At the recent SAMPE Conference and Exhibition (see “Related Content,” at left) Cornerstone Research Group’s (CRG, Dayton, Ohio) Reflexive composites were touted as a composite system that, according to CRG’s research and development engineer Thomas Margraf and chief engineer Ernie Havens, can detect and heal damage in load-bearing airframe structures in less than seven minutes. The system employs a trademarked “healable” version of CRG’s Veriflex shape memory polymer (SMP) resin. When activated by heat the thermoset SMP matrix reportedly recovers from deformation damage and physically closes any delaminations or cracks. At the same time and temperatures, linear polymer chains within the polymer become mobile with heat and bridge the cracks to “heal” the matrix.
This self-repair mechanism is triggered by an integrated structural health monitoring system, SMART Layer technology from Acellent Technologies Inc. (Sunnyvale, Calif.), which consists of a very thin Kapton polyimide material with embedded piezoceramic “pills” — tiny actuators that, when high-frequency electrical signals are applied, are able to sense changes in the composite structure caused by impact or damage, based on changes to the wave propagation through the composite.
A third component, an intelligent electronic control system developed in conjunction with Franklin Innovation Inc. (Dayton, Ohio), autonomously analyzes the change data and responds immediately by activating a Kapton-encapsulated foil etched heating grid, with each heating element capable of 100°C/212°F over a 1 in² area. The ability to heat specific elements in sequence allows the system to sweep through large areas of damage, limiting power consumption and maintaining load-bearing capability and modulus of the overall structure, notes Margraf.
Coupons tested to failure were heated in an oven above the SMP activation temperature, then cooled and retested. Those fabricated with carbon/SMP demonstrated up to 90 percent of their original flexure strength and modulus after healing, depending on the resin chemistry (photos show before/after comparison). Additional tests on dented samples have shown a complete recovery to pre-existing shape, notes Margraf.
CRG has produced working prototypes for NASA Langley Research Center and, notes Havens, is pursuing other applications.