Impact damage resistance and compression-after-impact strength of sandwich composites with graphite-epoxy facesheets and nomex honeycomb cores

Validated progressive Failure Dynamic analysis utilizing finite element models are developed for characterizing the impact damage resistance and compression-after-impact (CAI) residual strength of sandwich composites comprised of woven-fabric graphite-epoxy face sheets and foam cores. A multi-Scale micromechanically based failure technique is employed to predict local composite failure initiation and damage progression/growth in both face sheets and foam core. Numerical estimates of impact damage development, obtained using GENOA/LS-Dyna integrated software, are compared to experimental results for flat sandwich composite specimens subjected to drop weight normal impact with spherical steel impactors. Here a combination of ultrasonic C-scan images, visual inspections, and destructive sectioning measurements are used to assess test observed impact damage and comparison with numerical predictions. Impact damage estimates obtained from dynamic impact simulations are used to establish initial conditions in material and geometric nonlinearity aimed at predicting CAI residual strength, failure on damage resistance using a GENOA/Nastran or GENOA/Simulia finite element based Progressive Failure Analysis (PFA). Numerical predictions for impact damage resistance, damage progression, and CAI strength are obtained for a flat composite plate, and sandwich composite lay-up configurations. Finite element estimates for face sheet surface strains correlate well with strain gage measurements, as well with strain measurement obtained. Moreover, numerical estimates of CAI residual strength are consistent with experimental observations.