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The effect of microstructural variations upon the dynamic compressive and tensile strengths of aluminas

Seven aluminas of varying grain size, glass content and porosity were tested in plate-impact experiments over a range of impact stresses. Longitudinal stresses were measured using manganin gauges bonded between 8 mm thick tiles and polymethylmethacrylate (PMMA) backing blocks. Wave profiles were recorded by a 1 GS s–1 storage oscilloscope. Measurements were made of the Hugoniot elastic limit (HEL), rise time of the ‘plastic’ wave and of spall strength for each material. The spall strength was measured at impact velocities corresponding to less than 0.4 HEL stress for each material. The HEL was observed to decrease with increasing grain size while the ‘plastic’ wave rise time increased with increasing grain size. The spall strength increases with a decrease in porosity and an increase in alumina content. Variation in the spall strength was observed to occur over a range of stresses below half the elastic limit of the material.