November 06, 2017

Amazing slow-mo video shows why it cracks sometimes . . . and not others


The science behind the lottery of a shattered screen: Amazing slow-mo video shows why it cracks sometimes . . . and not others

Most of us will have felt the pain of dropping a mobile phone or tablet, only to find the screen is shattered beyond recognition or use.
The pain is further heightened when you receive the huge repair bill to fix or replace the screen of your smartphone.

But there are those happy moments when we retrieve our dropped mobiles from the floor to find that the screen has remained intact.
But what causes the screen to crack slightly on some occasions, and completely shatter on others?
When you drop your phone the impact causes a small amount of elastic energy to be converted into acoustic energy - the noise you hear. But the majority of the elastic energy stored in the glass will be converted into two (or more) new surface energies, which results in a crack (or several cracks). 

To find out exactly what happened, the YouTube channel Beyond Slow Motion decided to sacrifice an iPad.

There are two key factors to prevent your phone screen shattering, its hardness and its strength. Hardness is the resistance to abrasion, and stops the screen being covered in scratches whilst it moves about in your pocket/handbag.
Its strength is the relationship between the surface compression and inner tension, and it determines, how many blows a piece of glass can take before it shatters completely. 
Glass only breaks when the force of impact finally overcomes the surface compression. A scratch doesn’t shatter glass, but small stresses will eventually lead to it shattering.
New glass, especially made for tablets and mobile phone screens by Corning Gorilla Glass is hoping to change all this.

In tests it could withstand around 100,000 pounds of pressure per square inch.  It can withstand, without shattering or cracking, a 535g ball being dropped on it from 1.8 metres above.
The screen technology has already made its way into the Samsung Galaxy S3 smartphone.

If it is the first time your device has fallen to the floor the odds are it won't shatter, but if this is a repeated occurrence, then you won't be so lucky
How your phone or tablet falls to the ground is key to the shattering question.
If it falls face down you might escape without too much damage because the stress of impact is spread across the entire surface. It would almost certainly undergo damage, which you maybe couldn’t see with the naked eye, which would cause it to shatter on a subsequent occasion.

But if you drop your phone onto one of the corners, the uneven surface means the point of contact between the glass and ground is small and focused, directing the entire force of the impact onto one small point.
And this is where months or years of little bangs and bumps can become relevant. With every drop the invisible cracks become greater, until a major spillage will cause it to shatter.

It's impossible to determine the exact strength of a piece of glass, and there is no such thing as a flawless one.
This is why we can’t predict the exact or number of incidental drops that our devices can survive.  Some people suggest four is the magic number, but nothing is conclusive.

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