<a href=showthread.php?s=&postid=6313725#post6313725 target=_blank>Originally posted</a> by goby1
I have some ideas, but nothing conclusive. If there was a preexisting defect in the glass surface, such as small inclusions/voids from the manufacturing process, these defects would serve to raise the local stress in the glass. The urchin/ scraping might be enough to chip the glass like that, as it passed over the preexisting defect. Otherwise, the bow in the tank could do it. What leads me to suspect a manufacturing defect is that the chips are all aligned in the same way. You could have introduced the defects by scratching the glass with the scraper to begin with.
To clarify, brittle ceramic materials, like this glass, fail mechanically in a catastrophic manner. Cracks have no ability to blunt their tips like they do in metals, so once they get to a critical size, they propagate at the speed of sound in the material. Cracks form where the stress overcomes a critical stress of the material, which in glass happens at defects (there's always defects). So fracture in ceramics is statistical... what is the chance that there is a defect in the highest stress region? In fact, thicker ceramics in tensile structural situations will often fail under less load than thinner specimens. This is because with more volume, there are more defects. If you tested many ceramic samples, you'd find a large spread in the loads under which they failed. With metals, there is a much smaller window in which they always fail. Metals can plastically deform (yield) to lower stesses at crack tips. Defect free, a sheet of glass is much stronger than acrylic. Glass and other ceramics are terrible engineering materials for tensile situations, as defects are very unpredictable.
To this end, if this low iron glass is softer than other types, it is less likely to chip.
Again, are there any differences in the distribution of the chips across the panel? We should really be calling these things chips, not pits. Assuming the panel is not tempered, then the inside is under compression and the outside under tension. Tempering introduces a compressive stress on the outside thickness of the panel, so some deflection is required to reach a tensile situation, and in essence the panel is stronger. Chips like that on the outside would have me much more worried, as tensile stresses tend to widen/open cracks, where compressive stresses tend to close them.
It's really important to know if any new chips are forming, and if they are, if there is any correlation with scraping the algae off. If you get any new information, let us know...
G1
