Fibre and failure characterization in long glass fibre reinforced polypropylene by X-ray computed tomography

Abstract

Long fibre reinforced thermoplastics (LFTs) have become important in recent years, due to outstanding mechanical properties (such as high strength, high stiffness, and excellent impact behaviour). Nevertheless, there is still a lack of knowledge regarding the failure micromechanics of LFTs. This publication focuses on the qualitative and quantitative analysis of influencing factors for failure initiation in long fibre reinforced thermoplastics. Existing and new methodologies are combined for detailed analysis of the local microstructure and micromechanical behaviour. In situ investigations of the LFT via X-ray computed tomography (CT) and subsequent fibre and mechanical characterization based on the volume data were performed. A measurement workflow for the decision on the scanning area before starting a scan series is suggested. The difficulties in fibre orientation analysis of long, curved fibres are revealed by a comparison of different evaluation approaches. Fibre straightness, fibre volume fraction, fibre orientation and fibre end accumulations are investigated in detail. Local strains are determined with digital volume correlation (DVC) and related to the true stress along the measured specimen length. The investigations showed that the test specimens do not necessarily break at the position of the smallest cross-section under uniaxial loading conditions. The failure behaviour of LFTs is strongly influenced by the local microstructure and thus the combination of various microstructural factors such as fibre bundles, fibre ends, local fibre volume fraction and orientation.