Meso-Mechanical Analysis of Fatigue Crack Propagation in CFRP under Mixed-Mode Loading

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1. Mixed-mode (I+II) interlaminar fatigue tests were conducted by the mixed mode bending (MMB) method with unidirectional graphite/epoxy laminates. The effects of the mixed-mode ratio and the stress ratio on the crack propagation behavior were studied from a viewpoint of fracture mechanics.2. Devices for the in-situ SEM examination of the fatigue crack propagation under mode I,II and mixed-mode (I+II) loadings were developed.3. Fractographic observations were conducted to examine micromechanisms of the mixed-mode propagation of interlaminar fatigue cracks in graphite/epoxy laminates. A mode for the fatigue crack propagation was proposed on the basis of microprocesses of initiation, growth and coalescence of the matrix microcracks.4. A numerical technique for the meso-mechanical analysis of interlaminar cracks in FRP laminates was developed. The boundary element method is used for the meso-mechanical elastic analysis. In this analysis, the near-tip region of an interlaminar crack is modeled by the inhomogeneous FRP model composed of isotropic matrix and orthotropic fibers which is surrounded by the homogenized orthotropic FRP phase.5. By using the above meso-mechanical model of FRP laminates, the analysis of the elastic stress distribution near the tip of an interlaminar crack subjected to mode I and II loadings was conducted. For long cracks, the energy release rate obtained for the meso-mechanical FRP model is equal to that obtained for the homogeneous model. The distribution of the matrix-phase stress ahead of the crack tip for the meso-mechanical model is greatly different from that for the homogeneous model. Therefore, the meso-mechanical analysis is essential for investigation on microfracture of matrix and fiber/matrix interfaces of FRP composites.