Axial deformation behaviour of discontinuous aligned fibre reinforced commingled yarn preforms at thermoforming conditions

The axial flow behaviour of unidirectional discontinuous aligned fibre reinforced thermoplastic composites made from commingled yarns is investigated experimentally and modelled employing a micromechanical approach. One fibre that interacts with its immediate neighbourhood is considered and its motion as a result of an externally applied tensile force is simulated. Stress relaxation according to the rheological Herschel–Bulkley model is assumed to describe the flow curve of the micromodel. Based on the measured fibre length distribution and an assumed interaction length distribution the flow curve of a multi-fibre material is modelled as the statistically weighted sum of all considered micromodels each featuring one arbitrary combination of fibre length and interaction length. The modelled flow curves agree well with respective experimentally determined flow behaviour for all considered reinforcement fibre fractions, fibre length distributions, and test temperatures. However, the suggested model contains two independent scaling factors, determination of which requires knowledge of an experimental reference curve. The necessity of scaling is attributed to imperfections in terms of fibre alignment and inhomogeneous blending of the commingled yarn.