Three-dimensional strain sensing inside a structure is not feasible with conventional strain sensing techniques such as electrical strain gauges, which are limited to surface measurements. Three-dimensional strain measurement inside a structure would find uses in a variety of new applications: enhanced understanding and detection of composite failure modes, such as delamination; sensing for adaptive structural control; intelligent vehicle highway systems; and structural health monitoring systems for civil structures. The latter application could involve remotely monitoring structural integrity during and after an earthquake, for example. A fiber-optic strain sensor array (FOSSA) in a planar, patch-like configuration was developed, and accurate measurement of the three principal strains inside a simple structure was demonstrated. The planar configuration was chosen to avoid the difficulty and structural degradation of embedding optical sensors in three planes. Two extrinsic Fabry-Perot interferometric (EFPI) sensors and one polari-metric sensor form the planar sensor array. The two EFPI sensors were placed perpendicular to each other in the sensor plane to extract the two normal strain components along the x and y axes. The polarimetric sensor embedded in the plane was used to extract the third normal strain acting on the z axis. The sensor array was embedded in an epoxy resin cube and loaded to 454 kg (1,000 1b) with a loading machine. The strains that were measured correlated well with the external strains measured with surface-bonded electrical strain gauges. The variation in measured strain between the two sensor systems was less than 4 percent for all three principal axes.
The Solid Rocket Booster (SRB) Main Flame Deflector (MFD) at Launch Complex 39A was instrumented with sensors to measure heat rates, pressures, and temperatures on the final three Space Shuttle launches. Because the SRB plume is hot and erosive, a robust Tungsten Piston Calorimeter was developed to compliment measurements made by otT-theshelf sensors. Witness materials were installed and their melting and erosion response to the Mach 2 I 4000°F I 4-second duration plume was observed. The data show that the specification used for the design of the MFD thermal protection system over-predicts heat rates by a factor of 3 and under-predicts pressures by a factor of 2. These findings will be used to baseline NASA Computational Fluid Dynamics (CFD) models and develop innovative MFD designs for the Space Launch System (SLS) before this vehicle becomes operational in 2017.
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