*** 9714049 Sirkis A structurally embedded fiber optic transducer of 1 cm gage length will be developed, capable of simultaneously measuring the (1) temperature, (2)normal strain parallel to the optical fiber axis, (3) secondary principal strains in the plane perpendicular to the optical fiber axis, and (4) orientation of the secondary principal plane, inside a structural composite material. Successful development of the proposed fiber optic sensor technology will provide experimentalists with a capability that is simply unavailable today - the ability to measure key components of the strain state inside structural materials. This proposed transducer is based on cascading (1) an in-line fiber etalon (ILFE) sensor, (2) a saturated Bragg grating written into a standard low birefringent (LoBi) optical fiber, and (3) a normal (unsaturated) Bragg grating written into an elliptical clad, stress-induced high birefringent (1-liBi) optical fiber. The 11,FE sensor has very small intrinsic temperature sensitivity, and is sensitive to only the axial strain component in the host structure. As a result, the ILFE component of the proposed sensor leads to one of the five requisite independent optical measurements required to solve for the five sought after Thermo-mechanical field variables (three normal strains, temperature, and secondary principal direction). The saturated Bragg grating written in the LoBi-Fiber produces two reflected Bragg wavelengths, thereby yielding two more independent optical measurements. Finally, the Bragg grating written into the elliptical clad HiBi optical fiber takes advantage of residual birefringence coupled with this fiber's anisotropic mechanical response to yield the final requisite two independent optical measurements. The multiaxial thermo-mechanical strain sensor will be demodulated using a hybrid coherence division/wavelength division multiplexing technique coupled with scanning, dithered Fabry-Perot filter Bragg grating dem odulation and path-matched differential interferometry with pseudo heterodyne ILFE demodulation. The thermo-mechanical strains will be deduced from the resulting optical measurements through the use of composites micromechanics models based on concentric cylinders modeling techniques and nested multiple inclusions models, The sensor system will be evaluated through tests of progressively increasing complexity, starting with uniaxial tension of an unembedded sensor and ending with the combination of temperature and transverse compression applied to AS4/3501-6 graphite/epoxy laminates with embedded sensors. The potential uses of the proposed sensor assembly include: (1) material measurements to aid constitutive model development; (2) field measurements to verify micromechanics models; (3) real-time in-situ health monitoring; (4) process monitoring; and (5) residual stress measurements. This sensor can provide information to engineers and scientists that is Simply unavailable today, thus enabling the realization of more cost-effective, safe and highperformance systems.***
