Strain Sensing in Thin Composite Laminates with Embedded Fiber Bragg Grating Sensors

Deployable structures are popular for space applications as they enable large, complex spacecraft structures to overcome the size constraints of launch vehicle fairings. Such structures are increasingly manufactured out of thin (< 200 μm thick) composite laminates as they have a high stiffness-to-weight ratio, the ability to withstand high curvatures during stowage, and the potential for self-deployment using stored strain energy. To ensure the reliability of these thin composite spacecraft structures in operation, it is of interest to be able to continuously monitor their internal strain state to detect potential changes or damage that may compromise their integrity. Although there are a number of potential sensors that could be used for this, fiber Bragg grating (FBG) sensors are especially well suited for this task and have a track record of successfully monitoring both composite materials and large aerospace structures. However standard size FBG sensors, which have a cladding diameter of 125 μm, are too large to be integrated into the thin composite structures of interest. To overcome this, we worked with several suppliers to develop and manufacture ultra-thin FBG sensors (< 30 μm cladding diameter) for this work that are able to be successfully embedded into thin composite laminates. The primary objective of this thesis was to investigate the suitability of ultra-thin FBG sensors for the monitoring of strain changes in thin composite spacecraft structures. To this end, the work in this thesis first investigated how to best embed ultra-thin FBG sensors to be able to measure the internal strain changes of interest while minimizing their disruptions to the surrounding laminates. Second, mechanical testing was performed to assess the effect that the embedded ultra-thin FBG sensors have on the mechanical properties of thin laminates. Third, the ability of these sensors to detect and monitor for strain changes in thin composite laminates was assessed through further mechanical testing. Finally, the effects of temperature on ultra-thin FBG sensors were studied experimentally. Through this work, which was done at the coupon level, we sought to demonstrate the ability of these ultra-thin FBG sensors to monitor for strain changes in thin composite laminates and their potential for the health monitoring of thin composite spacecraft structures. It is our hope that our findings in this thesis help lay the groundwork for the future implementation of these sensors in not only thin composite spacecraft structures, but to many other composite materials and aerospace structures as well.