Embedded FBGs and 3-D DIC for the stress analysis of a structural specimen subjected to bending

Advanced optical measurement techniques are used to analyze the mechanical behavior of a composite structure submitted to bending tests. This is carried out by considering a beam type specimen which presents design singularities such as important thicknesses, ply drop off zones and a reinforced zone. This study concerns the central reinforced zone of the specimen. In order to estimate the strain distribution during a series of 3 and 4-point bending tests, several optical fiber Bragg grating sensors have been embedded in various levels of the ply stack. Simultaneously, surface strain field measurements by three dimensional digital image correlations are undertaken. Both techniques show a general linear distribution of longitudinal strains through the thickness of the thick zone but values are slightly different. A numerical model is developed and a test-calculation dialogue is carried out. The complementary information gathered by the two optical techniques for in-core and surface measurements reveals the importance of considering structural and edge effects.