Used in many applications ranging from microelectronics to aerospace structures, polymer-based adhesives are often considered as the weak link of bonded structures, including those subjected to cyclic loading. In this collaborative experimental and analytical effort, researchers from the University of Illinois at Urbana-Champaign are designing and demonstrating a new class of heterogeneous, multifunctional, epoxy-based adhesives that possess the unique ability to heal autonomically (i.e., without any external intervention) under fatigue loading, thereby substantially expanding the expected lifetime of adhesive joints. This new class of materials is inspired by living systems, in which damage (e.g., a cut or bruise) triggers an autonomic healing response. In biological systems, chemical signals released at the site of injury initiate a systemic response that transports repair agents to the site of damage and promotes healing. To achieve self-healing capability in the adhesive system studied in this project, sub-micron-size "nanocapsules" containing a monomer healing agent are embedded in the epoxy adhesive layer, together with a living catalyst dispersed in the epoxy matrix. As fatigue-induced microcracks appear and propagate in the adhesive layer, the nanocapsules rupture and release the healing agent. The monomer then mixes with the catalyst phase initiating polymerization and rebonding the crack faces. The research project focuses on (i) materials development with processing and characterization of nanocapsules and identification of viable self-healing chemistries for epoxy adhesives, and (ii) a multi-level numerical and experimental investigation of the fatigue response of a self-healing adhesive joint.
The successful completion of this project will lead to the development of a radically new type of adhesive system that present a much enhanced resistance to fatigue failure. Initial observations obtained on "bulk" self-healing composites indicate a five- to ten-fold increase in the fatigue life of epoxy-based components. Furthermore, by integrating multi-level experimental techniques with a new multiscale cohesive finite element framework, this project is expected to yield an integrated design tool for multifunctional adhesive joints with application well beyond the proposed self-healing epoxy-based adhesive system. This research project takes place at the Autonomic Materials Laboratories as part of an interdisciplinary research group at the Beckman Institute for Advanced Science and Technology that involves students and faculty from Aerospace Engineering, Engineering Mechanics, Chemistry and Materials Science.
