Fiber Optic Conjugate-Stress Sensor for Local Stiffness Changes under Quasistatic Loads

Abstract

A sensor capable of classifying damage due to fatigue without knowledge of the external excitation profile can be useful in many instances of structural health monitoring, like in aviation and offshore structures where external loads are random in nature. In this work, a fiber optic conjugate stress sensor (FOCSS) architecture able to detect local stiffness changes in host materials has been fabricated by pairing a fiber Bragg grating (FBG) sensor and an extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor. Conjugate stress sensing relies on having two strain sensors of significantly different stiffnesses to create a transfer function from sensor strains to host strain as well as host stress (and hence to host material stiffness and stress-strain hysteresis relationship) independent of the external load. In this work, the FBG performs as the stiff sensor. The optical fiber EFPI sensor is fabricated within a 3D printed polymer sleeve and performs as the compliant sensor. To determine this FOCSS architecture's sensitivity to changes in tangent host stiffness, quasi-static monotonic uniaxial tensile tests of steel were performed past the elastic limit. Finite element modeling is conducted to obtain insights into this FOCSS architecture's performance. We would like to acknowledge AlphaSTAR Corporation, Drexel University, Army Research Laboratory, and IBM for their support.