In situ measurement of strains at different locations in 3-D braided composites with FBG sensors

Due to the significance of the structural health monitoring in terms of the application 3-D braided composite materials, we utilized Fiber Bragg Grating (FBG) sensors embedded in various regions in 3-D braided composite to measure the in-situ strains under the uniaxial tensile test. We also developed a finite element analysis (FEA) model to calculate the strains and compared with those measured by the FBG sensors. We found that the internal strain varies in the regions of different distances to the sample end and of various unit-cells. In the region with a distance of 60 mm to the end (R-60), the strain of the interior unit-cell is greater than that both of the surface and corner unit-cell. While in the region with 100 mm to the end (R-100), the strains of all these three unit-cells are larger than those of the R-60, though the strains of these three unit-cells are even in this region. The different in-situ strains within a 3-D braided composite sample under uniaxial tension infer to that an appropriate failure criterion should be used for strength design.