MadSci Network: Engineering

Re: How does iron in reinforced glass help make the glass stronger?

Date: Mon Jan 11 09:50:49 1999
Posted By: David Ellis, Researcher, NASA Lewis Research Center
Area of science: Engineering
ID: 915736120.Eg

Since you asked two questions, I have divided my answer into two parts.

Safety Glass

Wire mesh reinforced glass is a form of safety glass. Safety glass is used in applications where you want to contain any glass shards that may be created when something strikes the glass and breaks it. One of the most common applications is automobile windshields. If you were to look at a cross-section of your windshield you would find two layers of glass with a thin layer of transparent plastic sandwiched between them.

Safety glass was originally patented by French poet-chemist Edouard Benedictus who observed that a test tube lined with a thin film of a nitrocellulose mixture broke but did not shatter. He patented this discovery in 1910. In 1926 the Triplex Safety Glass Co. of North America licensed the rights in America and began producing safety glass. Among their first customers were Henry Ford and the Ford Motor Co. The initial price of the glass was $8.80 per square foot.

Wire reinforcement is used where there is a desire for security as well as containing the glass if it breaks. The wire mesh is embedded in the glass during manufacturing. When the glass is broken the bond between the glass and wire keeps pieces for falling out. The wire mesh also stops someone from forcing their way trough the broken glass as would be possible if there was just a thin layer of plastic.

Currently there are several new products coming onto the market that use advanced plastics in place of the wire mesh. ShatterGard for example has a plastic film 0.014" thick that can be applied to glass. These new plastics can withstand the force of a hurricane such as Andrew which decimated southern Florida as well as stopping criminals.

Tempered Glass

Glass will only fail under a tensile (pulling) load. A crack will start at a defect in the surface and propagate through the thickness of the glass. In normal glass the cracks tend to go long distances quickly resulting in large pieces. Because of this, the strength of soda-lime glass has typically been around 4,000 pounds per square inch.

Tempered glass is made by heating ordinary glass in a furnace to the point where it almost sags under its own weight and then rapidly chilling it. The outer surfaces cool quicker than the interior. Therefore compressive stresses are formed at the surface. The rapid cooling essentially locks those stresses into place. There is a corresponding tensile stress in the center of the glass, but it is far away from flaws that will start a crack. Now when the glass is subjected to a tensile load the tensile load must overcome the compressive stresses and the inherent strength of the glass before the glass breaks. Because of this the glass is much stronger, often as much as five times stronger or 20,000 pounds per square inch. The tempering also makes the cracks branch rather than continue in a long line. So when the glass breaks it will break into small pieces rather than large pieces.

A more detailed explanation of the stresses and the processing including illustrations can be found in Introduction to Ceramics, 2nd Ed., W.D. Kingery, H.K. Bowen and D.R. Uhlmann, John Wiley & Sons, (1976), pp. 830-833

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