A thermally activated healing mechanism is proposed and experimentally validated to mitigate crack propagation damage in metallic materials. The protected structure is coated with a thin metallic film of a low-melting-temperature healing agent. To heal or mitigate crack damage, the structure is heated to the melting temperature of the healing agent, allowing it to flow into the crack opening. Once in the crack mouth, the healing agent has two benefits: (1) by adhering to the crack surfaces, the healing agent bridges the crack, reducing the amount of load at the crack tip; and (2) any voluminous substance in the crack mouth causes crack closure (premature crack-face contact during cyclic loading) that also reduces the crack-tip loading.

The technology is a coating that could slow crack propagation in metal aircraft and spacecraft structures. In practice, a structure is coated with a low-temperature healing agent. When a crack is detected, heat melts the healing agent and it flows into the crack. As the temperature is reduced, the healing agent hardens and heals the crack by reducing the crack-tip stress intensity factor through closure and bridging.

Currently, the technology is only applicable for cracks that have reached the surface. It requires an external heat source to heat the coated sheet to 250 to 300 °F(≈120 to 150 °C) and must be processed in a vacuum. The coating has been prototyped on a titanium alloy sheet with an indium-tin eutectic alloy coating. Development continues with the ultimate goal of developing the technology into an in situ healing mechanism that can work automatically with structural health monitoring detectors.

The goal of the technology is to heal cracks in metal on aircraft and spacecraft. Ultimately, the goal for the technology is to incorporate smart coatings that will enable truly self-healing materials, which would provide enormous safety and maintenance benefits. Falling short of the ultimate goal of true self-healing properties, the technology would still have value as an in-shop repair tool.

This work was done by Stephen W. Smith, John A. Newman, Robert S. Piascik, and Edward H. Glaessgen of Langley Research Center. LAR-17681-1

NASA Tech Briefs Magazine

This article first appeared in the June, 2014 issue of NASA Tech Briefs Magazine.

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