HOW THINGS WORK: Windscreens [VIDEO]

Peter explains the technology behind modern-day windscreens. We take a look at how windscreens have advanced over the years and how they are manufactured today. We also take a close look at the ways our windscreens can be damaged and how to repair them to ensure our safety. Watch the video now.

Estimated reading time: 5 minutes, 55 seconds.

If you can’t watch the video, you can read the video transcription below: Edited for clarity and readability

Welcome to this week's episode of How Things Work. 
In today's segment, we’re going to be looking at the remarkable technology behind the windscreens in our vehicles.

If we go back in history and look at what cars used in the early days, they used nothing. There were no windscreens, so you were subject to the elements and the wind. It was only in the 1915s that we began to see laminated safety glass windscreens. Prior to that, we just had single sheets of glass. You can imagine just how dangerous those were. There were a lot of injuries to drivers and passengers when the glass shattered.

In 1915, we saw the first laminated safety glass windscreens being used, which also provided a lot of structural strength. The disadvantage was that the cellulose used as the laminate would eventually start to discolour and go yellow. These first windscreens didn't have any UV protection, but as technology and processes advanced, we started seeing amazing technologies being used with plastic laminates. These new laminates provide a crystal clear view that doesn't fade over time and also offers UV light protection.In 1915, we saw the first laminated safety glass windscreens being used, which also provided a lot of structural strength. The disadvantage was that the cellulose used as the laminate would eventually start to discolour and go yellow. These first windscreens didn't have any UV protection, but as technology and processes advanced, we started seeing amazing technologies being used with plastic laminates. These new laminates provide a crystal clear view that doesn't fade over time and also offers UV light protection.

How is laminated glass manufactured? 
It's simple to explain, but more difficult in practice. We take two normal pieces of glass, heat them up to a high temperature, and place them on a mould called a lehr. The glass is then pressed into the shape of the windscreen we're creating, and then we add a plastic laminate to those two pieces of shaped glass. The laminate is more than just plastic, though. It's a very complex polymer, which is glued onto each surface of the glass. Once those processes have taken place, we put that whole assembly into a special oven called an autoclave. The autoclave will heat up the glass and laminate so that it bonds properly. The autoclave is also a pressure vessel, so we heat and pressurise the glass, resulting in a very strong piece of laminated glass.

This glass is designed to offer vehicle drivers and passengers a number of different protections. The first is protection from standard wind and a few objects that might be flung up, like stones and rocks. There's also a certain degree of protection from objects that penetrate the glass, for example, if a stick or a branch gets flung up, the windscreen will stop it from completely entering the cabin.

Something you might not realise is that the glass provides structural strength to the vehicle chassis itself. I'm going to illustrate it with a Pringles tin. In a frontal impact, the glass provides approximately 40% of the structural rigidity and in a rollover approximately 60% of the structural rigidity.
This is my windshield [Pringles tin lid], and this is my chassis [Pringles tin]. Without the windshield, it's quite easy to bend the chassis [Peter compresses the Pringles tin and it bends easily]. But if I've got my windscreen in place, it's actually quite tough to bend [Peter places lid on Pringles tin and tries to compress it. The tin doesn’t bend]. It's no exaggeration that the windscreen is a critical safety feature.

That brings me to the next few points. How can we repair windscreens? What causes cracks? Is it safe to drive a vehicle with a cracked windscreen?
 
Let’s have a look at the type of cracks and damages we see on windscreens. The most common is a stone chip. You may have noticed that depending on where the stone impacts your windscreen, the kind of damage caused differs. If the stone impacts somewhere in the middle of the windscreen, it usually remains a small crack.

We get four types of cracks. First is a bullseye, which is a deep stone chip. Second is a crater, which is semicircular in shape. Third is a half-moon shape. Fourth is a star shape. We can also have a combination of those. It depends on what type of crack we have as to how we fix it. Most windscreen aftermarket fitment centres have a process where they can fix these small chips and cracks. But, if the chip occurs on the extremities of the windscreen, the crack propagates very quickly forming a very large crack. When we apply heat in the process of manufacturing the glass, it pushes all the stresses locked into the glass, to the extremities of the glass, meaning that all of these stresses are in the outer band of the glass. If we chip that part of the windscreen, all those stresses are unleashed in one go, even if the chip is small. This results in the crack propagating very quickly.

As we've said before, the structural integrity of the windscreen is critical. It's providing approximately 60% of the chassis stiffness. If the windscreen is cracked, you need to replace it as soon as possible. If the chips are small enough, you can usually just get them fixed, unless they are within the driver's range of sight. By law, they have to be fixed which unfortunately usually means you’ll have to replace the windscreen.

In addition to the stress risers that exist on the perimeter of the windscreen that cause and worsen cracks, there are other ways that cracks can worsen. For example, the harsh vibrations from driving on a very rough road or hitting a pothole add stress to the windscreen and can worsen the crack. Another example is if there is an extreme temperature difference between the outside and the inside of the vehicle, the crack will propagate even quicker. This can also be caused if we pour hot water on a cold windscreen. So don't be tempted to pour hot water on an icy windscreen in winter! The best way to get rid of the ice on your windscreen is to simply scrape the ice off with a plastic credit card.

The most interesting way a crack propagates, to me, is by a process called dissociative chemisorption. It’s a mouthful, but it’s a simple process that occurs on an atomic level and relates to the free electrons and free orbitals that exist with the glass and the water itself. As electrons move between the free electrons and the free orbitals, they start to create a sharing effect that causes the crack to grow 20 times faster than normal.

That's it on windscreens, the technology behind them, and some of the ways we can maintain our windscreens.

We look forward to seeing you on future segments of How Things Work!

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