Investigations on the fragment morphology and fracture mechanisms of Al₂O₃ ceramics under dynamic and quasi-static compression

In order to investigate the mechanical response and damage mechanisms of Al₂O₃ ceramics, quasi-static and dynamic compression experiments are carried out on Al₂O₃ samples with a material test system and split Hopkinson pressure bar, respectively. In-situ optical imaging is adopted to capture the failure process of samples; synchrotron radiation CT and scanning electron microscopy (SEM) are, respectively, used to characterize the size and shape of recovered fragments and the micro fracture modes. Bulk strength data show that the compressive strength of Al₂O₃ ceramics conforms to a Weibull distribution and increases in a power law with the strain rate. In-situ optical imaging and SEM recovery analysis reveal that there exist obvious differences in crack nucleation and propagation between quasi-static and dynamic loading. Intergranular fracture around initial flaws is more likely to occur under quasi-static loading, macroscopically leading to fewer splitting cracks which tend to propagate along the loading direction and penetrate the sample; while transgranular fracture dominates micro cracking under dynamic loading, and the splitting cracks increases in number and interact with each other to form a large number of bifurcated, secondary cracks during the propagation process, which increases the crack density of sample. This is consistent with the three-dimensional CT characterizations. The mean of sphericity, convexity, elongation index and flatness index of fragments increase linearly with the logarithm of strain rate. The change in failure mode ultimately leads to the significantly enhanced strain rate sensitivity of ceramic materials at high strain rates.