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Performance of optical fiber sensors embedded in polymer matrix composites for 15 years

The objective of this paper is to describe the performance of optical fiber sensors that have been embedded within polymer matrix composites (PMCs) for more than 15 years. This paper was included in a session concerning the history of sensors for smart materials and structures, and was intended as an overview of early work in this area. Our group at Virginia Tech has been involved in the use of embedded fiber sensors since 1978 when R. Claus, then at the NASA Langley Research Center, helped embed sensors in PMCs to monitor cure and post-cure strain and temperature. Our oldest surviving cross-ply laminate composite specimen with embedded fiber sensors dates from 1982, and was fabricated on campus using a hot platen press. We have recently physically examined this specimen to study possible degradation of the material in the vicinity of the embedded fiber elements, and interrogated the embedded sensors using intensity, modal, interferometric and time-of-flight measurement systems. The basic conclusions of this work thus far are 1) the sensor fibers are still functional, 2) the sensor leads have not been sheared off the specimen after 15 years of use, 3) the composite specimen has not delaminated or otherwise showed signs of degradation, and 4) problems concerning the motion of sensor elements within curing systems, the interconnect problem, and cross-sensitivities that were difficulties in the early 1980s remain key issues today.