Autonomic healing of low-velocity impact damage in woven fiber-reinforced composites

Polymer-matrix fiber-reinforced composites have seen increasing use in applications requiring high specific strength and stiffness. These materials typically show excellent in-plane properties but are particularly susceptible to transverse impacts. Impact can significantly reduce strength and the extent of damage can grow under cyclic loading conditions. Because this type of damage often occurs below the surface, hidden from inspection, it is especially critical in structural applications. Traditionally, impact damage repair techniques have focused on increased factors of safety or use of toughened polymer matrices. In this work, a microcapsule-based self-healing epoxy is used in woven fiber-reinforced composite panels for the repair of matrix damage imparted by low-velocity impact. The initial work focused on self-healing in plain 2D woven S2 glass laminates with an epoxy matrix. The self-healing components, dicyclopentadiene (DCPD) microcapsules and wax-encapsulated first generation Grubbs' catalyst microspheres, were premixed into the liquid epoxy, and panels are fabricated using a hand layup technique and compression molding. Low-velocity impact damage was introduced to these panels by drop-weight impact testing. To visually assess the damage state, cracks on sections through the impact damage were marked with fluorescent dye penetrant. A 51% reduction in total crack length per imaged edge was observed for self-healing panels when compared to non-healing controls, indicating filling of damage with healed material. A reduction in damage resistance was also observed upon inclusion of both self-healing components. Recovery of mechanical properties after healing was investigated by conducting compression-after-impact tests. Self-healing panels showed full recovery of residual compressive strength up to a threshold impact energy nearly double that of non-healing controls. Above this threshold impact energy, residual compressive strength was partially recovered to a degree that diminished with increasing impact energy. The work on self-healing 2D woven composites indicated that catalyst microspheres significantly reduced damage resistance, while microcapsules did not have the same detrimental effect on damage resistance. Thus, potential improvements by the use of catalyst microspheres encapsulated by poly(urea-formaldehyde) (UF) were explored. To investigate the effect of encapsulating catalyst microspheres with UF on the mechanical properties, UF encapsulated wax microspheres were fabricated and investigated. Tapered double cantilever beam samples containing these UF encapsulated wax microspheres showed better mode I fracture toughness than samples containing wax microspheres. In addition, composite panels containing DCPD microcapsules and UF encapsulated microspheres exhibited higher impact damage resistance. The recovery of impact damage in 3D orthogonal woven composites was also explored. Low-velocity impact damage in 2D plain woven and 3D orthogonal woven glass/epoxy composites was compared by developing and implementing a semi-automated crack measurement program to obtain a statistical measure of damage state for both types of panels. The results indicate a modest reduction in total delamination length, total delamination cross-sectional area, and delamination separation in 3D composite panels compared to 2D woven composite panels. These findings are attributed to toughening mechanisms associated with through-thickness z-tow reinforcement. Self-healing functionality is incorporated into 3D orthogonal woven glass/epoxy composites via an aqueous impregnation suspension containing DCPD microcapsules and urea-formaldehyde encapsulated Grubbs' catalyst microspheres. The pre-impregnated fabric is infused with an epoxy matrix using vacuum bag resin infusion. A protocol based on double cantilever impact of beam samples and flexure after impact is used to characterize mechanical recovery. The lack of recovery of self-healing beam samples in four-point flexure after impact tests highlights the current challenges of incorporating a microcapsule-based DCPD-Grubbs' catalyst self-healing system into a 3D woven composite. An estimate of healing agent delivered to the crack plane indicated inadequate healing agent delivery to significantly fill the damage separations that were measured. In addition, it was demonstrated that the polymerization propagation distance of DCPD from the ruptured catalyst microspheres was insufficient to heal damage in regions where fiber tows overlapped. Finally, lap shear tests demonstrated that the adhesion of poly-DCPD to the epoxy matrix was significantly lower than the adhesion of the epoxy matrix to itself. This relatively poor adhesion limited the achievable recovery