A mechanical model to interpret distributed fiber optic strain measurement at displacement discontinuities

Distributed fiber optic (strain) sensing, which provides the unique advantage of sensing damage (e.g. cracking) at locations that are not known a priori, has been increasingly used in civil engineering. Quantitative crack measurement requires the translation of a discontinuous displacement field at the crack to a continuous strain deformation in the fiber. The main purpose of this article is to develop a mechanical model to explain the fiber deformation in the presence of a displacement discontinuity. The proposed mechanical model is validated with experimental results from cable calibration tests and concrete cracking tests. The model is extended to simulate the effects of multiple closely spaced cracks on fiber optic strain measurement, and this model is used to create an algorithm to automatically distinguish multiple cracks in distributed fiber optic (strain) sensing strain distributions. Using the model and two shape parameters, kurtosis and standard variation, the effects of cable properties (i.e. shear stiffness between cable and fiber, cable radius, elastic modulus, and interface cohesion) on the shape of fiber optic strain distributions across cracks are also quantified. The results provide an indication of beneficial cable properties for various measurement objectives.