Manufacturing-caused distortion of fibrous microstructure in bidirectional pultruded E-glass/polypropylene thermoplastic composites

• Distortions of fibres in bidirectional pultruded composites measured with XCT. • The distortions are higher (up to 30°) for higher proportion of transverse fibres. • Effect of the distortions is well-estimated using micromechanics. • Voids (∼1 %) appear only in laminates with higher proportion of transverse fibres. • The fibres retain their organisation in bundles, originating from the raw tapes. The paper investigates the microstructure of bidirectional pultruded thermoplastic (E-glass reinforced polypropylene) composites, defects (fibre misorientation and voids) and their effect on the mechanical properties. The pultruded profile with transversely reinforced plies meets the EN 13,706–3 requirements to mechanical performance of profiles for structural purposes. High pressure and deformations in the die create a danger of fibres distortions, which can compromise mechanical properties, putting them below the requirements. The purpose of the present work is evaluation of these distortions and the level of downgrading of the properties. The research uses X-ray computed tomography of 0°/90° reinforced pultruded plates with different ratio between the 0° and 90° layers. The following features are analysed: orientation of the fibres; void percentage, size; resin pockets; local fibre volume fraction. The specified fibre directions are well realised in the final profile, with a deviation of 90° fibres at the profile edges up to ∼30° for the sample with 5:6 proportion of 0° and 90° layers. Misalignment of 0° fibres is within 2° – 4° range. The stiffness of the profiles was estimated with orientation averaging method. The calculations show that the deviations from the ideal bidirectional microstructure bring significant changes of the laminate stiffness only for 5/6 laminate (drop in 0° stiffness about 20 %); for 7/4 and 9/2 laminates the effect of the fibre misalignment and other imperfections on the plate stiffness is not significant. The uncertainty in the mechanical properties is recommended to be used in quality assessment and parts design.