An update on monitoring moisture ingression with fiber optic sensors

Newly developed advanced aircraft structures are utilizing composite technology for improving stiffness, strength and weight properties. Such structures are commonly found in inaccessible regions where current NDE techniques are limited. The development of low profile, distributed, embeddable, real-time, optical fiber sensors capable of detecting the onset of composite failure in aircraft structures would eliminate a significant portion of related maintenance costs. Notable composite failures that are difficult to assess include delaminations and moisture ingression issues. Optical fiber-based sensors add the inherent advantages of being lightweight, low profile, immune to EMI, resistant to harsh environments, and highly sensitive to a variety of physical and chemical measurements. Optical fiber-based sensors can also be embedded directly into the composite part during manufacturing and co-cured. This creates a monitoring system that has little impact on the properties of the final part while providing significant benefits. Fiber optics embedded in composite honeycomb panels were fabricated and tested using ground - air - ground thermal cycles to determine moisture ingression monitoring capabilities of the sensors. Two different types of moisture sensing fiber optics were measured. One type of installed moisture sensor is based off of a Bragg grating system, while the other moisture sensor is based off of a long period grating system. Presented herein is a comparison of the two different types of fiber optic sensors that monitored the moisture ingression in honeycomb panels.