Range‐expansion technology and fatigue performance study on the self‐sensing steel strand with an embedded fibre Bragg grating sensor

Abstract

For the protection of a fragile optical fibre with a fibre Bragg grating (FBG) sensor, the encapsulating method of embedding the FBG sensor in a longitudinal groove set in the central wire of the steel strand is presented. According to the theoretical analysis of the strain transfer and sensing capacity between the embedded FBG sensor and substrate, the requirements for the cross‐sectional size of the groove and bond‐length of the optical fibre to the self‐sensing steel strand are discussed. A prepressure technology is proposed to overcome the problem of low ultimate tensile strain of FBGs. Experiments for range‐expansion with different prepressure values were carried out. The results showed that a prepressure of 30% Pc produced the optimum range expansion of the FBG sensor with a maximum strain measurement of 12,361 με, which was 1.3 times higher than the yield strain of the strands. The analyses of the fatigue performance and static load tests on self‐sensing steel strands were also carried out. The results indicated that the self‐sensing steel strand exhibited good performance after fatigue loading operation of 2 million cycles with a maximum stress of 0.45 σb and a stress amplitude of 250 MPa. Moreover, the decrease in the monitoring strain sensitivity of the FBG was calculated to be below 3.5%, whereas the experimental data showed good linearity and repeatability. Therefore, for effective lifetime monitoring of steel strands under fatigue loading, it is suggested that the FBG sensors need to be placed under a certain prepressure before the construction and service of self‐sensing steel strand.