Measurement of axial and bending strain in pipelines using Bragg grating sensors

The objective of this work was to demonstrate the feasibility of using fiber Optic Bragg grating sensors (BGS) to measure axial and bending strain in pipes. A special test section was used to characterize the performance of the BGS and compare the results with standard electrical resistance foil strain gages. Two sets of pipe strain tests were performed. In the first tests series, an optical fiber containing four BGS was used. The BGS were positioned along the pipe test section and directly attached to the outside surface of the pipe wall. In the second tests series, a second fiber was used and four BGS were encapsulated inside a stainless steel tube which was attached to the outside of the pipe wall. All tests were successfully completed and the results demonstrate that fiber optic Bragg grating sensors can be used in a practical configuration to measure axial and bending strain in pipes. The repeatability of the strain measurements was 2 microstrain in the first tests series and 6 microstrain in the second test series. The strain resolution was less than 1 microstrain. The test results also showed that as few as three BGS in one fiber can distinguish axial strain from bending strain.