This study focuses on interlayer hybridisation of flax with silk fibres and the resulting damage mechanisms controlled by the hybrid composite configuration, can lead to an improved balance between stiffness, strength and toughness/ductility. The results demonstrate that a sandwich design configuration with flax layers at the outside of the laminate exhibit the highest increase in (pseudo-)ductility compared to monolithic flax fibre composites. X-ray computed tomography (XCT) revealed that fragmentation and debonding of the flax fibre layers can be achieved by optimising the hybrid laminate configuration and the volume fraction ratio between the two fibres, explaining the increased toughness of the hybrid composites.

This study focuses on interlayer hybridisation of flax with silk fibres and the resulting damage mechanisms controlled by the hybrid composite configuration, can lead to an improved balance between stiffness, strength and toughness/ductility. The results demonstrate that a sandwich design configuration with flax layers at the outside of the laminate exhibit the highest increase in (pseudo-)ductility compared to monolithic flax fibre composites. X-ray computed tomography (XCT) revealed that fragmentation and debonding of the flax fibre layers can be achieved by optimising the hybrid laminate configuration and the volume fraction ratio between the two fibres, explaining the increased toughness of the hybrid composites.

In this study, the hybridization of flax and silk fibre reinforced composites, at ply level, were studied and compared to their monolithic counterparts. Hybrid FRP laminates were produced via the filmstacking compression moulding method using the highly ductile thermoplastic high-density polyethylene grafted with maleic-anhydride as the matrix. The fibre volume fraction ratio between the two fibres and the lay-up configuration were studied, resulting in hybrid composites with different degrees of distribution of the flax and silk plies within the laminate. The results showed that more balanced properties in terms of stiffness, strength, strain to failure, and impact energy absorption can be achieved by varying those two parameters, thus increasing the designing freedom tailored to specific engineering applications. Additionally, the findings revealed that by optimizing those parameters, multiple fractures of the flax plies or “fragmentation” can be achieved as a toughening mechanism, leading to a pseudo-ductile hybrid composite with a gradual or delayed failure development. The fibre fragmentation mechanism, apart from the increased ductility, can be potentially used as a failure detection criterion for structural health monitoring.

In this study, the tensile and bending properties of silk fibre composites using three (0, 90) woven fabrics with different architectures are investigated. The tensile results show that the silk composites can achieve high strain to failure (more than 20%) and toughness (up to 13 MJ/m3), which can be further manipulated based on the architecture of the fabrics, thus providing more tailored properties and design freedom in applications. XCT and SEM characterization are used to investigate and explain the outstanding toughness of these composites. In tension, a high density of microcracking was observed away from the failed location, which could explain the intrinsic high ductility and energy absorption of silk fibre composites by means of damage spreading throughout its volume in contrast to inherently brittle materials. In bending, significantly lower properties were observed with the more striking being the strain at failure reaching only 30% of the tensile value, thus limiting the potential of silk fibres in bending-dominated loading configurations. XCT revealed that the lower performance is due to failure on the compressive side of the composites, where a clear characteristic kink-band was observed in all composites subjected to bending, while there was no visible damage on the side under tension. This behaviour is also linked to the soft HDPE polymer matrix used, which provides little resistance to fibre buckling.