Smart Composites : evaluation of embedded sensors in composite materials
Palmer, Nathan Reed
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As an emerging form of renewable energy, horizontal wind turbines have experienced advancements in improving efficiency and reliability. These advances have pushed the limits of current technology used in wind turbines. Smart blades have been proposed as a method of addressing these limitations. Sensor integration within blade construction is the first step in development of smart blades. Thus, several low cost sensors were chosen, 1 axis strain gages, polyvinylidene fluoride films (PVDF), and single mode fiber optics either coated in acrylate or polyimide. To ensure successful bonding between sensor and composite two surface treatment techniques were developed. The first, dipping of the sensor into a bath of 20% by weight solution of nitric acid and the second was submersion of the sensor in the nitric acid for ten seconds prior to removal. These treatments were compared against sensors not surface treated prior to embedding. These sensors were embedded within samples created of fiberglass and epoxy or vinyl ester resin. Two different material tests were conducted. Tensile testing allowed for evaluation of sensor sensitivity, sensor failure point, material tensile modulus, and material tensile strength. Mode I fracture toughness evaluation, indicated the level of successful bonding which occurred during resin curing. Field Emission Scanning Electron Microscopy (FESEM) was conducted to further confirm the level of bonding between resin and sensor, post fracture. Results for embedded strain gages showed an adverse effect for vinyl ester samples. Epoxy samples fared better, thus concluding manufacturing success for epoxy samples, submersion being preferred, and alternative methods needed for vinyl ester samples. PVDF films had good qualitative FESEM images combined with increasing trends. It was concluded that integration for both resin groups with sensors submerged in nitric acid was successful. Fiber optics coated in acrylate also showed good bonding under FESEM imaging as well as testing. It was thus concluded that submersion was the preferred treatment. Lastly, fiber optics coated in polyimide embedded in vinyl ester composites showed significant drawbacks and it was concluded that alternative methods need exploration. Those embedded in epoxy were successfully integrated and submersion in nitric acid showed the most potential.