Evaluation of FBG strain sensor reliability through analytical, numerical and experimental results

The fiber Bragg grating (FBG) sensor is one of the most suitable sensors for structural health monitoring (SHM) of aircraft structures. In principle, the FBG, with adequate sampling and signal processing techniques, is usually more accurate than classical electrical resistive strain gauge. However, since the most common installation method is surface bonding, some significative differences between the strain in the host structure and the one experienced by the fiber may be observed. The aim of this work is the evaluation of the fiber Bragg grating optical fiber sensor reliability for strain detection in non-uniform strain conditions. A FBG sensor is surface bonded on a rectangular specimen with a centered hole under tensile loading. A multi device method for the strain detection is created with the use of optical fiber, strain gauge and DIC technologies at the same time. In addition, based on microscopic observations, a 3D finite element model of the bonded optical fiber is created to simulate the behavior of the test specimen and to have an insight into the strain transfer between the host material and the optical fiber core. Lastly, being the test specimen a well-known case in literature, the strain is also computed with closed-form expressions. All the experimental, numerical, and analytical results are then compared. Taking into account the possible sources of error, the comparison shows coherence between the different results. The optical fiber strain sensor reliability is verified through the multiple device method which represents the starting point for future research activities.