An Investigation of Optical Fiber Coating Performance in Embedded Sensing Applications
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AN INVESTIGATION OF OPTICAL FIBER COATING PERFORMANCE IN EMBEDDED SENSING APPLICATIONS REBECCA A. HAAKSMA, MARILYN J. CEHELNIK AND M. JONELL KERKHOFF Alcoa Laboratories, Alcoa Center, PA 15069. ABSTRACT The use of composite materials for aerospace applications has created an increased need for developing nondestructive methods for assessment of composite performance. Embedded optical fiber sensor technology provides the potential for monitoring parameters of interest during processing and testing of composite materials as well as the opportunity for tracking properties over the lifetime of composite parts in service. The successful development of this technology depends on designing optical fiber sensor systems suitable for embedding in composite structures. This paper focuses on the role played by optical fiber coatings in the design of embedded sensor systems. The performance of different optical fiber coatings under typical composite processing conditions will be discussed. Photomicrographs of test specimens containing embedded sensors will be presented which show delamination occurring at the coating/optical fiber interface in preference to the coating/epoxy resin interface. Coating performance criteria will be outlined for use in the selection of fiber optic sensors for composite applications. INTRODUCTION The growth of composite materials in aerospace markets has been slowed by limitations in the ability to assess the structural integrity of composite parts. As a result, the development of optical fiber sensor technology has recently become a major focus in the aerospace, military and defense industries.[l] Embedded sensors offer the potential for monitoring parameters of interest during the processing and the testing of composite materials as well as the opportunity for tracking properties over the lifetime of composite parts in service. Fiber optic sensors placed within a composite material during the manufacturing process would provide a means of optimizing fabrication of the material. This would be accomplished through measurement and control of relevant process parameters such as temperature and pressure. The same sensors could then be used in conjunction with conventional nondestructuive evaluation methods to test finished parts, serving as probes for internal defects, delaminations and voids. The ultimate application for fiber optic sensing in composites would be real time in-service monitoring of structural parameters allowing continuous measurement of properties over the lifetime of composite materials. A "smart" or "intelligent" composite structure would possess the capability of sensing conditions within the material and responding appropriately to changes. Embedded optical fiber sensors for composites is an emerging technology still in the very early stages of development. A great deal of research must be conducted before the sensing concepts described above can be demonstrated as viable outside the laboratory. Many theoretical and laboratory demonstrated sensors fail to perform when embedded in c
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