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Why and how is broken glass and irregular object?
Question Date: 2018-09-20
Answer 1:

Regular and irregular objects do not appear to be standard terms. However, a Platonic solid (3D) could be what is meant by a regular object. Such solids have faces which are identical and regular polygons.

Regular polygons means that the lengths of all sides are identical and the angles between sides are identical. There are only 5 Platonic solids. If we take those to be "regular" objects, then irregular objects are any which do not meet those criteria. The question then becomes why cracks in glass do not propagate along directions which produce those faces. By definition, atoms in glasses are not arranged on planes but are randomly positioned. One consequence of this is that their properties do not depend on direction. We can therefore expect that the material itself does not have a preferred direction for crack propagation. Random propagation directions are unlikely to produce the regular shapes described above.

The direction in which a crack grows also depends on the stresses present in the material. Many factors contribute to this, from residual stresses introduced during forming, chemical treatments to change the structure in some regions, differences in temperature across the piece of glass, etc. These effects are typically not spread uniformly throughout the entire piece of glass, so the direction of propagation will change with location. That being said, these principles can be used to cause glass to break into (relatively) regular pieces.

Tempered glass uses special processing to produce a stress state that causes glass to break into small pieces that can be roughly cuboid. [Although not part of the original question, manipulation of the stress state to change the crack direction is being investigated due to implications for certain types of small-scale fabrication.]

Answer 2:

Glass has a conchoidal fracture, which means that it does not break along planes the way that many minerals do. I don't know if that's what you mean by "irregular object".

Answer 3:

Broken glass might have flat parallel top and bottom sides if it comes from something flat like a window, but it's other sides don't form a square or a rectangle or a triangle or any regular object.

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