Bio-Inspired Mode-I Fracture and Fatigue Crack Healing in CFRP Composites Using Thermoplastic Healants

Laminated composites have outstanding in-plane strength but are highly susceptible to out-of-plane delamination. Hence, delamination due to static and fatigue loading is one of the most critical damage modes in composite structures that can lead to a catastrophic failure. Therefore, it is important to investigate and quantify the delamination crack growth behavior due to static and fatigue loading and explore methods to heal the damage due to delamination. In the present approach, a two-step bio-inspired healing mechanism was employed to heal the Mode I delamination crack in a CFRP composite: (i) close the cracked surfaces by thermally activating dispersed shape memory polymer (SMP) filaments, and (ii) heal the closed crack by melting the thermoplastic healant (PCL) dispersed in the thermoset polymer matrix and allowing it to flow into the cracked region and solidify. This concept is similar to the one used in the medical field to close an open wound using stitches or staples to facilitate the healing process. Heat is used as the stimulus for healing, which activates the shape memory property of SMP at 55 C and melts the healing agents (melt temperature of PCL is around 58–60 C). The source of this heating is discussed in the next paragraph.<br/>To test this concept, double cantilever beam (DCB) specimens of unidirectional carbon fiber-reinforced polymer (CFRP) composite containing dispersed SMP and PCL thermoplastic healants in the epoxy matrix were manufactured. Mode-I fracture and fatigue delamination experiments were carried out for virgin (initial case) and up to seven healing cycles. The main objective of using thermoplastic healants, i.e., polycaprolactone (PCL) and shape memory polymer (SMP), was to heal the cracks formed during static and fatigue loading and try to maintain the original fracture toughness and fatigue life of the DCB specimen. The in-situ healing was achieved by activating macro fiber composite (MFC) actuators bonded to the DCB specimen, where the high frequency vibration (~20 KHz) of the actuator provides the heat necessary to close and heal the delamination crack using thermoplastic healants. For fatigue loading, the in-situ healing was triggered using MFCs after 5000 cycles of initial loading to allow initial crack extension. From the experimental data of the virgin and healed specimens, the Paris law parameters were extracted, and the results obtained were shown to be repeatable. The substantial increase in maximum Mode-I strain energy release rate (Gmax) observed after in-situ healing is likely due to the increase in the bond stiffness of the DCB specimen material of the healed zone. More research is needed to investigate the exact mechanism for the increment of Gmax. While mode-I fracture toughness degraded slightly with the number of healing cycles, a two-fold increase in Mode-I fatigue life was observed for up to 7 healing cycles with respect to the baseline specimen, thereby vindicating our bio-inspired concept of self-healing. We envision that these findings will be helpful in extending the service life of aerospace composite structures and result in significant repair cost savings.