In this contribution the mechanical behaviour of polymeric matrix materials is analysed for both thermoplastics (Polypropylene) and thermosets (RTM6, RIM935). The results obtained from tensile tests carried out at different velocities indicate a nonlinear, inelastic material behaviour with strainrate dependence. For the clear identification and quantification of the nonlinearities, the experimental procedure has been extended to relaxation experiments and deformation controlled loading-unloadingprocesses with intermediate relaxations. Based on the experimental observations a small-strain viscoplastic material model is derived and material parameters are identified. The stress-strain-curves computed for different load histories are compared to the experimental results.