Experimental/numerical characterization of the dynamic fracture behavior of ceramic materials

An impact apparatus was developed in which the impactor strikes the end of a rectangular ceramic bar specimen with near perfect squareness at velocities of 5.8, 8.0, and 10. m/s. The impactor was identical to the specimen except that it was half the length of the specimen. The impact produced a sudden tension stress after the passage of the compressive wave at the middle of the specimen where a sharp 3 mm precrack had previously been introduced.; Dynamic fracture experiments were conducted on four ceramic materials; 99.5% dense {dollar}rm Alsb2Osb3{dollar}, hot pressed 99.9% dense {dollar}rm Alsb2Osb3{dollar}, 15% vol. SiC{dollar}sb{lcub}rm w{rcub}{dollar}/{dollar}rm Alsb2Osb3{dollar}, and 29% vol. SiC{dollar}sb{lcub}rm w{rcub}{dollar}/{dollar}rm Alsb2Osb3{dollar}. The stress wave was measured with a strain gage and the transient crack opening displacement (COD) was measured by the laser interferometric displacement gage (LIDG) technique. The crack length history, a(t), and the dynamic stress intensity factor, K{dollar}sb{lcub}rm ID{rcub}{dollar}, were calculated from the measured stress and the COD using the numerically determined calibration constants, {dollar}Omegasb{lcub}rm c{rcub}{dollar} and {dollar}Psisb{lcub}rm c{rcub}{dollar}. The calibration parameters, {dollar}Omega{dollar} and {dollar}Psi{dollar}, were derived from the theoretical equations governing the dynamic crack propagation in an infinite solid and were calibrated numerically for the finite specimen geometries used in this project by dynamic finite element analysis and were found to be constants for the given loading conditions and geometries.; The dynamic fracture initiation toughness, K{dollar}sb{lcub}rm Id{rcub}{dollar}, was consistent with published data for similar materials. The dynamic propagation stress intensity factor, K{dollar}sb{lcub}rm ID{rcub}{dollar}, and the crack velocity increased with increasing impact velocity. K{dollar}sb{lcub}rm ID{rcub}{dollar} initiated at the expected value of K{dollar}sb{lcub}rm Id{rcub}{dollar} and rose to as high as 4 times K{dollar}sb{lcub}rm Id{rcub}{dollar}. Crack branching occurred in all four materials impacted at 10 m/s and the hot pressed {dollar}rm Alsb2Osb3{dollar} impacted at 8.0 m/s. Crack branching occurred at roughly 0.4 times the shear wave speed and the dynamic crack branching stress intensity factor, K{dollar}sb{lcub}rm Ib{rcub}{dollar}, was 2 to 2.5 times K{dollar}sb{lcub}rm Id{rcub}{dollar}. The hot pressed {dollar}rm Alsb2Osb3{dollar} and the two composites had essentially the same crack velocities and K{dollar}sb{lcub}rm ID{rcub}{dollar}.