Experimental investigation of high velocity impacts on brittle materials

Experiments were conducted on soda lime glass and AS800 grade silicon nitride. Soda lime glass is often used in windows of military vehicles and aircraft where integrity in the event of shrapnel impacts is of vital concern. AS800 grade silicon nitride is considered one of the leading material candidates for the next generation of aircraft engine turbine blades because of its superior high temperature properties when compared with nickel based super-alloys. The suitability of these materials for their applications depends upon their response to point and planar dynamic impact loading. An experimental apparatus was constructed to fire one-sixteenth inch diameter hardened chrome steel ball bearings at 50 mm square soda lime glass blocks of thicknesses between 3 mm and 25.4 mm. Inelasticity due to the crushed zone effects the coefficients of restitution and the surface strains. The change in severity of cracking with velocity and specimen thickness is observed. Shock compression and pressure-shear experiments were conducted by means of a single stage gas gun capable of attaining impact velocities of 600 m/s. High velocity planar shock compression experiments on soda lime glass reveal a lack of spall strength, and a decrease of shear impedance and shear strength in the presence of a failure wave. The longitudinal impedance remains nearly constant. The spall strength of glass is 3.49 GPa and is sensitive to the presence of shear. Shock compression studies on silicon nitride using normal shock compression show that the material has a Hugoniot Elastic Limit of 12 GPa and that the spall strength decreases with increasing impact velocity due to damage below the HEL. The presence of inelastic deformation stops this trend, while the presence of shear increases the rate of spall strength drop by five times because of more severe microscopic damage. Experiments involving multiple shocks on silicon nitride show that material loading and unloading follows the shock Hugoniot closely. The HEL of the shocked material is decreased by 6%, while the residual strength remains high. This indicates that the longitudinal properties of silicon nitride including its shock impedance are not dramatically affected during the initial shock compression.