Progressive flexural failure analysis of laminated composites with knitted fabric reinforcement

Abstract

Prediction of the flexural strength of a laminated composite is important for engineering application yet difficult in nature. The purpose of this paper is to analyse the progressive failure process of laminated composites subjected to a bending load, with application to those reinforced by knitted fabrics. The analysis is based on the classical lamination theory and a bridging micromechanics model [Compos. Part A 32 (2) (2001) 143–172]. Only the mechanical properties of constituent fiber and matrix materials under the bending load condition and the laminate geometric parameters are required. All these data can be measured independently before composite fabrication. As the internal stresses in the fiber and matrix have been explicitly determined using the bridging model, a lamina in the laminate is considered to have failed whenever any of its constituents fails, according to a stress failure criterion. Then, a stiffness discount is applied to the failed lamina, and a predicted progressive failure process results. Unlike in an in-plane load situation where the ultimate tensile strength occurs when the laminate’s last ply fails, the ultimate bending strength of the laminate is attained generally before its last-ply failure. As one does not know a priori which ply failure corresponds to the ultimate failure, the use of only the stress failure criterion is no longer sufficient for the determination of the laminate ultimate strength. An additional critical deflection or curvature condition must be adopted either. The critical deflection or curvature is the laminate deflection or curvature corresponding to which the ultimate bending strength is measured. Experiments have been carried out to obtain the bending stiffness and strength of laminated beams reinforced with six layers of plain weft knitted fabrics under four-point bending. The laminate lay-ups used are: [0/0/0/0/0/0], [90/90/90/90/90/90], [0/90/90/90/90/0], [90/0/0/0/0/90], and [0/45/−45/−45/45/0], where 0 denotes that the fabric wale direction is along the beam axial direction. Using a matrix combustion and fabric peeling out method, it has been found that only some of the layers in the laminates failed after the bending test. For these laminates and based on the stress failure criterion, the predicted load–deflection curves till the fourth-ply failure agree reasonably with the experimental data. By incorporating with the critical deflection condition, the laminate ultimate bending strengths thus obtained correlate favourably with the measured results.