Development of calculation methods for the mechanical properties of bonded joints using micromechanical models

Mechanical properties of polymeric materials with hyperelastic behavior can be reinforced by the introduction of active fillers. Polyurethane (PU) is a versatile class of these materials with different applications in engineering. The primary aim of this dissertation is to (i) study the reinforcing effect of active fillers in different concentrations on the mechanical properties of a PU system and (ii) to develop a micromechanical approach to model the hyperelastic behavior of (un)filled PU. For this purpose, mainly three models are considered: without strain amplification, with constant strain amplification and with a deformation-dependent strain amplification. The stress-strain data from the conducted uniaxial tests on filled PU nanocomposites indicate a clear reinforcement, brought about by the incorporation of carbon black at 5, 10, 15 and 20 wt%. In the case of a low filler content (1 wt%), the addition of two grades of carbon black and a fumed silica partly results in a deterioration of mechanical properties. The measured stress-strain curves for a low filler content (1 wt%) is well described by the micromechanical model without considering a strain amplification. Consideration of a constant strain amplification results in a better predictive performance of the micromechanical model for samples with higher filler contents (5-10 wt%). For PU nanocomposites containing a larger filler concentration (20 wt%) as well as for most of the investigated commercial PU adhesives, consideration of a deformation-dependent strain amplification effect results in the best prediction. This is implied by the highest R2 value resulted from the curve fitting.