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Brittle materials

wine glass
An important characteristic of brittle materials is that they can break suddenly and without warning. Materials can be brittle due to their bonding, like covalently bonded glasses, or as a result of their processing and resultant microstructure, like cast iron.
wine glass fracture
The stress needed to fracture a brittle material depends on the size of flaws in the materials. The flaws can be internal or external inhomogeneities, like cracks or chips in the material, or regions of strain. Brittle material strength depends on the size of flaws; the larger the flaws, the lower the strength.
wine glass closeup
Strain in glass, introduced during fabrication, is typically removed by annealing. If not relieved, this residual strain can weaken the brittle glass.

The polariscope is a tool used by glassmakers to locate strained regions in glass. It consists of a uniform source of polarized light, which is used to illuminate a work piece. When the work piece is then viewed through a second polarizer, areas of strain are indicated by bright illumination. The strained areas of the glass rotate the incident light rays from the polarized orientation, causing a bright light in the strained regions.
wine glass magnify
In glass, the direction of the fracture movement from the origin can be determined by observing the rib marks, also called Wallner lines, which are found on the fracture surface. The rib marks, which appear as concentric ridges, are almost always concave toward the origin, in the direction from which the fracture initiated.