Non-linear photomechanics

Many transparent materials when deformed and viewed in polarized light exhibit a birfringent effect. The present study is an attempt to consider, within a consistent framework, many of the non-linear aspects of the phenomenon of birefringence in polymers and how they may be used in the development of a method for the stress analysis of non-linear problems.

The kinematics in a rate form for the large deformations of a medium exhibiting permanent as well as recoverable deformations is first given. The constitutive problem for rate dependent materials is then treated. The proposed differential model can exhibit a very wide range of material behavior and is in a form that satisfies the requirements for the proper formulation of constitutive relations. A mechanical-optical constitutive relation is derived that can model rate dependent behavior without itself having any explicit dependence on the rates. The use of the birefringent effect for solving non-linear time dependent problems is discussed and the inapplicability of some of the existing photoelastic procedures pointed out. The theory and techniques of analysis are checked by first performing the quasistatic and dynamic characterization of a polyester blend to about 25% strain, and then solving some stress analysis problems.