The double cantilever beam (DCB) tests were executed for stitched CFRP laminates to evaluate the interlaminar fracture toughness (G_IC). The stitch parameters (stitch line spacing and pitch spacing) were changed and their effects on interlaminar fracture toughness were examined by means of DCB tests and FEM analysis. Different shape patterns in load-crack opening displacement (COD) curves were obtained at one level of equivalent stitch density for several specimens. It can be concluded that the difference in load-COD curve shape was affected by the stitch parameters. On the other hand, G_ICvalues themselves were not so seriously affected by the shape of load-COD curves. Interlaminar fracture toughness is mainly governed by the stitch density, in other words, increase in stitch density in laminates leads to the improvement in G_ICt. Simple FEM model tuned for unstitched and single stitch line DCB specimens was presented. FEM analysis results in linear relationship the between interlaminar fracture toughness of stitched CFRP laminates and the stitch density. Moreover, G_ICA of CFRP laminates with arbitrary stitched parameter can be predicted by the simple FEM model.

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The interphase between fiber and resin in a composite material has a three-dimensional region. The interphase has an important role, namely, supports the stress transmission between fiber and resin. However the influence of this interphase on the mechanical properties of the whole laminated composite has not been clarified. In this study, the more flexible interphase than the matrix resin was proposed. The fabrication method of unidirectional CFRP laminates with flexible fiber/matrix interphase has been established. The influences of the flexible interphase thickness on the flexural modulus and strength were experimentally investigated. Also the relationship between the interphase rigidity and the flexural properties were found out quantitatively. Based on these experimental results, a new designing parameter of flexible interphase was introduced for the improvement of flexural properties of unidirectional CFRP laminates.

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The interphase has an important role, that is, conducts the stress transmission between fiber and resin. Authors introduced the concept of flexible interphase and established the fabrication method of prepregs. In this study, a new numerical model considering the fiber/resin interphase was proposed, in order to design the flexible interphase. For the mechanical behaviors of composite structures were predicted by the finite element method, the consideration of the heterogeneity is an important key. Authors have established a numerical modeling method, namely, a quasi-threedimensional model which can consider the heterogeneity. Based on the quasi-three-dimensional modeling concept, a new numerical model which can independently consider the fiber/matrix interphase was proposed. The mechanical behaviors of unidirectional CFRP laminates with various flexural interphase thickness under the three-point bending load were simulated. By comparing the prediction with experimental results, the validity of the proposed model was examined. It was concluded that the model was effective for the fiber/resin interphase designing in the CFRP composite structures.

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