Inelastic behavior of polymer composites

Sammanfattning: In the present thesis, the inelastic behavior of polymer composites is investigated. This investigation concerns three fields of study; time dependent behavior during the lifetime of the composite material, influence of micro-damage on its overall mechanical performance and development of chemical shrinkage strains during the curing process. The significance of this work is related to the nature of all composite materials. All polymer composites tend to indicate an inelastic behaviour. This behaviour can be either linear or non-linear. No matter what it is, is very important to be taken into account in the analysis, since it is related to strain rate effects, micro-damage induced to the structure of the composite and/or irreversible plastic strains. The first part of this thesis consists of the time dependent behavior of polymer composites. There are two main assumptions; irreversible strains in a damaged state are higher and that the strains can be decoupled into viscoelastic and viscoplastic response. Each assumption is investigated and a material model that includes all the above is compiled. In order to examine its validity, different material categories have been examined. Pure polymer (paper I), polymer reinforced with short fibres (Paper II), polymer reinforced with continuous fibres (Paper III). As a step further on, the time dependent behavior within a ply level was examined. A [45/-45]s laminate was used and the non-linear shear stress strain response was studied (Paper IV). In the first part of the thesis, damage was only quantified in terms of elastic modulus development after high stress application without going into detail in what is causing it. In Paper V, the effect of damage, in terms of crack density on shear elastic modulus was studied. More accurate expressions for stress calculations in the damaged lamina were suggested, by incorporating shape functions and by checking validity with the principle of minimum complementary energy. Finally, the results from the suggested model are compared with existing models and with results from finite element analysis. A small improvement is observed at all cases. Finally, in Paper VI, the effect of curing parameters and development of chemical strains during the curing process was investigated. A relation between curing time, degree of cure and mechanical performance was drawn. What is more, different procedures for measuring chemical strains were used and a testing methodology is suggested.