Polymer fiber optic sensors for strain monitoring in Solid Rocket Motors´ propellant

Summary form only given. Polymer Optical Fibers embedded in the propellant of Solid Rocket Motors are demonstrated for monitoring strains higher than 10%. A new architecture incorporating a closed-loop fiber is proposed and its theoretical behaviour is experimentally verified. The propulsion system of Solid Rocket Motors (SRM) is considered as the most critical system in a guided missile as it powers the missile with predetermined characteristics. A key issue for the safety and reliability of those SRM missiles and consequently for the effective and economic management of missiles´ inventory is the efficient Structural Health Monitoring of the energetic propellant grain. This work proposes the use of photonic sensors [1,2] for SRMs´ continuous SHM as the preferable solution in such environments, due to their electromagnetic interference immunity and inherent electrical and explosion safe characteristics. Polymer Optical Fibers embedded with excellent bonding characteristics to the propellant material of SRM are demonstrated as an efficient sensing platform. The use of POFs for SHM applications is considered as more robust in terms of installation and operational reliability compared to silica fibers where their fragility leads to a high rejection ratio (around 60%) of unsuccessful installations due to fiber breakage during handling and this ratio is prohibitive when installing fibers in highly expensive units such as SRMs. PMMA POFs used both as jacketed cables or unjacketed bare fibers, were successfully embedded into solid propellant materials and afterwards mechanically loaded in special tensile tests equipment. The experiments were realised with a simple optical setup of a high power LED of 1 mW at 650nm and a photodetector operating at linear regime. By using special primers an excellent bonding between fibers and propellant was achieved allowing thus efficient strain transfer from the energetic material and avoiding relative slipping [3]. Two different conf- gurations were employed, one of axial longitudinal incorporation of the POF along the specimen and the other by incorporation of a fiber loop segment in the longitudinally embedded fiber. The applied strain in the specimen is transferred partially axially to the straight fiber sections while it also allows the geometrical deformation of the embedded loop from the circular to elliptical shape. This deformation results in change of local bend radii along the looped fiber inducing thus additional bend losses. The intrinsic elasticity of the POFs and also the macro-bend originated losses under strain provide monitoring capability for strain and stress much higher than 10%. Additionally resulted plastic deformation of POFs in excessively high strains, is proposed to serve as a data/event logger important for recording materials´ stress history.