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

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.