The effect of microstructure on the tensile and impact behaviour of short-glass fibre-reinforced polyamide 6.6 as assessed by micro-computed tomography

Injection moulding of short-fibre-reinforced thermoplastics opens a new dimension in the field of mass production of complicated net-shaped parts with accurate dimensions and the new challenge is to produce parts with tailored properties. However, the layered structure frequently observed in these composites strongly affects their mechanical behaviour and constitutes the main difficulty in transferring the results of tests performed on standard specimens to actual components and parts. In addition, to use injection-moulded composite materials safely, their mechanical behaviour under different loading conditions must be well understood. In the present work, the effect of microstructure, in terms of fibre length and orientation, on the tensile and impact behaviour of injection-moulded short-glass fibre-reinforced polyamide 6.6 was investigated. Digital reconstruction of the three-dimensional structure of samples, differently oriented with respect to the melt flow path, was obtained by the high-spatial-resolution non-destructive technique of synchrotron radiation micro-computed tomography (micro-CT). Automatic evaluation of the fibre length distribution was developed by a global method based on the mean fibre length distribution, computed from the star length distribution (SLD). The results of uniaxial tensile tests and Izod impact experiments were successfully correlated with morphological analysis of fractured surfaces and the results of SLD. These studies revealed important changes in fibre orientation distribution when the sample orientation is changed with respect to melt flow direction, which also strongly influenced the composite mechanical response.