that glass is not broken

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Diagnosing Glass Breakage