Mechanical characterization and self-healing in synthetic vascularized materials

Inspired by natural healing processes, a variety of synthetic self-healing materials have been developed that mechanically recover from structural damage autonomously. One strategy for creating self-healing materials is to distribute reactive fluids throughout the material volume using a microvascular network of microchannels that serve as conduits of flow and permit fluid transport throughout the material volume, in much the same way that vascular systems transport nutrients and biochemical components throughout living organisms. Artificial vascular systems have been successfully incorporated into materials with low load-bearing requirements to heal cracks in coating materials and skin-core debonding with foam crushing in composite sandwich structures. In these applications, cracks are confined to the coating material or near the skin-core interface; the vascular system is intentionally placed within regions with a low probability of failure and terminates where damage is expected to occur. This dissertation explores the interaction of cracks with a vascular system embedded within a structural material and the delivery of healing agents to these sites of internal damage. The mechanical impact of incorporating a synthetic vascular system into a load-bearing material is assessed through the measurement of the bulk material stiffness and fracture toughness, as well as the influence on crack propagation and the distribution of strain. As expected, the bulk stiffness decreases and strain is concentrated in regions surrounding the vascular features. The bulk fracture toughness of the material decreases with the addition of a high volume fraction vascular system, but individual vascular features impede crack propagation under certain conditions. Test protocols are developed to characterize the ability to repeatedly repair large damage volumes under both quasi-static and fatigue loading conditions. Damage events that occurred in the same location are healed multiple times owing to the interconnectivity of the vascular system, which allows the flow of liquid healing agents from undamaged regions of material to the sites of damage. Pressurized vascular systems improve the delivery of healing agents by allowing a larger damage volume to be serviced by a smaller vascular system, making flow less susceptible to obstruction, and providing a means of directing flow to enhance mixing of two liquid healing agents. The result of pressure-driven flow is a higher degree of mechanical recovery and sustained repeatability of healing events. In addition to addressing quasi-static fracture damage, crack propagation under cyclic fatigue is slowed or completely arrested using pressurized vascular systems to deliver rapidly curing healing agents to actively growing cracks