After having read the previous pages, you know that almost all flaws contain residual stress. However, do not ignore the fundamentals of the residual-stress-free case, lest you later be overwhelmed. While residual stress can be relieved by annealing, chemical etch, or even moisture aging, most of these techniques are impractical after precision polishing of optics; nonetheless, the residual-stress-free theory provides some usefulness in application, in addition to providing the basis for the residual stress theory. For nonoptical precision surfaces, such as in lightweight core construction of ceramics or glass, acid etch can greatly improve strength by both removal of material and rounding of flaw tips (even those 3X beneath a surface, without removing 3X the material).
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