A Novel Self-Healing Agent for Autonomous Healing in FRPs

Abstract

The use of composite materials in advanced structural applications has been increasing exponentially over the last few decades. Despite their high specific strength and stiffness their limited ability to undergo plastic deformation results in a low tolerance to damage. For this reason low energy impact can lead to matrix cracking, internal delaminations and fibre fracture through the thickness of CFRP laminates. New approaches to mitigate damage growth can be explored by mimicking nature and integrating self-healing functionality within laminates. Novel self-healing materials offer an innovative approach to the challenge of extending composites life-time. The concept of self-healing whereby internal damage can be sensed and repaired autonomously may challenge the current zero damage growth philosophy. The need for an engineered healing agent is critical to effect a successful repair vis-a-vis the function of blood components in the healing of wounds within the human body. In terms of structural application, a suitable self-healing agent is required to interact with the propagating damage ensuring that any internal crack is promptly repaired and/or stress fields readjusted. A low viscosity healing agent is critical to the infiltration of damage, and a high reactivity is required under service conditions in order to rapidly mitigate stress concentration at the crack tip via the adhesive action between the cracked surfaces. The whole self-healing system must have a lifetime which is comparable to that of the host structure. In this study, different bespoke self-healing agents are formulated and compared for their suitability to an aerospace application. Among the different systems considered only those able to remain in liquid form down to typical high altitude temperature are investigated. An additional criterion is that any resulting formulation must have a gel time of less than eight hours. Finally, their suitability to facilitate strength recovery in CFRP is assessed by DCB (Double Cantilever Beam) testing where the strain energy release rates of adhesively bonded and pristine laminated composites are compared.