The purpose of this study is to develop hybrid FRP sheets as structural composites or reinforcements with the specified requirements synthetically in stiffness, strength and ductility for strengthening and retrofitting infrastructures by consisting of different types of continuous fiber sheets. Encountering general infrastructures such as different concrete structures, three types of fiber sheets with superior tensile strength, stiffness and ductility such as carbon fiber sheets of high strength and modulus types, glass or polyethylene fiber sheets with high ductility are considered in the hybridization in order to realize high performances surpassing steel reinforcements in stiffness and structural concrete in equivalent ductility. The tensile behavior of hybridized composites is investigated through tensile tests on different compositions. The experimental results are compared with calculated ones based on the rule of mixtures, and the difference is discussed. Finally, some optimum hybridizations are suggested.

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This study focused on mechanical properties under through-the-thickness tension loads for stitched CFRP laminates. Tension tests in this direction for stitched CFRP laminates were carried out using the test specimens with a single stitch thread. The used Kevlar thread thickness was 1, 000 denier and modified lock stitch by an industrial sewing machine was employed for the present laminates. The test specimen was glued to aluminum T angles and loaded by a screw driven test machine in the displacement control mode. Load-displacement curves were obtained for all the cases and slice pictures along the stitch thread direction were taken by micro focused X-ray CT system for some specimens at several displacement levels. It was confirmed that interlaminar crack propagation through-out the test section occurred first. In the stitched laminate cases, the upper stitch thread broke secondarily at inter-locked point. Finally, the upper thread pulled out from CFRP laminates with some friction force. It was also found that the energies absorbed by the break lead of the stitch thread (W₂) and during pull-out (W₃) contributed mainly to the total energy consumption in the through-the-thickness tension tests of stitched CFRP laminates. FEM analysis were conducted where the present stitched CFRP tension test data were introduced into element properties. It was confirmed that numerical load vs. crack opening displacement curves and relationships between Mode I energy release rate and volume fraction of stitch threads were in good agreement with experimental data by DCB test, when both effects of W₂ and W₃ are taken into account. Fictitious analysis without the effect of thread pull-out provided lower estimation of the Mode I energy release rate.

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