Fatigue of self-healing polymers: Multiscale analysis and experiments

Self-healing polymers were recently developed at the University of Illinois by incorporating a microencapsulated healing agent and chemical catalyst in a polymer matrix. Whenever damage occurs, the repair process is triggered and after sufficient healing time the inherent strength and toughness of the material is recovered. Self-healing polymers are designed to heal the microcracks that occur naturally during fatigue, thereby preventing large-scale cracks from forming. In this paper we report on the fatigue response of self-healing epoxy and show that under certain conditions fatigue cracks can be permanently arrested when the chemical kinetics of healing are much faster than the mechanical kinetics of crack growth. To understand the complex role that various kinetic processes have on the ultimate fatigue behavior of self-healing polymers we have developed a multiscale analytical model of fatigue crack growth from molecular dynamic (MD) simulations of the curing reaction of the healing agent to a cohesive volumetric finite element (CVFE) description of the self-healing polymer system. We show that good agreement with experiments can be obtained by properly calibrating the cohesive law that is used in the CVFE simulations. Our cohesive law describes the natural competition between progressive degradation of the cohesive properties associated with the fatigue process and their restoration through self-healing. Ultimately, the cohesive law and its kinetic description will be directly linked to MD level simulations of the healing process.