Nobody knows more about windows.
Jun20Filed under: About windows & glass, Glass breakage, personal injury; Tagged as: broken glass, broken glass injury, exploding tempered glass, glass breakage, glass breakage pattern, glass consultant, glass impact, glass impact breakage, glass shower door shatter, glass shower doors, glass stress cracks, photos of broken glass, shattered glass, spontaneous glass breakage, stress in glass
Mark Meshulam is a glass consultant involved with glass breakage and an
expert witness for broken glass injury.
Sometimes it’s easy to know why glass breaks.
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
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 glass 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.
The resulting shards between these cracks are dangerous! Broken glass injuries can be serious, even deadly. If broken glass shards 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, head, 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 glass impact, with cracks radiating away from the impact. Due to the aspect ratio (relationship between width and height), shards are long and narrow.
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.
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.
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 cubic 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 inclusions. Inclusions are tiny impurities in the glass. The most well known are nickel sulfide, however there are also ferrous, silica and gaseous inclusions which look like tiny bubbles.
Normally, when tempered glass breaks, it falls down into a pile of little cubes. Only the most patient glass consultants with the most generous client would ever consider piecing the cubes together to determine the cause of breakage. That being said, I have personally spent many hours picking through broken glass looking for an important clue: a pair of adjacent hexagons, known as a butterfly pattern, that borders a nickel sulfide inclusion.
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.
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.
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. But the story gets worse. Nickel sulfide grows an additional 4% of its size over time, especially in the presence of heat. If it is located in the strata in the glass between tension and compression, and it grows, kaboom!
Exploding Glass Shower Doors
One of the scariest, yet somewhat common type of tempered glass spontaneous breakage is in glass shower doors. They are exposed to banging against bumpers, heat from the shower, wrenching action of through-mounted towel bars and of course, nickel sulfide inclusions. In hotels, multiply the risk factors by the number of rooms and the lack of care typically taken by a hotel patron.
There can be a delay between impact and crack propagation in tempered glass, just like any other glass, and sometimes the time the glass finally explodes seems ironic and and Machiavellian. Quite often the perverted glass will explode while the unsuspecting victim is naked and in the shower.
Let’s count the problems: 1. The victim is naked. 2. There are sharp cubes of glass projectiles flying around. 3. The victim is bare footed. 4. The victim must walk barefooted over a field of freshly shattered glass shards. So here’s a tip for readers who actually do take showers: If your shower door shatters, stand still for a moment and take stock of the situation. Hopefully you are not cut too badly. Without moving your feet too much, look for a towel. Try to grab it and lay it down on the glass so you can walk out. Then get the hell out and never shower again.
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 in the glass. However, the term “stress crack” can be misleading.
If annealed glass is subjected to thermal fluctuations that create glass stress beyond its capabilities, the glass 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 normal stresses, such as thermal, are applied. In this case the edge damage, not the thermal stress is the culprit.
To tell the difference between a true glass stress crack and a crack due to edge damage, look at the edge of the glass for a chip, which we window linguists sometimes 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
Need a glass consultant to diagnose glass breakage or investigate a broken glass injury?
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