Tapered [0°] laminates of S2/CE9000 and S2/SP250 glass/epoxies, and two different specimen types of IM6/1827I, a graphite/epoxy with a toughened interleaf, were tested. Specimens were subjected to cyclic tension in a hydraulic load frame. The specimens usually showed some initial stable delaminations in the tapered region, but these did not affect the stiffness of the specimens, and loading was continued until the specimens either delaminated unstably, or reached 106 to 2 × 107 cycles with no unstable delamination. The final unstable delamination originated at the junction of the thin and tapered regions, and extended into both the tapered and thin regions.
A finite element (FE) model was developed for the tapered laminate, both for the laminate with no initial delamination, and for the tapered laminate with the initial stable delaminations observed in the tests. The analysis showed that for both cases the most likely place for an opening (Mode I) delamination to originate is at the junction of the tapered and thin regions. For each material type, the models were used to calculate the strain energy release rate, G, associated with delaminations originating at that junction and growing either into the thin region between the belt and core plies, or into the tapered region, between the belt and dropped plies. For delamination growth in either direction, calculated values of G reached a peak at a delamination length equal to few ply thicknesses, and then decreased. The highest values of G were calculated for the laminate with an initial stable delamination in the tapered region and an opening mode delamination growing from the junction into the tapered region.
For the materials tested, cyclic GImax values from double cantilever beam (DCB) tests were used with the maximum G values calculated from the FE analysis for delamination growth in the tapered and thin regions, to predict the onset of unstable delamination at the junction as a function of fatigue cycles. The predictions were compared to experimental values of maximum cyclic load as a function of cycles to unstable delamination from fatigue tests in tapered laminates. The predictions, assuming delamination initiated in the tapered region, agreed reasonably well with the test data, although the correlation was slightly conservative for all except the S2/SP250 material. The measured and calculated results, assuming delamination initiated in the thin region, showed good agreement, although the predictions were slightly unconservative for the S2/SP250 laminates