Impact testing of ophthalmic lenses: stress distribution and the "search" theory

Elementary concepts in fracture mechanics, stress analysis, and impact are discussed as they pertain to fracture of lenses in drop ball testing. Quasi-static estimates for stress distributions during impact are obtained. These profiles show a sharp peak in the tensile stress under the impact point. Identical groups of lenses were impact tested in two different modes: 1) 25 impacts at a single spot on the lens, 2) 1 impact at each of 25 different spots. Much more breakage was observed in the latter group. Both the experimental and analytical considerations lend support to the "search" theory of drop ball testing--fracture occurs when the ball strikes a weak spot (where there exists a sufficiently severe flaw) on the lens. Lenses which pass an impact test may well have dangerous flaws away from the impact point.