A soft-recovery plate impact experiment for studying microcracking in ceramics

Abstract

Plate impact experiments are presented for generating microcracks in ceramics under well-characterized loading conditions which also allow recovery of the specimen to examine the induced microcracking. A star-shaped aluminum flyer plate impacts a ceramic plate backed up by a steel plate. Compressive stress pulses and reflected tensile pulses propagate through these plates and eventually leave the aluminum and ceramic plates at rest while the steel plate carries away the momentum. The central region of the specimen is loaded by plane waves which are monitored by means of laser interferometry. The recorded velocity-time profiles provide an indication of the evolution of microcracking in the ceramic. Electron microscopy of the recovered specimens shows microcracks along grain boundaries in the ceramic. Their lengths can be measured easily and accurately with a transmission electron microscope. These experiments have proven to be successful in causing controlled microcracking in α-Al_2O_3 under well known stress conditions, while still allowing microstructural examination of the specimen. The information obtained from the experiments is used to evaluate a simple model for predicting the cracks resulting from the tensile interval of the stress history, showing that these tests can be used to develop a stress dependent theory of microcracking. Such a theory would contribute to a fundamental understanding of the phenomenon of microcracking in ceramics and thereby allow further progress towards toughening these highly brittle materials.