PSA With Regards to the Proliferation of Tempered Glass

[Curufinwe_wins]

Hello everyone, with the recent move by numerous manufacturers to include tempered glass into their product lines, I think it is extremely important to take this time to explain some of the physics behind tempered glass and its advantages/disadvantages as a side panel material.

 

Tempered glass ( poorly and often vaguely called toughened glass or safety glass) is a glass that has been heat treated in a fashion where the glass is pre-stressed such that the bulk strength of the glass is increased and the energy of any impact can be dispersed throughout the entire pane, resulting in thousands of tiny particles of glass.

 

 

For many applications this is AMAZING! For a vehicle that could potentially be exposed to a crash, this means you are less likely to be impaled by a pane of glass. For a overhead window, a failure means a gentle rain of pebbles instead of a potentially fatal heavy/sharp load. And truly a piece of tempered glass can withstand far more pressure than standard glass (although still less than things like PVC and Acrylic).

 

Well then what is the issue?!? Tempered glass can and does fail without warning. It can shatter due to moisture, heat, or even completely acausally due to the pent up stresses (and manufacturing defects) in the glass.

 

 

Classically speaking the so-called Prince Rupert's droplet is an exact example of the issues of tempered glass. While the bulb is extremely resistant to impacts (similar to the relative strength of tempered glass over float glass), the strip is very brittle and causes the entire piece to fail if broken. This is due to stress concentrations, and unfortunately is a exact analogy to what happens if you drop tempered glass on it's edges...

 

While all glass (baring extremely specific different chemically strengthened and heat-treated glasses like Pyrex, outside the US) fails when dropped from large heights, thin sheets of tempered glass can fail falling under its weight from even a few inches off the ground. As a standard rule of thumb, any fall that has the potential to chip or crack the edge of regular glass (an annoying but not necessarily fatal issue) will COMPLETELY RUIN a similar pane of tempered glass. 

 

Thus it should come as no surprise tempered glass is one of the WORST glass-like side panel materials possible. In fact, if the glass is frame-less, float glass and any clear plastics are safer to use, less likely to fail, and most probably cheaper. The move towards it in the consumer market is almost entirely founded in the ability of the material to command a higher price at market, and thus improve case manufacturer margins.

 

Small sample of sources to take a look at:

http://miami.cbslocal.com/2015/09/25/exploding-glass-doors-a-growing-problem/ <- video discussing the increasing failure rates caused by cheap/poorly made tempered glass doors in the US market exploding without warning/incident.

http://www1.eere.energy.gov/solar/pdfs/pvrw2010_barry.pdf <- great ppt of different failure modes of tempered glass (sometimes laminated tempered glass.)

https://www.youtube.com/watch?v=xPwaq_6aXbo <- video in slow mo to demonstrate tempered glass failing. NOTE the extremely low amount of force applied on the edge of the glass to cause the entire thing to fail.

  

Random notes/notices:

• When I say tempered glass, I mean explicitly non-laminated tempered glass, the likes of which is used by case makers today. Laminated glass (like seen in Windshields) allows the glass to sometimes not completely fail on a slight fracture, and to not spew particles everywhere in the case of failure. Laminated glass is much more expensive, but is almost always superior, esp when combined with thick plastic layers for increased shock resistance. This is what all ballistic glass is made of. Laminated glass would make an absolutely fantastic side-panel glass (minus the cost).
• Tempered glass is more expensive to make, but the main reason glass is cheap compared to tempered glass is that standard float glass is sold at razor thin margins (being made a commodity decades prior).
• It is physically impossible for tempered glasses of the same composition to exhibit better clarity than similar quality float glass, although average clarity on non-tinted panels is often quite similar.
• If you do decide to make/use a large tempered glass side panel for ANY reason (this is actually good practice for standard glass as well), the minimum best practice behavior framing should be as follows:
  • Holding the physical pane of glass (blue), you should have a soft silicon/rubber c channel frame. This provides both a tight fit, and a degree of cushion should a small drop occur like when removing the panel.
  • Surrounding the soft c-channel is a metal/hard plastic c-channel. This provides a clean aesthetic, while also providing further protection from small bumps (just as two part phone cases do).
  • Under no circumstances should the edge of the glass ever be exposed. This is the most vulnerable portion of a tempered pane.
  • Both parts are readily available online and/or at local retailers due to their use in window applications.
• I have omitted the stress concentration issue of current side panels in use with mounting holes from the general discussion, but simple math allows you to prove that such holes increase the local stress on the material by no less than 2x making it even more susceptible to shock failure. This is a truly idiotic decision by case manufactures and is extremely discouraged in real world practice, even with float glass.
• If you do own a tempered glass case, I would highly recommend turning the case on its side when removing or applying a TG side panel (such that the glass is set horizontally on the largest face). This reduces your chance of dropping it on an edge, and thus obliterating the panel.

 

[byalexandr]

For the car crash thing, windshields use laminated glass. They use sheets of regular soda lime glass and polycarbonate/vinyl layers in between. Side windows on the other hand use tempered glass, which makes them extremely hard to break (even with a hammer, unless you break the edge first).

 

I don't really understand the point of this topic, yeah it provides some info but most people should already know how tempered glass works. The reason it's used in side panels is because it has a much higher breaking point than regular soda lime. Same reason glass tables are tempered glass. They shatter yes, but they take a LOT more force to do so.

[Curufinwe_wins]

15 minutes ago, byalexandr said:

snip

You didn't read the notices (see below).... The issue is that tempered glass is just worse for side panels. You aren't setting things on side panels, but take it on or off enough and you are quite likely to drop them a few inches (instant failure of a tempered panel, potential crack of float glass.) The higher breaking point is completely irrelevant in this application because the weakness of tempered glass is the edges, and those are not being protected at all on current models (again tables and almost all other tempered glass applications often use channels/ and other edge cushions/supports to address the issue, and explicitly warn against striking the edge.) 

 

Even this first comment explains why it needs to be here (although I do probably need to clean it up to make those points come home).

 

Additionally, while I haven't tested this part, I tend to think that in a system with many fans/moving parts, the type of shock failures exhibited with tempered panes are much more likely to damage or ruin parts of the system esp fans/heatsinks.

 

24 minutes ago, Curufinwe_wins said:

 

• When I say tempered glass, I mean explicitly non-laminated tempered glass, the likes of which is used by case makers today. Laminated glass (like seen in Windshields) allows the glass to sometimes not completely fail on a slight fracture, and to not spew particles everywhere in the case of failure. Laminated glass is much more expensive, but is almost always superior, esp when combined with thick plastic layers for increased shock resistance. This is what all ballistic glass is made of. Laminated glass would make an absolutely fantastic side-panel glass.

Oh, and while front/rear windshields use laminated glass, side windows almost always use standard tempered glass (which was what I was referencing.)

[byalexandr]

Just now, Curufinwe_wins said:

You didn't read the notices (see below).... The issue is that tempered glass is just 100% worse for side panels. You aren't setting things on side panels, but take it on or off enough and you are quite likely to drop them a few inches (instant failure of a tempered panel, potential crack of float glass.) The higher breaking point is completely irrelevant in this application because the weakness of tempered glass is the edges, and those are not being protected at all on current models (again tables and almost all other tempered glass applications often use channels/ and other edge cushions/supports to address the issue, and explicitly warn against striking the edge.) 

 

Even this first comment explains why it needs to be here (although I do probably need to clean it up to make those points come home).

 

Additionally, while I haven't tested this part, I tend to think that in a system with many fans/moving parts, the type of shock failures exhibited with tempered panes are much more likely to damage or ruin parts of the system esp fans/heatsinks.

 

 

But the higher breaking point ISN'T irrelevant. At all. It's what makes it BETTER for side panels. Dropping it a few inches is likely not enough stress to break it. Dropping it a couple feet onto a concrete floor on it's corner is enough, for example. Dropping a regular soda lime sheet of glass a few inches off the table will break it.

 

You're not understanding the clear difference here. It's not that soda lime glass is superior, it's just different. It's far better for windshields, because it stays together. But it cracks much, much more easily. The use of tempered glass is much better for side panels, because it's much harder to break. Doesn't matter if it shatters.

 

Think of it this way:

 

Would you rather have an easy to crack piece of glass that you would end up having to throw away and replace if it breaks?

 

Or would you rather have a much harder to crack piece of glass that you would end up having to throw away and replace if it breaks?

 

Both break, one just breaks at a much higher stress point in exchange for breaking a lot more. I'd rather take the one that can take a beating before it breaks over the one that is way more fragile, even if it only cracks a little (I'd end up having to replace it anyway).

[Curufinwe_wins]

21 minutes ago, byalexandr said:

snip.

Lots of issues here. First laminated glass isn't exclusive to float glass (soda-lime is a composition not a process, and is erroneous because it can be both tempered and float), for example, REAR windshields use laminated TEMPERED glass. Front windshields use laminated float glass (because you need as much front visibility as possible more than you need the additional protection of tempering.) Side windows nearly exclusively use STANDARD tempered glass.

 

The Prince Rupert's droplet issue should explain to you the logical issue with tempered glass... It is only as strong as the absolute weakest link (in more than just the standard cliche way.) A crack anywhere OBLITERATES it. Thus thin panes (esp with non-rounded edges) are extremely susceptible to edge drops (more so than float glass which has the ability to chip locally).

 

The only feasible time in which you would have a side panel exposed to any force which distinguishes the two (no impacts on a side panel, no load bearing), is removal and application. Thus the "harder to crack" thing isn't a factor at all (since small drops on hard surfaces quite easily chip glass and tempered glass due to stress concentrations). 

 

 

Additionally, you are completely ignoring the moments where tempered glass (often due to minute manufacturing flaws) have a history (and currently increasing frequency) of completely random failures...

 

http://miami.cbslocal.com/2015/09/25/exploding-glass-doors-a-growing-problem/

http://chicagowindowexpert.com/windowtags/shattered-glass-shower-door/

 

EDIT: This sort of failure, the common and classic "oyster chip", is often not something you need/want to replace an entire piece on, and cannot occur with tempered pieces that catastrophically fail.

 

[W-L]

Personally I just replaced a stock acrylic panel with regular window glass but in general though tempered glass is still a better option as it doesn't slice like regular glass can when it shatters into large shards. However as you did mention you must be carefully when you place it down you can't drop it on the edge or slightly hit it with anything or it will just break. 

[Curufinwe_wins]

1 hour ago, W-L said:

Snip

Tempered glass: safer for hands, worse for feet (assuming you are on a hard surface you can see anyways...)

 

For a dyi, it's super hard to recommend TG as you can't cut or drill it, it's harder to find and is 5-10x more expensive for consumers. 

 

I'd be less upset about the use of TG side panels if these case companies didn't insist on borderless designs and actually used proper attaching mechanism's that didn't amplify stress.

[W-L]

2 hours ago, Curufinwe_wins said:

Tempered glass: safer for hands, worse for feet (assuming you are on a hard surface you can see anyways...)

For a dyi, it's super hard to recommend TG as you can't cut or drill it, it's harder to find and is 5-10x more expensive for consumers. 

I'd be less upset about the use of TG side panels if these case companies didn't insist on borderless designs and actually used proper attaching mechanism's that didn't amplify stress.

Very true if you need a custom size it always will need to be special ordered, but for mounting I guess it depends who's the manufacture but most use a large knurled thumb wheel that bolts through the glass which works quite well. That being said I guess it would come with the nature that users would be expected to handle the panels much more gingerly, personally I've never had a problem with regular glass on a metal frame just don't drop it

 

[Curufinwe_wins]

3 hours ago, W-L said:

Very true if you need a custom size it always will need to be special ordered, but for mounting I guess it depends who's the manufacture but most use a large knurled thumb wheel that bolts through the glass which works quite well. That being said I guess it would come with the nature that users would be expected to handle the panels much more gingerly, personally I've never had a problem with regular glass on a metal frame just don't drop it

 

The large knurled thumb screw is a really ideal thing for float glass, but is pretty bad for tempered (although I don't know of too many cases of failure so far) either due to over tightening or general stress concentration.

 

Just framing the glass, as you say, helps SO SO much.

[Curufinwe_wins]

LTT learning the hard way on this....