Strain Profile Measurement for Structural Health Monitoring of Woven Carbon-fiber Reinforced Polymer Composite Bonded joints by Fiber Optic Sensing Using an Optical Backscatter Reflectometer

Abstract

In order to monitor crack initiation and propagation under static and fatigue loading in adhesively bonded joints, strain profile measurement such as backface strain (BFS) is a very efficient technique. In single lap (SL) joints, crack initiation and propagation in the glue line can be monitored by detecting the negative peak of the strain profile. Therefore, the accuracy of the monitoring system greatly depends on detecting the strain profile correctly and accurately. Previously, an array of electrical strain gages as well as fiber Bragg grating (FBG) sensors had been used successfully to capture the profile of BFS of a SL joint by applying sensors on the backface of an adherend, near the overlap zone. In this work, the backface technique is improved by replacing an array of strain sensors by an ordinary optical fiber (without FBG sensors) connected to an optical backscatter reflectometer. The great advantage of this system over the more conventional arrays of FBG is that the entire length of the fiber can be used for sensing strains, and hence it provides a better spatial resolution. The experimental results are compared with finite element analyses, which were further validated by two-dimensional digital image correlation measurements

Deformation and damage evolution of a full-scale adhesive joint between a steel bracket and a sandwich panel for naval application

“…Longitudinal strain measurement with DIC and FBG over the thickness of the upper part of the specimen showed a perfect agreement between both techniques; in the lower part of the specimen some values differed by less than 10%. Bernasconi et al 22 studied strain profile measurement and crack propagation in SLJ specimens of CFRP laminates adhesively bonded by structural epoxy Scotch Weld 9323 B/A, subjected to tensile and fatigue loadings. FBG and strain gauges were embedded at the back face of the adherend near the overlap region.…”

Interfacial adhesion between embedded fibre optic sensors and epoxy matrix in composites

“…tanh 𝛽𝑙 𝑒 𝛽𝑙 𝑒 (5) In this step, it is necessary to determine the value of r2. According to the theory proposed by Nairn [25], r2 corresponds to the external radius of the cylinder formed by the surface of the matrix.…”

“…Monitoring systems with fibre optic sensors are used for both damage detection and usage monitoring in a wide range of applications, such as: the study and prediction of crack propagation in adhesive joints [4,5,6,7], strain and displacement monitoring of aeronautical structures [8,9,10,11,12] and civil infrastructures [13,14,15].…”

Fatigue crack growth analysis in composite bonded joints by back face distributed strain sensing and comparison with X-ray microtomography

“…The application of the BF strain technique using OBR distributed sensing, with one single optical fibre as sensing unit, was demonstrated in [22] for a woven CFRP SL joint. BF strains were measured by OBR at different levels of static loading.…”