Design, manufacturing and testing of 3D printed variable-stiffness laminates for improved open-hole tensile behaviour

Using 3D printing, continuous fibre can be steered in high curvature paths to create variable-stiffness (VS) laminates for applications such as notched laminates. In the present study, VS 3D printed continuous carbon fibre-reinforced thermoplastic laminates were designed, manufactured, and tested, to improve open-hole tensile behaviour. The design was carried out taking into account the current limitations of fused filament fabrication. A curvilinear function was used to describe the fibre trajectory of the VS laminates, which enabled the parametrisation of the design-oriented meso-scale model based on the finite element method. Conventional stress-based models for the estimation of damage initiation and ultimate failure strengths were adapted to the behaviour of the 3D printed material. These models successfully reproduced the fibre-dominated behaviour of the printed laminate. An as-manufactured model was also developed to analyse the effect of defects caused by manufacturing constraints. VS laminates presented higher strength than quasi-isotropic reference laminates for holes larger than null 1 mm, increasing to more than 30% for holes of null 6 mm or larger. In addition, this increase in strength combined with the slight decrease in stiffness significantly increased the energy absorbed up to failure (up to 200%).