Thermoplastic Self-healing of Mode I and Mode II Quasi-static and Fatigue Crack Growth in CFRP Composite Using PCL/SMP Dual Phase Healant

Interlaminar fracture is one of the most notable limiting factors for fiber reinforced composite in the high-performance structural application, which makes interlaminar crack growth study an active research topic. Several methodologies have been developed over the years to improve the interlaminar fracture toughness, delamination resistance and crack growth stability such as toughening with nanomaterials (e.g., nanographene, CNT), modification of the fiber by different functionalization techniques to improve the fiber matrix bonding, and Z-pinning. However, none of these methods have the ability to improve the desired properties in case of repeated fracture. The studied self-healing methodology employs the biomimetic concept of closing a flaw before healing. In this study, a blend of PCL (polycaprolactone) -SMP (shape memory polymer) mixture was used in the DGEBA epoxy system for stabilizing the Mode I quasi-static and fatigue crack propagation and healing of Mode II interlaminar fracture. The quasi-static experimental results showed that for each double cantilever beam (DCB) specimen the fracture process in the virgin cycle is more unstable compared to subsequent healing cycles which can be attributed to the ductile nature of PCL-SMP fibrils bridging the crack, whereas DGEBA epoxy exhibited brittle fracture in the virgin cycle. The crack growth rate (da/dt) and the slope of crack driving force with respect to crack length (dGI/da) also supported the crack growth stability in healing cycles. The biphasic PCL and SMP blend also exhibited extraordinary healing performances (recovered ~146.2 % Mode I fracture toughness and ~73.8 % Mode II fracture toughness). Mode I fatigue crack growth resistance was recovered in subsequent healing cycles. The comparative slope (m) of linear fits through log(da/dN) vs. log(Gmax), for all cycles showed the fatigue crack growth rate is slowed by the presence of healants in the composite in subsequent healing cycles.