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

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.

Finite Element Simulation of Delamination in Carbon Fiber/Epoxy Laminate Using Cohesive Zone Model: Effect of Meshing Variation

Delamination or interlaminar fracture often occurs in composite laminate due to several factors such as high interlaminar stress, stress concentration, impact stress as well as imperfections in manufacturing processes. In this study, finite element (FE) simulation of mode I delamination in double cantilever beam (DCB) specimen of carbon fiber/epoxy laminate HTA/6376C is investigated using cohesive zone model (CZM). 3D geometry of DCB specimen is developed in ANSYS Mechanical software and 8-node interface elements with bi-linear formulation are employed to connect the upper and lower parts of DCB. Effect of variation of number of elements on the laminate critical force is particularly examined. The mesh variation includes coarse, fine, and finest mesh. Simulation results show that the finest mesh needs to be employed to produce an accurate assessment of laminate critical force, which is compared with the one obtained from exact solution. This study hence addresses suitable number of elements as a reference to be used for 3D simulation of delamination progress in the composite laminate, which is less explored in existing studies of delamination of composites so far.