Biomimicry of plantae vascules in the development of self-healing composite structures

This paper presents the first investigation into the concept of creating a Plantae inspired vascular network within a fibre reinforced polymer composite laminate which provides an ongoing self-healing functionality but does not incur a mass penalty. Through the application of a 'lost-wax' technique, parallel hollow vascules, similar to 'ray cells' in ring porous hardwoods, were successfully introduced within a carbon fibre reinforced epoxy polymer composite laminate. The influence of single and dual vascules on the surrounding composite fibre architecture (i.e. introduction of fibre waviness and resin rich pockets) was characterised experimentally using a compression-after-impact test methodology. It has been shown that the vascules interact with the impact damage but they do not promote a preferential failure path whether aligned parallel or normal to the host plies. The compressive failure strength was found to be comparable in the baseline orientation and parallel (0°) configurations for both the undamaged and damaged (10J) cases. In the case of the normal (90°) vascule the formation of resin rich pockets does not appear to have degraded the relative compressive performance of the laminate, although intuitively the out-of-plane ply variation would be expected to trigger the early onset of compression failure. The interlaminar tensile strength tests illustrated that although a knockdown in performance occurs as a result of the vascules inclusion (39% for the 90-degree vascule, 43% for the 45-degree vascule and 61% for the inclusion of the aligned vascule), the off-axis vascule (and their associated resin-rich pockets) perform better than the vascules aligned to their host ply. This observation may explain the difference in performance observed in the compression tests where the aligned vascules not damage by the impact event, but in close proximity to the damage site, help alter the stress state ahead of the crack thus initiating earlier failure. In conclusion, the research undertaken here illustrates the positive and negative influences the vascules have on the global compressive failure mode as a minimum-mass self-healing solution. Following this new understanding, better bioinspired composite materials can be designed which harness the inherent damage tolerant capabilities to 'direct' a propagating crack into a predetermined healing feature.