This paper presents the fatigue failure properties of adhesive lap joints of GFRP under cyclic tension-shear loading. The fatigue crack growth in FRP adherends as well as in the adhesive layer is focused in this study. Results are summarized as follows. 1) The fatigue failure occurs mostly in the FRP adherend. This phenomenon is independent of lap length. 2) It is found that the main crack initiates and grows in the matrix layer of the FRP adherend nearest the adhesive layer. The failure process is examined by the calculation of the energy release rate using FEM. This failure process does not depend on lap length. 3) Fatigue life of FRP lap joints having a certain lap length can be roughly estimated based on the master da/dN-G_T diagram for the joint with 25 mm lap length. The lap length little affects the fatigue crack growth. 4) In-situ microscopical observation shows there exist interlaminar delamination in FRP adherends and micro matrix cracks. The concept of an equivalent crack length a^* (=na) considered as internal micro damage ahead the crack tip enables the quantitative prediction of crack growth and fatigue life.

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In sporting goods, especially in skis, optimization has been done by the combination of dissimilar materials and the design of stiffness distribution. Flexural stiffness which is given as functions of elastic moduli (E, G), Poisson's ratio, and dimensions is very complicated. Composite structures are integrally shaped by laminating prepreg with the other members in the mold. However, the FRP has the thickness distribution because of the flow of prepreg. This distribution which is determined by the component, the structure, and the mold can not be theoretically predicted. Therefore, a simple design method only depending on the flexural elastic moduli and dimensions of the members was proposed by substituting a Poisson's ratio, a shear deformation, and a thickness distribution for a correction factor. Also, in order to test the accuracy of this method, the apparatus for measurement of the flexural stiffness was developed.

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