Aspects of the Failure and Postfailure of Fiber Composites in Compression

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Abstract
In this paper the analysis of the compressive failure of an AS4/PEEK composite started in [10] is continued. The compressive strength of this material is typically 60% of its tensile strength. It has been long suspected that this is related to fiber misalignment. Two types of fiber imperfections were found in the material analyzed: imperfections introduced during the manufacturing of the prepreg, and fiber waviness introduced during the lamination and final curing of the composite. Failure due to uniaxial compression takes the form of kink bands of distinct widths and inclinations. Kink bands were preserved by conducting the experiments on circular cross section cylindrical rods which were radially confined. Post-failure examination of the kink bands showed them to be at angles ranging from 11°-16° to the x2 direction of the material. Furthermore, all kink planes were found to have normals in the two orthogonal planes containing the imperfections. The problem was idealized as a two-dimensional solid with alternating layers of matrix and fibers. The models account for material and geometric nonlinearities and the two types of fiber imperfections seen in this composite. The response is characterized by a load maximum which represents the strength of the material. The strength is strongly influenced by the imperfection wavelength, amplitude and spatial distribution. The load maximum is followed by fiber bending confined to narrow inclined zones. The fibers eventually break into distinct kink bands because of excessive bending. The inclinations and widths of the highly deformed zones just before fiber breaking is expected to start were found to be similar to those of the kink bands measured in the experiments.

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