23 May 1997 Remotely queried wireless embedded microsensors in composites
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Embedding sensors in structural composites has been a topic of research in recent years. Embedded sensors can be used to monitor and optimize the manufacturing process, to monitor performance during use, and for structural health monitoring in high-performance applications. To date, optical fiber sensors have been the principal sensing technique for these applications. There are well-known problems with optical fiber sensors, including high manufacturing costs, fragility, the need to provide ingress and egress from the structure, and the interdependence of strain and temperature measurements. The US Naval Research Laboratory is funding a multi-disciplinary team to develop micro-machined sensors and an associated remote-querying capability to allow self- contained microsensors to be embedded in a composite structure and queried using methods that do not require physical connections. The sensors are to be left in place for the lifetime of the structure, are powered by the querying apparatus, and require no penetrations through the surface of the structure. Part of this work included studying electromagnetic propagation into graphite-epoxy (conductive) composites. A key part of this research has been the development of embeddable antennae that can operate within a conductive composite matrix with the efficiency required to both absorb power for the circuitry and to transmit and receive data. This paper describes the integrated approach taken to realize the goal of an interrogatable strain rosette that is embedded 0.25' into a graphite composite plate. Aspects of the sensors, the transponder, and the antenna are also covered.
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Donald G. Krantz, Donald G. Krantz, John H. Belk, John H. Belk, "Remotely queried wireless embedded microsensors in composites", Proc. SPIE 3044, Smart Structures and Materials 1997: Industrial and Commercial Applications of Smart Structures Technologies, (23 May 1997); doi: 10.1117/12.274666; https://doi.org/10.1117/12.274666

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