Dissertation - Vladimir Kolupaev
Summary
At the beginning of this study non-linear deformation models for the description of the multi-axial creep are investigated and classified. The well-known models were extended to describe the compressible creep of thermoplastics.
These phenomenological models are based upon the invariant theory and built up of the three irreducible invariants of the stress tensor. In order to analyse the convexity of the potentials the three dimensional problem was reduced to a two dimensional one. The resulting surface lines are discussed. The practicability of the models was studied on the basis of creep transverse contraction. Thereby
incorrect values of the transverse contraction number in compression occurred for some applications. In this cases, the material was assumed to show incompressible stress behaviour in compression. In tension however the compressible behaviour was taken into account. Therefore the potentials in those cases consist of two surfaces depending on the sign of the first invariant. The transition is continuously differentiable. It is demonstrated, that the proposed models are capable of describing a variety of material properties. The number of the experimentally determined parameters can be fitted to the effects expected. The presented method is practice-oriented. It allows to design the components from thermoplastics safely, even if there is some missing information about their properties.