Mark Meshulam is aglass consultant and expert witnessinvolved withglass breakage and glass damage
Sometimes it's easy to know why a glass breaks.
For example, a dropped ball combined with neighborhood children running can often be considered symptoms of impact-related glass breakage.
Today we will look at four types of glass breakage:
- impact break
- stress cracks
- gunwale damage
- spontaneous rupture
If you know anything about glass, you know that it can break, and when it does, it's no good.
Going back to the ball through the window example, the pattern of glass breaking 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 before the game. Ball.
A very well-hit hard ball or a well-throwed rock directly striking a piece of annealed glass will shatter the glass with a circular puncture with cracks extending from the point of impact.
The resulting fragments between these cracks are dangerous! Injuries caused by broken glass can be serious and even fatal. If shards of broken glass land on your arm (as they often do during cleanup), you'll soon be in the emergency room.
Experienced glaziers often tape the shards together and then remove the entire panel. If you need to delete these fragments, delete the upper ones first, then the lower ones.
Wear thick rubber gloves, protect your arms, head, eyes and feet, and place the fragments in a cardboard box, not a garbage bag.
Blunt or distributed impact on long narrow annealed glass
In this example, we see a horizontal crack at the center of the blunt glass impact, with cracks radiating from the impact. Due to the aspect ratio (ratio between width and height), the snippets are long and narrow.
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high speed small rock
Kudos to this vandal who was able to select the right size small rock and throw it with enough speed to completely puncture this glass. The combination of size and speed resulted in a localized pattern of glass breaking.
Bigger stone Less speed
This hooligan's eyes were bigger than his throwing arm. Although it marked a large area of glass damage, it did not achieve the goal of full glass penetration. However, the impact was large enough to shatter the inner glass of the insulating glass unit. In the image below, you can see two sets of impact break patterns. You can also see the rich source of projectiles: the train tracks. A week after we finished installing the windows in a new school, the local kids had a picnic with the rocks and our new windows.
broken tempered glass
When tempered glass breaks, the energy trapped in the glass due to internal stress/compression is explosively released, producing a glass breaking pattern sometimes referred to as “cubes”.
Seeing a cubic breakage pattern doesn't tell you why the glass broke, it just says the glass was tempered. In general, there are three reasons why tempered glass can break: impact, edge damage, or inclusions. Inclusions are small impurities in the glass. The best known are nickel sulfide, however, there are also ferrous, silica and gaseous inclusions that look like tiny bubbles.
Normally, when tempered glass breaks, it falls out in a pile of small cubes. Only the most patient glass consultants with the most generous customer would consider putting the cubes together to determine the cause of breakage. That said, I've personally spent many hours combing through broken glass looking for one important clue: a pair of adjacent hexagons, known as a butterfly pattern, bordering a nickel sulfide inclusion.
However, occasionally broken pieces of tempered glass will remain in the opening, stuck together like blocks in a masonry arch. And just like a masonry arch, if you remove the key, the arch, or the glass in this case, it collapses.
This image shows broken glass that was part of a laminated unit. The PVB (polyvinyl butyral) interlayer held the pieces in place, giving us the opportunity to see that impact-related glass breakage can be visible, even on tempered glass.
Spontaneous Breakage in Tempered Glass
Glass, especially tempered glass, sometimes breaks on its own. This can be quite unnerving when, as happened in an unnamed public place here in our big city, large thick panels of tempered glass basically exploded quite often. The unusual cause in this rare case: The glass contractor tried to polish the edges of the glass after tempering it, creating a series of time bombs. It is a very bad idea to modify the glass once it has been tempered!
A better known, but also quite rare, cause of spontaneous glass breakage is nickel sulfide inclusions. If you read the previous post, you must have noticed that glass is made from fused powders. A nickel sulfide inclusion is a small rock of material that remains in the glass. See an artist's graphic representation of a nickel sulfide inclusion below.
You can imagine that a small rock embedded in a glass slab that is under high tensile/compressive forces can weaken the glass and eventually cause the glass to break. But the story gets worse. Nickel sulfide grows another 4% in size over time. If it stays in the glass layers between tension and compression and grows, kaboom!
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Exploded glass shower doors
One of the scariest, yet somewhat common, types of spontaneous tempered glass breakage is in glass shower doors. They are exposed to bumper impacts, the heat of the shower, the tearing action of cross towel racks, and of course, nickel sulfide inclusions. In hotels, multiply the risk factors by the number of rooms and the lack of care a hotel customer tends to have.
There can be a delay between impact and crack propagation in tempered glass, as with any other glass, and sometimes the moment when the glass finally explodes seems ironic and Machiavellian. Often, perverted glass explodes 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 flying glass projectiles. 3. The victim is barefoot. 4. The victim must walk barefoot over a field of freshly broken glass. So here's a tip for readers who actually shower: if the shower door breaks, stand still for a moment and take stock. Let's hope you don't cut yourself too much. Without moving your feet too much, find a towel. Try to grab him and put him on the glass so he can get out. Then get out and never shower again.
Normally, a "stress crack" will only occur in annealed or heat-strengthened glass. Stress cracks emanate from the edge of the glass and snake seemingly aimlessly. But there is a purpose: to relieve stress in the glass. However, the term "stress crack" can be misleading.
If the annealed glass is subjected to thermal fluctuations that stress the glass beyond its capabilities, the glass will break in such a way as to alleviate the stresses induced by the thermal changes. This type of failure is often a design issue. Thermally strengthened glass may have been a better choice for the application.
However, there can be an almost identical pattern of breakage from damage to the edges of the glass that fails when normal stresses, such as thermal, are applied. In this case, edge damage rather than heat stress is to blame.
To tell the difference between a true stress crack in glass and glass breakage due to edge damage, look on the edge of the glass for a sliver, which we window linguists sometimes call an "oyster." You may have to look hard because the oyster may be buried in the main seal on surface #2 or #3.
Another clue would be the distribution of broken glass in the building. It would be normal to find stress-type cracks in elevations with greater temperature changes. But does tearing also coincide with the use of reflective interior blinds, especially in the partially open position? This would be indicative of a true stress crack rather than a crack induced by edge damage. Also, look outside. Is there something that partially darkens the glass? That could be a factor.
New Photos: Nickel sulfide inclusions that can spontaneously break tempered glass
Want to learn more about nickel sulfide inclusions that can spontaneously break tempered glass? See this photo album:Inclusion of nickel sulfide: a tiny particle that destroys tempered glass
Great resource from Viracon
Viracon technical information:Heat stress fracture
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