Investigations on the fragment morphology and fracture mechanisms of Al2O3 ceramics under dynamic and quasi-static compression
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摘要: 为探究Al2O3陶瓷的宏观力学响应与破坏机理，分别利用材料试验机和分离式霍普金森压杆对其进行准静态和动态压缩实验，同时通过原位光学成像观测试样的破坏过程，并利用同步辐射CT和扫描电镜（SEM）对回收碎片的尺寸和形状以及微观破坏模式进行表征分析。宏观强度数据表明，Al2O3陶瓷的抗压强度符合Weibull分布，且与加载应变率呈现指数增长关系。原位光学成像和SEM回收分析共同揭示了动静态加载下裂纹成核与扩展模式存在明显差异。准静态加载时材料微观上更易发生沿晶断裂，宏观表现为劈裂裂纹较少，且倾向于沿加载方向传播并贯穿整个试样；而动态加载时穿晶断裂占主导地位，劈裂裂纹明显增加并发生相互作用，因此在传播过程中容易分叉而形成大量次生裂纹，提高了试样内裂纹密度。这与碎片的CT表征结果一致，即碎片平均球形度和伸长、扁平指数等均随应变率对数线性增加。破坏模式的改变最终导致高应变率下陶瓷材料应变率敏感性显著增强。Abstract: 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.
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Key words:
- Al2O3 ceramics /
- strain rate effects /
- synchrotron radiation CT /
- fragment morphology
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图 1 分离式霍普金森压杆及原位光学成像系统示意图
Figure 1. Schematic diagram of the SHPB device implemented with an in-situ optical imaging system
图 4 Al2O3陶瓷在应变率为0.001 s−1的准静态 压缩下的原位光学成像结果
Figure 4. In-situ optical imaging of the Al2O3 ceramics under quasi-static compression at 0.001 s−1
图 5 Al2O3陶瓷在应变率为1 000 s −1的动态压缩下的原位光学成像结果
Figure 5. In-situ optical imaging of the Al2O3 ceramics under dynamic compression at 1 000 s −1
图 6 不同应变率下碎片的三维图像与等效直径分布情况
Figure 6. Volume renderings and the equivalent diameter of fragments at different strain rates
图 7 碎片形状参数（均值和标准差）随应变率的变化关系
Figure 7. Evolution of the shape parameters (mean and standard deviations) of the fragments with the strain rate
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