investigated. STRAIN DISTRIBUTION IN 0° PLY OF A CFRP CROSS-PLY LAMINATE The FBG sensor has been embedded in 0° ply on the border of 90° ply in a CFRP cross-ply laminates for the detection of transverse cracks that run through the thickness and width of the 90° ply, as shown in Figure 1(a) [1]. The occurrence of the transverse cracks can be detected from the change in the form of the reflection spectrum from the FBG sensor owing to the non-uniform strain distribution. Hence, the longitudinal strain distribution in 0° ply was calculated theoretically. As shown in Figure 1, McCartney’s theory [2] was applied for each region between two neighboring cracks. In this analysis, generalized plane strain conditions are assumed, and calculated longitudinal strains in 0° ply are the values averaged through the thickness of the 0° ply. r z t(ri) t(ri-1) si si +dsi dz r z t(ri-1) si-1 si-1 +dsi-1 dz Figure 2: Free-body diagrams of a multiple cylinder model: (a) the ith layer; (b) the inner cylinder assembly of the ith layer. (a) (b) STRAIN TRANSFER TO THE CORE OF AN OPTICAL FIBER Duck et. al. proposed a derivation that could predict the axial strain field of an embedded optical fiber sensor from a given arbitrary varying axial strain field in the surrounding material [3]. From the calculation results, they indicated that the in-fiber strain could not be assumed to be equal to the strain field present in the surrounding material. In this research, we modified the method to apply to multiple cylinder models and calculate the strain transfer more accurately. The optical fiber coated with resin is assumed to be axisymmetric and divided into thin concentric cylindrical layers. The layers are numbered from the innermost core, so that the ith cylinder occupies the region ri-1 £ r £ ri for i = 1…N, where ri denotes the outer radius of the ith cylinder and r0 = 0. Figure 2(a) shows a free-body diagram of the ith layer. Since the radial and azimuthal stresses are assumed to be negligible, equilibrium equation is expressed as follows: 0 x z L L s s Transverse Cracks Figure 1: Schematic diagram for the calculation of the non-uniform strain distribution caused by transverse cracks: (a) positions of transverse cracks obtained from an experiment; (b) a region between two neighboring cracks where McCartney’s theory is applied. (a) (b) Positionz (mm) 0o Ply 0o Ply 90o Ply Transverse Cracks FBGSensor 0 10
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