Investigations On The Fragment Morphology And Fracture Mechanisms Of Al₂O₃ Ceramics Under Dynamic And Quasi-static Compression

Al₂O₃ ceramics own good sintering property,sample size stablility and high surface finish quality,and are of promising application prospect in industrial production,military construction,aerospace and other fields.They can provide satisfactory defense capability at a relatively low manufacturing cost,and thus have been widely used as ballistic missile materials of different sizes and in the protection of important parts of armored vehicles,aircraft,etc.They were received great attention in the field of national defense industry.Al₂O₃ceramics in such applications are concurrently exposed to impact loading.In order to investigate the mechanical response and damage mechanisms of Al₂O₃ ceramics under different loading conditions,and to better reveal the micro-mechanism of strain rate effect,the dynamic and quasi-static compression tests are carried out on ceramic samples along with the in-situ and ex-situ characterizations in this paper.The change in failure mode ultimately leads to the significantly enhanced strain rate sensitivity of ceramic materials at high strain rates.The main work is as follows:?1?The quasi-static and dynamic compression experiments are carried out on Al₂O₃samples with a material test system and split Hopkinson pressure bar,respectively.Bulk strength data under quasi-static loading show that the compressive strength of Al₂O₃ ceramics conforms to a Weibull distribution and increases in a power law with the strain rate.That is,with the increase of strain rate,Al₂O₃ ceramics exhibit a higher strain rate sensitivity under high strain rate loading.?2?In-situ optical imaging and digital image correlation are used to characterize,in real-time,the quasi-static and dynamic damage and fracture process of Al₂O₃ samples.There exist obvious differences in crack nucleation and propagation between quasi-static and dynamic loading.Under quasi-static loading,fewer splitting cracks nucleate in the sample;they tend to propagate along the loading direction and penetrate the sample.However,the splitting cracks increase 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.The strain fields of Al₂O₃ ceramics during quasi-static and dynamic loading are obtained via digital image correlation and imply that tensile fracture dominates the failure of ceramic samples.?3?The fragments of samples are recovered,and the 2D morphology and fractography of fragments are obtained by scanning electron microscopy?SEM?.The SEM micrographs reveal that intergranular fracture around initial flaws is more likely to occur under quasi-static loading,while transgranular fracture dominates micro cracking under dynamic loading.It is interesting to note that the surface of fragments under dynamic loading is smoother than that under quasi-static loading.?4?Three-dimensional morphology of the fragments are characterized by synchrotron radiation CT.The fragments under quasi-static loading is slender or more flat.The shape of fragments under high rate loading is closer to sphere than that under quasi-static loading.The mean of sphericity,convexity,elongation index and flatness index of fragments increase linearly with the logarithm of strain rate.The evolution of fragment morphology with strain rate is consistent with the macroscopic mechanical properties.