Many fatigue damage detection system for CFRP composite structures has been developed recently, but they were not successful. In this paper a new fatigue damage detection method by measuring Poisson's ratio change is described. The phenomena that Poisson's ratio change is more sensitive than stiffness change was discovered by a simple FEM analysis. Then more precise FEM analysis has been carried out and confirmed the phenomena. And, Poisson's ratio change on 0/90 and quasi-isotropic CFRP laminates were calculated. From the static strength test results, damages such as transverse cracks could not be detected by Poisson's ratio change, because of the non-linearity of the longitudinal stiffness of CFRP laminates. However, in case of fatigue tests, it was observed that a liner relation exists between the transverse crack density and Poisson's ratio change, for example, 0.5 crack/mm transverse crack density led to 3-5% Poisson's ratio change. This system would be useful not only for damage detection of coupon tests, but also for a health monitoring system of actual aircraft structures in the future.

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A new analytical method for calculation of transverse lamina cracking in multidirectionally laminated composites has been proposed in the present paper. Both energy balance criterion and stress criterion have been applied simultaneously for evaluation of initiation and extension of transverse lamina cracks with special consideration of residual stress. A micro model of [ θ/90 ] s laminate with a transverse crack in 90 deg. layer has been numerically analyzed and the results have been applied to evaluate the stress distribution and the stiffness reduction of a multidirectional laminate. Mixed mode fracture criterion based on Mode I and II energy release rates and Hashin's stress criterion were used to predict the initiation and extension of transverse lamina cracks in the multi layers. The analytical results were compared with experimental results of the cross-plied and quasi-isotropic laminates. Effects of ply thickness and stacking sequence on behavior of transverse lamina cracking were well explained by the present method. The present method has been expected to be a useful analytical tool for damage tolerance design of laminated composite structures.

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Many models concernig the nonlinear stress-strain (S-S) relationship of FRP have been proposed. To the authors' knowledge, no interactions between normal and shear stress were assumed in most of them. However, in the previous our study shows that the interaction between normal and shear stesses exists. In this study we propose a new model considering the interactions between normal and shear stresses for the nonlinear S-S relationship of fiber composites using fabrics. This model consists of springs and friction elements which are parallelly combined for each stress axis. The friction elements represent the interaction between normal and shear stresses. Both surfaces on yield and failure criteria are assumed ellipsoidal to estimate the S-S curves and stresses as failure of composites at multi-axial stess state. Stress field is also assumed that the stress state expanded as a concentric ellpsoid from yield to failure stresses. The shape of ellipsoid was determined by the experimental results in both uniaxial tension and pure tosion loading tests. Moreover, the S-S relationship under low cyclic loading was also represented by this spring-friction model. And the calculated results by using this model well agreed with the experimental results under biaxial proportional and low cyclic loading.

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