(357g) Failure Processes Governing High Rate Ballistic Impact Performance of Epoxy Resins Toughened with Soft Nanoparticles

We demonstrate that commercially available core-shell rubber (CSR) nanoparticle additives can improve ballistic impact performance by over 180% for epoxy resins with glass transition temperatures in a range between 60 and 110°C, without large reductions in T_g or stiffness.  The ballistic impact performance of modified epoxies covers a transition from brittle to tough behavior which is shown using SEM fractographs to be due to a change in failure mechanism.  For brittle resins, failure is dominated by initiation of Hertzian cone fracture which depends strongly on fracture toughness K_IC, while for tough resins, failure is dominated by plastic yield at the impact site and is independent of fracture toughness above a minimum K_IC value of about 1.1 MPa-m^1/2.  The insights gained from this study are valuable for design of next generation composite matrices for lightweight protective applications.