Discontinuous glass fibre reinforced polypropylene – A macro- and microstructural comparison of the mechanical response for different fibre lengths and test geometries

The basic principles of stress transfer and fracture in discontinuous glass fibre reinforced polymers are well established. However, quantitative experimental validation of these mechanisms on a local three–dimensional scale remains limited. This publication provides a three-dimensional quantitative analysis of strain and fibre microstructure, along with a qualitative defect analysis ranging from a global macrostructural to a local microstructural level, based on X-ray computed tomography data. From these quantitative evaluations, volumetric strain and apparent Poisson’s ratios were calculated. The stress–strain behaviour of short (approx. 450 µm mean fibre length in the final part) and long (approx. 950 µm) glass fibre reinforced polypropylene was compared for different main fibre orientations (0°, 45° and 90°) and test geometries (constant radius and notched). The results demonstrate that strain localisation and failure behaviour are governed primarily by test specimen geometry rather than microstructure. Locally measured strains exceed the global failure strain by up to one order of magnitude and are already established at low load levels, persisting until fracture. Microstructural effects related to fibre length and orientation mainly influence transverse strain components and apparent Poisson’s ratios in unconstrained geometries. Furthermore, long glass fibres show a more quasi–isotropic deformation behaviour, while global failure strains remain comparable.