a) Digital image correlation showing the recovery strain due to the shape memory effect around the crack location.
b) Schematic of complete flow for the combination of active toughening with active healing. Credit Garcia, Lin and Sodano; AIP.

Researchers out of the Multiscale Adaptive Sensors and Structures Lab at Arizona State University have developed an interesting but relatively simple approach to self-healing material, one that somewhat mimics the natural healing mechanism found in bones.

Other approaches to self-healing materials have been demonstrated. The ones I’ve seen with concretes and polymers have depended on a chemical reaction that is triggered by a physical force that then cause the release of a binding agent. Biological systems however, don’t just heal: They also typically act swiftly to stop the spread of damage. Bones, for example, are able to respond to, and minimize, damage through mechanisms familiar to ceramists and other materials scientists and engineers: crack deflection, crack bridging, microcracking ad viscoplastic flow. The body responds to bone damage in a way that simultaneously limits the spread of the injury and re-toughens the damaged region.

The MASS lab group, led by Henry Sodano, use a similar concept in a novel autonomous system, based on shape memory polymers containing a closed-loop fiber optic network, that is the first SHM approach I have seen that successfully pairs healing with a method to prevent damage propagation.

Generally speaking the way their system works is that when the polymer is damaged, one or more fractured fiber optics have reduced transmission. This signals that damage has occurred and immediately initiates a high-power laser diode that provides photothermal energy at the fracture sight. The author, in a paper that appears in the Journal of Applied Physics, describe the process:

“The transmission loss from the fractured fiber optic obtained via a thermal power meter not only signifies a crack in the material, but also initiates a control algorithm to adjust the amount of thermal energy applied, thereby reducing the modulus at the crack location and dissipating the fracture energy. The technique only heats only the polymer locally around the crack tip. This preserves the global properties of the material while adding significant toughness. The shape memory property of the shape memory polymer SMP also allows the crack to be closed such that other healing modalities can be used … The autonomous adaptive material also demonstrated that it could effectively reverse the strain induced by the crack while altering the crack front and effectively healing the induced damage.”

The MASS lab is also doing work in other interesting areas including Active Structural Fibers (SiC fibers with piezoceramic shell for structural health monitoring, power harvesting, vibrational control, etc.), active piezoelectric nanocomposites (to increase the electroelastic properties of the nanocomposites) and  multifunctional piezoelectric carbon fibers (piezoceramic shell on carbon fiber for sensors and actuators).