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Jun
20

Diagnosing Glass Breakage

Filed under: About windows & glass; Tagged as: broken glass, glass breakage, glass impact breakage, glass stress cracks
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.

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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. They are so tiny that I have never seen one. I never really looked, either. 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

Need to diagnose glass breakage?

Contact me, Mark Meshulam,
the Chicago Window Expert
for the expert attention you deserve.

My phone: 847-945-9200 ext 229
My email: mark@chicagowindowexpert.com

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11 Comments

11 Responses to “Diagnosing Glass Breakage”

Lacy June 23rd, 2009 at 11:57 pm

Pretty good post. I just found your site and wanted to say
that I have really liked reading your posts. Any way
I’ll be subscribing to your feed and I hope you post again soon!
Mark Meshulam June 24th, 2009 at 9:55 pm

Thank you for your kind comment. Feel free to ask questions or suggest future articles.
Hampton Fillingame June 25th, 2009 at 7:09 pm

I know the family that ownes Florida Crush Stone. About 80% of the aggregate for cement for the whole US comes from them. Do you want me to help forward this site to people in related industries? Do you work with companies around the country? How small is too small?

Hampton
Mark Meshulam June 26th, 2009 at 7:18 am

I would be grateful for readers forwarding this site to other interested parties. Yes, we do work with companies around the country, despite the ChicagoWindowExpert name. In the consulting part of our business, there is no job too large or small. Thanks for writing!
Victor June 29th, 2009 at 3:28 am

Nice post by smart M and I was enjoyed. Hope new posts would be on the site and serving Chicago.
Mark Meshulam June 30th, 2009 at 7:13 am

Thanks for the nice comment from my friend Victor Wei of Yuanda. Yuanda is the Chinese curtainwall company responsible for two of the most memorable buildings of the Beijing 2008 Olympic Games, the Bird’s Nest and the Water Cube. Congratulations to Yuanda for those two wonders!
Kim May 26th, 2010 at 12:04 am

Hi Mark,

I came across your website and found it very helpful. I was curious if you had any photos of nickel-sulfide inclusions? My window spontaneously shattered one night and the window supplier is claiming it broke because of impact — I know I didn’t hit it! Based on the tempered glass breakage photo on your website, it would appear that my living room window has a point of impact. But would spontaneous breakage also have the same radial, “point of impact” pattern? I’ve read on other websites about butterfly patterns that appear because of nickel-sulfide inclusions — I think mine shows that, but I would like to get a professional opinion.

I would be very interested in having you diagnose, but I’m an American expat living all the way in China, so I’m not sure if you can help or not. I’ve attached photos of the broken window taken by the window supplier (it’s a curtain wall, floor-to-ceiling residential window). The outside window pane is intact, according to the supplier.

Sincerely,

Kim
Mark Meshulam May 26th, 2010 at 12:12 am

Hi Kim,
According to my glass experts, a nickel sulfide inclusion should be visible to the naked eye.
Attached please find an interesting photo I recently came across.
Kumar June 6th, 2010 at 2:16 pm

Hi, Kim,
I am working in a window glass production plant. Recently I am getting more complaint from my customers regarding temperness. But I found very good cutting in size before packing. will you please be able to help me in this regard. what are the general breakages styles of different reasons in related to stress, strain vice versa.if you have any pictures related to that subject please enclose with details. your co-operation is highly obliged.

with regards

kumar
Mark Meshulam June 22nd, 2010 at 10:56 pm

Kumar,

The pattern you see on the background of this page (scroll all the way down) is a clasic breakage pattern of tempered glass. I recently learned that it is possible for a lite of glass to be incompletely tempered, i.e. tempered in some areas and not fully tempered in other areas.

Although I have never seen it, this condition could result in a hybrid breakage pattern that transitioned from the tempered cubes to larger shards. Has anyone out there ever seen such a thing?

Since tempering occurs when glass is heated then rapidly cooled, I would imagine an uneven temper could result from uneven heating or cooling.

What is the breakage pattern your customers are complaining about, Kumar?
Francis July 14th, 2010 at 11:56 pm

I have experienced the incomplete tempering process. It is a laminated glass, the breakage happened at surface no.3 however surface no. 4 is intact. The breakage pattern is as shown in the background, just like a tempered.