Ultrasonic measurements with embedded Fiber Bragg Gratings in polyurethane resins

In geophysics, Distributed Acoustic Sensing (DAS) optical fibers are increasingly being exploited to probe the subsurface using seismic waves. However, the integration of the deformation undergone by the fiber over a gauge length (typically a few meters) and a limited sampling frequency prohibit the use of this technology for ultrasonic applications. On the other hand, fiber Bragg gratings (FBGs) offer a sufficiently small sensitive element (500 μm to several cm) to measure the dynamic deformation field at ultrasonic frequencies. Recently, it has been shown that FBGs bonded to a surface or embedded in a structure can be used to measure the dynamic deformation field caused by the propagation of an ultrasonic wave. However, when inserting an optical fiber into a material, the effects of the structure on the fiber mechanical and optical properties need to be taken into account. For FBGs embedded in a reacting material, such as a polymer or concrete, the polymerization stage can affect the FBG spectrum leading to a variation of the Bragg wavelength, a decrease of the grating amplitude and, in the worst case, a modification of the FBG signature (lobe broadening, introduction of harmonics, etc.) due to the material’s shrinkage inducing strain on the fiber. In order to prepare for large-scale on-site measurements with embedded FBGs, it is necessary to carry out preliminary laboratory tests to address the instrumentation issues associated with inserting optical fibers into a curing material. For this purpose, several FBGs with different characteristics (insensitive to microbending, acrylate/polymide coating, multicore, etc.) have been integrated into different polyurethane resins to compare their response to insertion, heat transfer and the measurement of dynamic deformations at ultrasonic frequencies. In this contribution, the reduce scale models produced in polyurethane resins, and the results related to fiber insertion and ultrasound measurements, are presented in detail.