Development of Static and Fatigue Strength Characterization System for Composite Interface using In-situ Observation

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Macroscopic fracture properties of fiber reinforced composite materials depend on the properties of mesoscopic components such as fiber, matrix, fiber/matrix interface and their spacial distribution. Then, the direct measurements of the fracture behavior of these mesoscopic components are important for the understanding of the fracture process in mesoscopic scale. In the present study, two types of model composites were used to evaluate the inter facial strength of glass fiber/epoxy composites. One is microcomposite, in which one central fiber was pulled out from six surrounding fibers was The other is couple fiber shear specimen, in which a pair of fibers was pulled away. The latter was specially designed for in-situ observation. Tests were carried out under static and fatigue loadings using low-load capacity fatigue testing machine attached to scanning electron microscope. Two and three dimensional finite element analyses were carried out to find out the mechanism and criterion of th … More e inter facial fracture.The results are summarized as follows :(1) In microcomposite tests, the fracture surface indicated the transition point of stable opening mode crack propagation to unstable shear mode crack propagation. This can be correlated to the maximum pullout load in the experiments. This condition can be expressed by the constant intensity of stress singularity without respect to the fiber diameter and the resin impregnated length.(2) In couple fiber shear tests, in-situ observation showed stable crack growth before final unstable fracture. The three-dimensional finite element analysis indicated that the controlling mode of the fracture was mode I without respect to the crack length. The transition of stable to unstable crack growth was correlated to the minimum values of the mode I stress intensity factor with the crack length. .(3) Couple fiber tests were carried out with fibers of different fiber diameters. The comparison of the results indicated that the above criterion was valid without respect to the fiber diameter.(4) The tests were also carried out under fatigue loading with automated controlling and data acquisition systems. Less