Analysis of the Strain Transfer Mechanism between a Truly Distributed Optical Fiber Sensor and the Surrounding Medium

Distributed Optical Fiber Sensors (DOFS) are a key tool for monitoring health of large structures. An optoelectronic device, paired with an optical fiber in a cable, provides strain profiles over several kilometers with few micro-strains accuracy. However, due to shear deformation of the cable’s protective coating, strain measurements provided by DOFS may differ from actual strains in the embedding medium. A methodology was thus developed to determine the relationship between measured/actual strains, called Mechanical Transfer Function (MTF) of the cable. In a first step, tensile and pull-out tests were carried out to assess the mechanical properties of the cable components (optical fiber, multiple coating layers) and the interface laws. These parameters were then introduced into a finite element model to determine the cable’s MTF and its domain of validity. Finally, an inverse analysis based on this MTF made it possible to evaluate the strain profile within the host structure from the DOFS measurements, and to quantify the crack openings as well.