Fracture behaviour of thermosetting polymers for ophthalmic lenses

In the worldwide market we found strongly positioned two thermosetting materials for ophthalmic lenses: CR-39 and Superfin. In this work, the fracture behaviour of both materials is studied through the application of Linear Elastic Fracture Mechanics (LEFM) using two geometries: double torsion and three point bending. The fracture toughness has been determined by the double torsion technique. CR-39 showed a continuous and stable crack propagation, whereas Superfin exhibited a stick-slip behaviour whose instability decreased when the strain rate increased. The values at crack initiation agreed with the fracture toughness results of single edge notched tests in bending geometry.