Compressive damage and fracture modeling of ceramic subjected to high-velocity impact

Ceramics are often used as protective measures against impact damage in applications such as spacecraft and military vehicles. The processes of impact and penetration of a high velocity projectile against ceramic are not well understood, and they have not been adequately modeled. Since ceramics are much weaker in tension than in compression, most failure work for ceramics deals with tensile crack growth. At high impact velocities, however, compressive failure occurs directly under the impact point and well ahead of the actual penetration path because of the extreme loading and the high sound speed of the ceramic.; The objectives of this research program were to develop, implement, and demonstrate a failure model for aluminum oxide ceramic that considers compressive failure under impact loading. The necessary data for the development of such a model were not available, so a comprehensive test program for a single type of aluminum oxide (Coors AD-85) was conducted. Four types of experiments provided a basis for the development of the ceramic failure model. Quasi-static compressive tests and dynamic compressive tests on a split Hopkinson pressure bar were used to evaluate strain rate effects. Plate impact recovery tests were used to evaluate damage and fragment size versus loading magnitude and duration of load. Rod-on-rod impact tests were used to provide supplemental data on high strain rate strength, and a macroscopic view of the propagation speed of failure. A fifth type of experiment, metallic rod impacts against confined ceramic targets with preperforated front metal cover plates, was used to demonstrate an application of the model.; The experimental program did not provide sufficient recovered compressively damaged ceramic data in a form suitable for a microstatistical failure model. Therefore, a phenomenological damage-based failure model for compressive fracture of impacted aluminum oxide was developed with emphasis placed on predicting fragment sizes of failed ceramic. Test data did support a fragment size correlation with loading rate. A model was developed incorporating these data and implemented in a finite difference wave propagation continuum mechanics code. Comparisons of the results of this model to penetration tests were positive.