title: Low-energy dynamic indentation method for analysis of ophthalmic materials.
abstract: OBJECTIVE
  The purpose of the present work is to understand and study the mechanical behavior and critical parameters of ophthalmic polymers. The article introduces a novel low-energy indentation method that can be used to study and optimize the mechanical properties of ophthalmic materials. The technique has been developed in the frame of a larger study on the impact resistance of materials.
  
  
  METHODS
  The low-energy dynamic indentation method is based on a lumped mass-spring model solved by a 4th-order Runge-Kutta numerical method. The model can be used to predict the material response to the indentation of a hemi-spherical tip and calculate the elasticity modulus of materials, dissipated energy during impact, residual deformation after impact, indentation depth and their conservative and nonconservative components.
  
  
  RESULTS
  As an example, two ophthalmic polymers were compared: CR-39 as the universal ophthalmic standard, and Superfin as Indo Lens U.S., standard. Results showed the model is in good agreement with experimental data and allowed to obtain elasticity moduli for both materials, which showed similar values. A larger conservative component of the displacement for Superfin was also obtained and a smaller calculated residual displacement, which is indicative of less deformed material after low energy impacts.
  
  
  CONCLUSIONS
  The model can satisfactorily predict the behavior of materials under low energy indentation situations. In addition, it can be used to distinguish two apparently similar materials, such as CR-39 and Superfin, and classify them according to their response to these kind of indentations. The technique could be used as a very powerful tool to improve ophthalmic materials.
authors.affiliation: Industrias de Optica S.A., Sant Cugat del Vallès, Spain. pau.artus@indo.es
authors.forename: Pau
authors.initials: P
authors.lastname: Artús
authors.forename: Juan C
authors.initials: JC
authors.lastname: Dürsteler
authors.forename: Antonio B
authors.initials: AB
authors.lastname: Martínez
mesh_terms: D003261:Contact Lenses; D006801:Humans; D008422:Materials Testing / Q000379:methods*; D008954:Models, Biological; D010969:Plastics
chemical_list: D010969:Plastics