Despite the fact that today plastic materials are widespread, knowledge about them is still insufficient. There also aren't many standards that would be dealing with properties of plastic materials or would prescribe methods for measuring its mechanical properties. In order to assess the damages of plastic materials, which are the result of mechanical loads, knowledge of the mechanical properties of plastic materials is unavoidable. Mechanical loads, which are considered in this thesis, are the consequence of the thermal expansion of the material. We regard any permanent deformation, that remains when the material is unloaded, as damage to the plastic material. In the case of instability the possibility of buckling is considered as a potential possibility for damage to the material. Permanent deformations can be the result of plastic as well as viscoelastic deformation of the material. Elastic deformations are completely reversible and do not cause permanent changes in the shape or structure of the plastic material. Phenomena that can lead to permanent deformations and are discussed in this thesis are the instability or buckling of the plates, the stresses in two-dimensional and three-dimensional plates. Since we are dealing with the heating of plastic material as a way of causing mechanical stresses, it is necessary to take into account the temperature dependence of the mechanical properties of the plastic material. Analytical and numerical methods are used to determine the solution. Of the numerical methods, the method of integration of systems of differential equations and the finite element method are used. As a method of heating, a simplified principle of heating of the outer surface and the interior absorption of light in plastic material is used.
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