ASPECTS OF THE FAILURE AND POSTFAILURE OF FIBER COMPOSITES IN COMPRESSION

In this paper the analysis of the compressive failure of an AS4/PEEK c omposite started in [10] is continued. The compressive strength of thi s material is typically 60% of its tensile strength. It has been long suspected that this is related to fiber misalignment. Two types of fib er imperfections were found in the material analyzed: imperfections in troduced during the manufacturing of the prepreg, and fiber waviness i ntroduced during the lamination and final curing of the composite. Fai lure due to uniaxial compression takes the form of kink bands of disti nct widths and inclinations. Kink bands were preserved by conducting t he experiments on circular cross section cylindrical rods which were r adially confined. Post-failure examination of the kink bands showed th em to be at angles ranging from 11 degrees-16 degrees to the x(2) dire ction of the material. Furthermore, all kink planes were found to have normals in the two orthogonal planes containing the imperfections. Th e problem was idealized as a two-dimensional solid with alternating la yers of matrix and fibers. The models account for material and geometr ic nonlinearities and the two types of fiber imperfections seen in thi s composite. The response is characterized by a load maximum which rep resents the strength of the material. The strength is strongly influen ced by the imperfection wavelength, amplitude and spatial distribution . The load maximum is followed by fiber bending confined to narrow inc lined zones. The fibers eventually break into distinct kink bands beca use of excessive bending. The inclinations and widths of the highly de formed zones just before fiber breaking is expected to start were foun d to be similar to those of the kink bands measured in the experiments .