An important characteristic is strength. Most people don’t realize that glass is one of the strongest composites available in the world – even stronger than steel! The tensile stress of virgin, or untouched glass, is 1 million psi. Tensile stress is a pulling or stretching force, as opposed to compressive stress, which is a squeezing, or compacting force. Steel has a tensile stress of 60 thousand psi.
As heat is applied, steel ring expands.
The glass melts and fuses with the steel.
As the steel cools, it contracts and compresses the glass, strengthening it.
Glass almost always fails under a tensile stress and not during compression – similar to concrete. Glass is not ductile like steel, and cannot stretch like steel. Therefore, imperfections will create stress concentrations. In fact, just the touch of your fingers will reduce its tensile stress from 1 million psi to 1 thousand psi. In comparison, the compressive strength of glass is approximately 200 thousand psi. This is nearly 200 times its tensile strength.
Unfortunately, when glass fails under pressure, it is sudden and catastrophic. In fact, research has shown that glass can fracture at 5 miles per second. This is why safety is a critical factor in sight glass selection.