Chris Barry Explains the Origins of Glass BreakageSeptember 13th, 2012 by DWM Magazine
by Casey Neeley, firstname.lastname@example.org
In his seminar “Glass & Glass Breakage” for the Insulating Glass Manufacturers Alliance (IGMA) Educational Seminar, Chris Barry, director of technical services for building products with Pilkington North America posed the question, when glass breaks, does it mean that “the load too great or was the glass too weak?”
According to Barry, breakage causes include tensile stress from bending, temperature and NiS inclusion expansion, impact from a hard or soft body, crushing, acts of God and Mother Nature.
The strength of glass varies, however. Strength tests conducted on batches of glass show variations in the amount of pressure needed to break the glass. Because of the inability to determine a uniform breakage resistance for a specific batch of glass, Barry said, “We talk about a probability of breakage, not an absolute strength.”
“Glass does not break everywhere all at once. Breakage starts at an ‘origin,’” continued Barry in his presentation. Bending and thermal origins in fractures are most common.
Barry cited “incorrect ‘clamped edges’ [which] create very high bending stresses at low temperatures in insulating glass (IG)” as an example of cause for bending origins.
“Temperature doesn’t break glass: it is temperature difference that causes the stress,” Barry said. As glass heats and cools, the difference in temperature can cause fractures. An example of a window with a running crack was shown during the presentation, and Barry said he was able to deduce by the fracture origin and situation of the specific scenario that the glass cracked because of variations in temperature caused by half-lowered window blinds.
An interesting point Barry noted is that a crack in glass does not end where it appears to stop. Once a fracture has run along the glass, though it may not be visible, at least on a molecular level it has continued to crack.
The IGMA Educational Seminar was held over the last two days at the Las Vegas Hotel and Casino in Las Vegas.