We investigate strain transmission ratio of a distributed optical fiber sensor bonded to a structure’s surface. Optical fiber sensors are widely used for structural health monitoring, because of its light weight and superior corrosion resistance. Also it is immune to electromagnetic interference. However, it is susceptible to damage during installation of the sensor or health monitoring. For the protection of the optical fiber sensors, they are usually packaged or coated with high toughness materials such as polymer or metals, before they are installed on a structure’s surface with adhesive materials. The packaging/coating layers and the adhesive layers reduce the strain transmission from the host structure to the optical fiber sensor, which results in distortion of the strain distribution in the optical fiber.
In this study, we first analytically obtain strain transmission ratio of the distributed optical fiber sensor with a coating layer, which is bonded on a base structure with adhesives. We find that the strain transmission ratio depends on the wavelength of the strain distribution. As the wavelength of the strain distribution decreases the strain transmission ratio decreases due to local shear deformations of the adhesive and coating layers. Also, as the thickness of the coating and adhesive layers increases, the strain transmission ratio decreases. This is verified with finite element method using a commercial FE software ABAQUS. In the FE analysis we also consider plastic deformation of the coating and adhesive layers and find that local plastic deformation of the coating and adhesive layers locally changes the strain transmission ratios. Based on the strain transmission ratios we propose a way to precisely estimate the strain distribution of the base structures from the signals of the optical fiber sensor using Fourier and inverse Fourier transform.
For the practical applications of the distributed optical fiber sensors in structural health monitoring, safety of the sensor is a primary concern. However, increasing the thickness of the protective layers (coating or adhesive layers) for the protection reduces the strain transmission ratio and distort the strain distribution of the optical fiber. Therefore, it is hard to get accuracy and safety of the installed distributed optical fiber sensor at the same time. In this research, we propose a way to solve this issue; one install the optical fiber sensor with highly protective coating and adhesive layers and then estimate the accurate strain distribution of the base structure with the proposed strain estimation method using (inverse) Fourier transform and strain transmission ratio.