Effect of Resin on Impact Damage Tolerance of Graphite/Epoxy Laminates

An experimental investigation was conducted to evaluate the effect of the matrix resin on the impact damage tolerance of graphite-epoxy composite laminates. The materials were evaluated on the basis of the damage incurred due to local impact and on their ability to retain compression strength in the presence of impact damage. Twenty-four different resin systems supplied by seven manufacturers in prepreg tape form were evaluated. Materials were not competitively submitted for the purpose of selecting a best material, but were voluntarily provided to study what improvements in damage tolerance the various resin improvements might make. Most of the materials tested had a smaller interior damage region, as determined by C-scan inspection, and less damage on the specimen surface than a baseline control resin which is widely used and whose damage tolerance characteristics have been documented. Five of the systems demonstrated substantial improvements compared with the baseline system, including retention of compression strength in the presence of impact damage. Examination of the neat resin mechanical properties indicates the resin tensile properties influence significantly the laminate damage tolerance, and that improvements in laminate damage tolerance are not necessarily made at the expense of room-temperature mechanical properties. Preliminary results indicate that a resin volume fraction on the order of 40 percent or greater may be required to permit the plastic flow between fibers necessary for improved damage tolerance. Test techniques for evaluating damage tolerance are discussed, including a newly developed multi-span beam shear test for which test results correlate well with damage tolerance trends from impact tests.