Future Work

The work on self-healing 3D woven glass/epoxy composites highlights the limitations of using a system based on DCPD and first generation Grubbs’ catalyst. Two major drawbacks of this

system are the low adhesion to the epoxy matrix and the requirement that first generation Grubbs’ catalyst be protected in wax microspheres. However, Wilson et al. [108–110] have demonstrated a self-healing system based on second generation Grubbs’ catalyst that shows better adhesion to epoxy and does not require that the catalyst be wax-encapsulated. Second generation Grubbs’ catalyst has been shown to react with amines to form stable complexes that are still reactive with DCPD, thus eliminating the need to wax-encapsulate it [110]. In addition, 2nd generation Grubbs’ has been shown to be reactive with dimethyl norbornene ester, which can act as a adhesion promoter [108]. This system should be explored because it provides an immediate option for improving the performance of self-healing 3D woven composites with a DCPD-Grubbs’ catalyst healing system.

More generally, the work on woven composites points to some important remaining challenges and future directions for self-healing woven composites. In any healing system, the two key factors in determining successful mechanical recovery are the healing agent to damage volume ratio and the adhesive strength of the healed material to the matrix. In fact, these two factors can be interrelated, as an increased adhesion can offset lack of complete damage filling. In terms of damage volume, the completed work demonstrated that the damage from low-velocity impact is often too large in volume and separation to heal with a microcapsule-based self-healing system. Increased healing agent delivery can be achieved by increased microcapsule loading, but this can result in a reduction of damage resistance. In addition, there are fundamental and processing limits on the amount of microcapsules one can embed in a composite system of sufficient fiber content. Thus, it would be desirable to incorporate a self-healing system that has a much higher healing agent delivery volume, while still maintaining a low volume fraction. Such a system is possible with a microvascular approach. While microcapsule-based self-healing systems present a simple method of incorporation into woven composites, microvascular systems typically do not. However, the method of incorporating microchannels via the use of a sacrificial filament or fiber is being developed and should be investigated for use in self-healing of composites.

While the amount of delivered healing agent can be greatly increased by use of a microvascular networks, increasing the adhesion of the healed material to the epoxy matrix is essential to maximizing performance. While epoxy-containing microcapsules have already been shown [111–113], work is still ongoing to encapsulate amine curing agents. When amine encapsulation and the subsequent epoxy-amine healing system are successfully demonstrated, composite panels containing this system should be testing for recovery of impact damage. The increased adhesion may result in greater recovery, despite a low degree of damage filling. A microvascular approach with an

epoxy-amine healing system has already been shown to heal internal damage [114], so the epoxy systems used can be translated to a microvascular healing system in a fiber-reinforced composite. Thus, composite panels with microvascular systems fabricated using a sacrificial fiber approach can be filled with epoxy-amine healing chemistries. These types of panels should be investigated for recovery of impact damage. Because this type of system can theoretically deliver a large amount of healing agent that bonds well with the matrix, the primary technical challenge will be to ensure adequate mixing of the two-component healing system in the crack plane.

While low-velocity impact usually results in relatively large damage volumes, there are other types of damage that may be more amenable to healing with a microcapsule-based system. Transverse cracks in cross-ply laminates is such an example, and work is ongoing to investigate healing in these systems with an epoxy-based healing system. Another potential application is the healing of fatigue damage in fiber-reinforced composites. The growth of existing damage or the initiation of new damage under cyclic loading can significantly reduce the lifetime of a composite part and often requires conservative design of structures. For 3D composite systems in particular, Mouritz and coworkers [115, 116] showed that increasing z-direction reinforcement in stitched, 3D woven, and z-pinned composites led to decreasing fatigue life in tension. Previous work has successfully demonstrated retardation and arrest of fatigue cracks in epoxy [8–10] using the microcapsule-based DCPD-Grubbs’ catalyst system. There is potential to extend these experiments to fiber-reinforced composite materials.

Finally, UF encapsulation of catalyst microspheres was explored for improvements in fracture toughness and damage resistance, but another potential benefit is increased chemical protection. This material should be investigated for its barrier properties to amine curing agents and other deactivating matrix components. Other shell wall materials (e.g. poly(melamine-formaldehyde) or polyurethane) could also be explored for improvements to mechanical performance and chemical protection.

APPENDIX A

CODE FOR CRACK ANALYSIS

This appendix contains the MATLAB® code used to implement a semi-automated scheme for finding crack routes and separations.