We develop an analytical model for the relationship between the strain measured by a fiber Bragg grating sensor and the actual structural strain. The values of the average strain transfer rates calculated from the analytical model agree well with available experiment data. Based on the analytical model, the critical adherence length of an optical fiber sensor can be calculated and is determined by a strain lag parameter, which contains both the effects of the geometry and the relative stiffness of the structural components. The analysis shows that the critical adherence length of a fiber sensing segment is the minimum length with which the fiber must be tightly bonded to a structure for adequate sensing. The strain transfer rate of an optical fiber sensor embedded in a multilayered structure is developed in a similar way, and the factors that influence the efficiency of optical fiber sensor strain transferring are discussed. It is concluded that the strain sensed by a fiber Bragg grating must be magnified by a factor (strain transfer rate) to be equal to the actual structural strain. This is of interest for the application of fiber Bragg grating sensors.

Threaded connections are the most common pipe fittings used in oil and gas transportation systems. Due to external vibrations, cyclic loads, and pollution, the fitting parts may start getting loose, which could result in pipeline leaks and other environmental disasters. It is of great significance to develop a reliable technique that could provide real-time monitoring of the looseness of pipeline fittings. In this paper, a piezoceramic-based active sensing method combined with the electro-mechanical impedance (EMI) technique was developed to monitor the health condition of threaded pipe connections in real time. Two pipe segments coupled with a threaded coupling fitting were assembled in the laboratory, and a lead zirconate titanate (PZT) patch was surface bonded onto the coupling part. In the experiment, the PZT impedance signatures were measured at each simulated loosening condition. A root-mean-square deviation (RMSD) method was employed to build a looseness index from the measured impedance signatures. To verify the effectiveness of the developed EMI technique, the experimental results were compared with those computed from a numerical simulation. The good agreement from experimental and numerical results highlights that the developed piezoceramic-based EMI technique has great potential for determining early looseness, as well as for monitoring the health status of the pipeline fitting during its service life.