A modified double torsion test has been developed to measure the fracture toughness of polymeric ophthalmic lenses. Measurements on PMMA sheet using both conventional double torsion and compact tension geometries gave the same fracture toughnesses as lenses made from the PMMA sheet and tested in the modified double torsion test. Tests on plano lenses made of commercial ophthalmic lens materials showed that the measured fracture toughness was independent of lens diameter and thickness and also crack length provided the notch length was within specified bounds. Test results showed quite small scatter. Because of the reproducibility and simplicity of the test, the modified double torsion test should provide a more reliable and cheaper measure of the impact performance of the lenses than does the existing drop weight test.
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McAuliffe, P.J., Truss, R.W. A modified double-torsion method for measuring the fracture toughness of polymeric ophthalmic lenses. J Mater Sci: Mater Med 5, 138–143 (1994). https://doi.org/10.1007/BF00053333
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DOI: https://doi.org/10.1007/BF00053333