Measurement accuracy of distributed sensing fibers embedded within capillaries of solid structures

As an initial step of research 'Embedded fiber optic sensor (FOS) within additive layer manufactured part for structural health monitoring', this work investigates strain measurement accuracy of the embedded FOS. There are different factors that could influence strain measurement accuracy, and this study singles out fiber position variation within capillaries by using milled Aluminum 6082. To be more specific, three Aluminum 6082 parts are milled, each of which has a straight capillary inside, with diameters 2mm, 4mm and 6mm respectively. Optical fibers are placed into capillaries centers manually and adhesives are filled in between to bond the fiber and specimens. Subsequently, strain measurement accuracy by embedded FOSs is studied both analytically and experimentally. An analytical model is used to calculate theoretical strain of specimens with embedded FOSs. Static four-point bending tests are performed on the three Aluminum 6082 parts with embedded FOSs under 8KN. Based on the analytical model, theoretical strain distribution along the capillary center is plotted and two strain distribution extremes corresponding to fibers on upper and lower edges of capillaries respectively are plotted as well. Comparison of results from both the model and test shows that test results lie within the two strain distribution extremes, and yet discrepancy exists between the test measurement and theoretical strain distribution, which increases when capillaries holding the embedded FOS gets larger.