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The paper deals with the photoelastic method of solving the problems of non-linear elasticity and plasticity under the large strains with the help of the birefringent materials, i.e. a polyurethane resin and a composition on the basis of epoxy. The transparent models and photoelastic coating method are used. The optical-mechanical dependencies of the non-linear photoelasticity under the strain in the range of 50 - 150% have been obtained. The interpretation of the optical patterns has been worked out when the object is under the large strains. There exist two ways of decoding. The first one uses Muny-Rilvin and Bartenev-Khazanovich's elastic potentials. The second one is much easier and describes the behavior of polyurethane in the whole range of the given strains, so it was used in the present investigation. It's worth mentioning that the processing of experimental data in non-linear problems is more complex than in classical photoelasticity, as it's necessary to take into account the change of the specimen's shape and geometry during the deformation. In this case the principle extent of lengthening lambda (a difference between the length of a deformed specimen and its initial length) was used as a strain value. The method of nonlinear photoelasticity gives the possibility to investigate the fundamental problems of the mechanics of solids. These problems have physical and geometrical non-linear properties, i.e. large elastic and plastic strains, mechanics of destruction. Siberian scientists have already investigated some problems of this kind. But they haven't reached the theoretical solution and solved it in the first approximation. So they need an experimental test. You'll find some of them below, i.e. the problem of thermal stresses in the multi-layered panels, determination of stresses in the reinforced beams with the naturally developing cracks, determination of forms and sizes of plastic zones near the sharp-notches tips in the metallic specimens, the problems of stress-strain condition in the necking of the plane steel specimens.
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