Non-Destructive Evaluation of Fiber-Reinforced Composites for In-Service Maintenance and Fabrication Control via Embedded NQR Strain Sensors

Fiber-reinforced composites are lightweight, high-performance materials that play a critical role in the design of space structures, surface launched interceptors and other ground and space based systems. Quality control in-service and during fabrication is of paramount importance to maximizing the capability of advanced composites and to increase cost effectiveness. What is needed is a non-destructive evaluation (NDE) method to easily monitor the condition of the composite during its entire lifetime, preferably with embedded sensors which must not perturb the properties of the composite. Quantum Magnetics is proposing a novel nondestructive strain sensor for this application. By measuring internal and surface strains, flaws in the composite structure can be detected at their onset. Early flaw detection will prevent catastrophic failures, costly repairs and serve as quality control during fabrication. No external stress needs to be applied to the tested structure. The proposed strain gage is based on the principle of Nuclear Quadrupole Resonance (NQR). 1-5wt% of crystalline additives is blended into the resin during fabrication of the composite structure. For testing, the composite is irradiated with radio frequencies to evoke an NQR response from the embedded crystals, which is a linear function of strain.