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Mechanics and Design of Self-healing Materials to Complement SHM



Structural Health Monitoring pursues the goals of making engineered structures safer and more efficient by enabling automated detection of damage. This can provide a warning before catastrophic failure and supports a paradigm shift in design, enabling lighter weight structures by trading design margins and damage tolerance for increased inspection capabilities. Development of self-healing materials, with an innate capability to repair damage, is pursuing the same goals of preventing catastrophic failures and increasing efficiency by offering a paradigm shift from traditional damage tolerance. Self-healing materials generally fall into several categories; autonomic (selfactuating) vs. non-autonomic (requiring external actuation) and crack closing vs. bonding. Crack closing self-healing materials have the capability to restore large scale geometry after fractures occur but are generally non-autonomic. This form of selfhealing material and SHM are complementary because the materials have the ability to mend damage, preventing catastrophic failure, but requires information of the existence and location of damage. SHM provides a means to detect damage and inform and initiate actuation of self-healing. In order to realize the vision of merging these technologies there are multiple hurdles to overcome. Non-autonomic, crack closing self-healing materials generally consist of shape memory wires / fibers embedded within a structural matrix, similar to a composite. Prior work with this form of self-healing has been limited in capability, only able to restore geometry in a free state. Healing has not resulted in a closing load across the fracture and therefore structures have been unable to withstand external loading during or after healing, nor has ultrasonic inspection of healed structures been informative This paper will present a design process intended to create self-healing structures suited to ultrasonic SHM by pre-straining shape memory wires before fabrication. This results in constrained recovery that produces a closing load across the fracture face in the healed state. The fracture closing load gives the healed structure load carrying capabilities was intended to support ultrasonic wave propagation & inspection. Mechanics based analysis of these structures along with their properties in a “healed” state are compared to experimental results. Finally, continuing challenges to ultrasonic inspection are discussed. Integrating Structural Health Monitoring with Self-healing capabilities will improve understanding and capabilities in each field individually while building toward their mutual goals of improving safety and increasing the efficiency of engineered structures.


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