Page 24 - Curriculum Visions Dynamic Book

Glass is very strong when compressed but much weaker when it is stretched by being bent, pulled, or twisted. There is not much difference between the strength of all glasses due to the different chemicals
in them, but strength can be greatly affected by surface imperfections. Just as a scratch mark cut by a diamond tip creates a line of weakness along which the glass will break cleanly, so a scratch on the surface of a glass will be a place at which stress builds up.
Defect-free glass is much stronger than any metal. However, glass is rarely defect free. As mentioned above, it has surface flaws and minute scratch marks or contamination of the surface, for example, by handling with fingers. They can reduce the strength of glass by a thousand times when it is pulled, although they do not affect it when the glass is squashed.
The strength of glass also varies depending on how long it has to support a weight.
When glass breaks, or fractures, the crack starts at the weakest point and then travels outwards. That is true even when a piece of glass appears to explode or shatter.
Strengthening glass
There are several ways to make glass stronger and less liable to break.
One way is to remove all the surface scratches (including the ones you cannot see) by polishing the glass. Glass can also be “glazed,” that is, a thin layer of another glass is fused onto the surface. This thin layer is made of glass that swells and shrinks less than the main piece of glass.
When the glass that has been glazed is heated or cooled quickly, the surface swells or shrinks less and so puts the surface under compression – which is when glass is strongest.
Remember that glass cannot be broken by compression forces, only by pulling (tension) ones. In glass that is compressed on the surface, the middle of the glass compensates by going under tension. But since this region of tension is in the middle of the
(Below) There is almost nowhere more vulnerable than the glass used in car headlights. The glass catches the force of the wind and rain as well as dirt and stones from the road. If the glass were soft, it might scratch easily and so scatter the light. Headlight glass has to be hard and tough.
In this picture you are looking through the headlight glass to the glass of the bulb. The other materials protected by the headlight glass are the silvered plastic reflector and the orange plastic diffuser used to turn a light sideways for road safety.
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