MODE-I FATIGUE CRACK HEALING IN CFRP COMPOSITES USING THERMOPLASTIC HEALANT

Abstract

Mode-I fatigue crack healing in carbon fiber-reinforced polymer (CFRP) composites subjected to fatigue loading is investigated in this study. Laminated composites are highly susceptible to delamination, and delamination due to fatigue loading is one of the most critical damage modes in composite structures that may lead to a catastrophic failure. Hence, it is paramount to investigate and quantify the delamination crack growth behavior due to fatigue loading and explore methods to heal the delamination. Therefore, double cantilever beam (DCB) specimens of a carbon fiber-reinforced thermoset polymer (CFRP) composite containing thermoplastic healants were manufactured. Mode- I fatigue delamination experiments were carried out for virgin (initial case) and up to seven repeated healing cycles. The main objective of using thermoplastic healants, i.e., polycaprolactone (PCL) and shape memory polymer (SMP), was to close and then heal the cracks formed during fatigue loading and retain the 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 of the actuator provides the heat necessary to close the cracks using thermoplastic healants. The insitu healing was triggered using MFCs after 5000 cycles of initial loading to allow initial crack extension. The DCB specimen was then loaded up to half a million cycles to study the effect of healing on fatigue life. From the experimental data of the virgin and healed specimens, the Paris law parameters were extracted, and the results obtained were repeatable. Significant increase in maximum Mode-I strain energy release rate (G ) 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 increase of G . Mode-I fatigue life improvement of up to a factor of 2 was observed after in-situ healing for the same delamination crack growth with respect to the virgin cycle prior to healing. We envision that these findings will be helpful in extending the service life of composites and result in significant repair cost savings.