PREDICTION OF THE COMPRESSIVE STRENGTH OF THICK-SECTION ADVANCED COMPOSITE LAMINATES

An analytical study, with the twin objectives of (1) development of a simple, yet comprehensive approach for prediction of the compressive strength of thick-section advanced composite (graphite/epoxy) cross-ply laminates and (2) explanation of unacceptably low strength of the (90-degrees-N/0-degrees)K cylindrical specimens tested at David Taylor Research Center (DTRC) under hydrostatic compression, has been undertaken. A simple closed-form expression for the kink mode compressive strength has been derived, under the simplifying assumptions of small deformation, material linearity, local flatness and piecewise linear distribution of the surface-parallel displacements through the thickness. using the minimum potential energy approach. Initial fiber misalignment (maximum tangent angle), ultimate fiber shear strain and the two transverse shear moduli are found to be the key parameters, which limit the compressive strength of the aforementioned thick-section composite laminates. An analysis pertaining to the elastic plane strain inextensional deformation of the compressed composite reveals that kink band formation is primarily due to rotational instability of the fibers, and regardless of the event or sequence of events that may act as precursor(s), kink band formation, once triggered, will, in general, be the dominant (lowest energy) failure mode, especially in the presence of such defect as fiber waviness or misalignment. Critical kink band angle has been found to be bounded well away from zero (a fact consistent with experimental observations), and has been shown to be dependent on the ultimate shear strain of the fiber, maximum initial fiber misalignment tangent angle, phi-0 and transverse moduli, E(T), G(LT), and G(TT), of the composite laminate. Numerical results for graphite/epoxy and glass/epoxy laminates demonstrate the extreme sensitivity of the former to the initial fiber misalignment defects, responsible for the lowering of the compressive strength of the thick cylinders tested at DTRC and the observed scatter in the test data. These results clearly indicate the sensitivity of kink band angle, beta, for graphite/epoxy composites, to the maximum initial fiber misalignment tangent angle, phi-0, in the lower range of values (2-3-degrees or less).