We present results from using optical frequency domain reflectometry for high-density distributed fiber optic measurement of strain in a composite wind blade during dynamic fatigue testing. This work illustrates the potential of distributed fiber optic strain measurement for early defect detection in large-scale composite structural health monitoring.

High resolution fiber optic strain sensing is used to monitor the distributed strain throughout the manufacturing process of a 9-meter wind turbine blade with intentionally introduced defects.  Standard telecommunications-grade optical fiber was embedded in several layers of the carbon fiber spar cap and used to sense distributed strain during the VARTM process.  The amplitude and phase of the light reflected from the fibers are measured using a commercial optical frequency domain reflectometer (OFDR). Changes in the amplitude and phase of the backscattered light were measured to determine the strain along the entire length of the spar cap with 5 millimeter resolution.  Distributed strain measurements throughout the depth of the spar cap provide valuable information at intermediate points in the manufacturing process which elucidate defects both prior to and during infusion.  The embedded sensors will subsequently be used to measure strain during fatigue testing of the blade to provide a cradle-to-grave method for non-destructive testing of composite structures.

S. M. Klute et al., “Defect Detection During Manufacture of Composite Wind Turbine Blade with Embedded Fiber Optic Distributed Strain Sensor,” 43rd Proc. International SAMPE Tech. Conf, Ft. Worth, TX, 2011.