Mode I delamination propagation and R-ratio effects in woven composite DCB specimens for a multi-directional layup

In this paper, a method is proposed for evaluating the delamination growth rate da/dN for any given R-ratio (load or displacement ratio) using data from a relatively small number of tests. To this end, double cantilever beam (DCB) specimens fabricated from a plain woven prepreg (G0814/913) arranged in a multi-directional (MD) layup were tested. First, quasi-static tests were carried out to obtain the resistance GIR-curve. Then, constant amplitude fatigue tests were preformed to determine the delamination growth rate da/dN. In both tests, displacement control was imposed. Four different cyclic displacement ratios Rd, namely 0.1, 0.33, 0.5 and 0.75, were used for the fatigue tests. When the delamination growth rate da/dN, calculated from the experimental data, was plotted with respect to GImax and ΔGIeff=(GImax-GImin)2, different behavior of the da/dN curves was observed. In addition, modification of the Hartman-Schijve representation was made. When the da/dN data was plotted with respect to the new modified representation which was denoted here as ΔK‾I, all of the data obtained for different displacement ratios collapsed into one master curve. From this master curve, da/dN values may be calculated for any given R-ratio.