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The role of edge failures in impact testing of ophthalmic lenses.

The role of edge mode failures in drop ball testing of ophthalmic lenses was investigated. The relative effects of lens shape and type of edging operation were determined by comparing the impact performance of groups of lenses. Strain gages were mounted on the edges of several lenses to determine the applied tensile stresses. The results were reviewed in the context of the flaw distribution theory. As expected, more breadage was observed for those edging processes which introduce more severe flaws and for those lens shapes for which higher edge stresses were measured. It was also observed that the search theory of impact testing is applicable for edge flaws as well as for surface flaws (a lens sustains many impacts before it fractures; the ball is searching for a weak spot on the lens).