Experimental and Numerical Methods for Characterization of Impact Damage in Titanium-Graphite Laminates

TiGr (Titanium-Graphite) is a relatively new aerospace material composed of layers of titanium alloy and graphite-reinforced thermoplastic.  This material is in a class of materials called FMLs (Fiber-Metal-Laminates) which are characterized by high stiffness, fatigue resistance, and strength-to-weight ratios when compared to traditional composites.  One of the particularly advantageous properties of TiGr is its damage tolerance; specifically its resistance to impact damage.  While traditional graphite composites may sustain significant sub-surface impact damage in the form of delamination, fiber breakage, and matrix cracking that can be very difficult to detect, TiGr sustains damage primarily by absorbing energy in the form of plastic metal deformation and re-direction of impact energy into the plane of the metallic layers.  This is useful both in that the damage is readily detectable by visual inspection, characterized by local plastic yielding, and that the mechanism for energy dissipation allows for absorption of greater energy with less effect to residual compressive strength compared to traditional graphite fiber composites.  Finite element models have been developed to show failure mechanism dependence on impact energy, layup sequence, and titanium thickness, and have been validated with empirical data.  Initial results show that dynamic finite element simulation can accurately portray the various failure mechanisms associated with low-velocity impact.  Additionally, subsequent dynamic analysis can be used to simulate compression-after-impact (CAI) testing of the damaged TiGr specimens.  This analysis gives a quantitative measure of how damage sustained from an impact affects lasting performance, and is critical to establishing the in-service reliability of a structural material.  Together with the impact data, these results demonstrate a valuable tool for comparing and contrasting various TiGr layups, as well as understanding how TiGr’s unique properties can be exploited to make aerospace structures stronger, lighter, and more reliable.