A fiber optic conjugate stress sensor for instantaneous tangent modulus detection targeting prognostic health monitoring applications

Abstract

In this work, a surface-mounted fiber optic conjugate stress sensor (FOCSS) is developed, fabricated, and experimentally demonstrated to accurately evaluate the instantaneous tangent modulus of various metals, thus allowing measurement of the elastic-plastic stress–strain behavior of the host material. The originally proposed embedded version of the conjugate stress (CS) sensor measured the deformations experienced in two fully embedded, ellipsoidal, collocated sensors of significantly different stiffnesses, and related those sensor deformations to host modulus via Eshelby’s equivalent inclusion method. In contrast, the current experimental study focuses on a surface-mounted implementation of the CS sensor. As Eshelby’s analysis is not applicable in this case, a simplified lumped-element spring model is developed to instantaneously relate the deformations in the CS sensor to the local stress–strain state of the host. Therefore, tangent modulus of the host can be estimated from the output of the FOCSS, which can serve as an important damage precursor in structural and prognostic health monitoring for a prominent set of failure mechanisms, including mechanical overstress, fatigue, and corrosion. In this study, the FOCSS data is used in conjunction with the lumped-element spring model to measure the elastic modulus of three different materials of significantly different stiffness: aluminum, copper, and steel. Additionally, as validation of instantaneous tangent modulus tracking, the elastic-plastic stress–strain curves of copper and steel are reconstructed from the FOCSS outputs during uniaxial tensile tests (with a goodness of fit R 2 > 0.98). The results demonstrate the ability of the FOCSS to detect instantaneous modulus as materials experience plastic deformation.