Field-mountable fiber optic sensors for long term strain monitoring in

Abstract

A metallized glass fiber optic is braze-bonded or solder-bonded to a metal shim to form a sensor unit that may be spot welded to a structure for monitoring structural strain. Preferably shim thermal characteristics are selected to match those of the structure being monitored, and a cover plate protects the fiber after attachment to the shim. Alternatively, a non-metallized optical fiber is attached to the metal shim using a special ceramic adhesive coat whose metal constituents thermally match the thermal expansion co-efficient of the shim. After adhesive curing, a protective cover plate is attached. An embodiment for use with an extrinsic Fabry-Perot type sensor provides a metal plate with a fiber-sized groove that guides the target and the transmit/receive fiber optic elements together to form an air gap. Another embodiment metallizes the fiber optic elements with a mask ring having a depressed region into which a cover plate is attached. Each of the embodiments may be used with a white light interferometer to provide long term strain measurements at temperatures up to about 1,000.degree. F., and may be used in radiation environments.