Testing the Strain-Rate-Dependence of Alumina Ceramics

By Bhavna KavetiReviewed by Susha Cheriyedath, M.Sc.Jun 20 2022

Aluminum oxide (alumina) ceramics with favorable properties are extensively used in engineering applications. In an article recently published in the journal Ceramics International, the researchers performed strain-rate-dependent compression and indirect tension experiments on CeramTec Alotec 98% alumina ceramic. The results revealed that the peak strength was 10 times higher than tension and the compression had greater rate sensitivity than tension. Moreover, the primary crack speed in tension was two times higher than the compression.

Study: An experimental study on the strain-rate-dependent compressive and tensile response of an alumina ceramic. Image Credit: SEVENNINE_79/Shutterstock.com

Background

Alumina is a brittle material that undergoes fracture on impact or blast loads. Thus, studies of alumina ceramics on their mechanical response under varying mechanical loads are the current research focus. Additionally, the material's microstructural features, including pores, grain size, and impurities, were reported to be influenced by the material's properties. Hence, prominent research is being carried out to understand the influence of strain rate on mechanical properties.

Many reports were precedented, focusing on the advanced ceramic's compressive response. However, limited antecedents were reported in tensile response studies due to difficulties in generating uniaxial tension. Split-Hopkinson pressure bar (SHPB)-based Brazilian disk and flattened Brazilian disk (FBD) were reported to determine tensile strength, especially for brittle materials. The study of brittle material's tensile and compressive responses at varying strain rates is thus critical.

About the Study

In the present study, the researchers investigated the strain-rate-dependent mechanical properties and failure of CeramTec Alotec 98% alumina ceramic by applying indirect tension and uniaxial compression approaches. They carried out mechanical testing by employing split-Hopkinson pressure bar (SHPB) and quasi-static strain rates for dynamic strain rates, in combination with ultra-high-speed photography and digital image correlation (DIC) technique by investigating strain-rate-dependent failure behavior.

Further, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), electron backscatter diffraction, and X-ray microscopy were employed to determine the failure and microstructural features. From the experimental results, the researchers observed more than 10 times higher strength in compression than in tension.

Additionally, compression's strain-rate sensitivity of strength was higher than that of tension, which was associated with interaction and crack growth. The fracture surface's post-mortem analysis revealed the appearance of intergranular fracture which appeared in quasi-static loading and alumina's dynamic loading transgranular fracture, which generates more micro-cracks.

Research Findings

EDS maps showed the distribution and elemental composition of alumina, where the brighter areas corroborated the higher concentration of the elements. The EDS map showed the presence of oxygen (O) and aluminum (Al) distributed throughout the material. At the same time, spots of silicon (Si), calcium (Ca), magnesium (Mg), and carbon (C) were weakly scattered.

The researchers believed that the Si, Ca, and Mg came from the manufacturing process and C was expected to be caused due to environmental contamination or during the sample preparation where C powder was entrapped on the inside of the deep spots during the carbon coating process.

The alumina's average grain size was computed as approximately 1.85±0.98 µm, which was smaller than the previously reported grain size. The inverse pole figure (IPF) indicated that the material microstructure consisted of numerous nearly circular small grains distributed randomly in the whole IPF map area and comprised of a small number of horizontal and large grains with high-aspect-ratio.

Large columnar and small equiaxed grains had no preferred crystal orientation and most of the boundaries were of high disorientated angles. Moreover, the IPF map showed unindexed areas with an irregular shape that either correspond to the regions or pores with impurities in EDS maps. The grain structure in the present study differed from the previously reported studies due to its high aspect ratio due to Ca, as indicated by EDS.

From the experimental results of compressive stress and strain curves, the team realized that during compression experiments, the strain curves from all AOIs overlapped with each other closely and linearly increased at around 18 µs, which indicated the uniform distribution and constant strain rate achieved during the experiments of SHPB compression.

Conclusion

In conclusion, the current study investigated the mechanical behavior and mechanisms of failure in CeramTec Alotec 98% alumina under indirect tension and uniaxial compression under both dynamic and quasi-static conditions. The microstructural features of CeramTec Alotec 98% alumina were determined by employing multiple techniques and were associated with the material's strength. The indirect tension and uniaxial compression experiments were validated through DIC analysis.

The overlapping of strain-time from stress-time curves and all the areas of interest (AOIs) demonstrated the effectiveness of the experiments. For materials under compression, the peak strength was 10 times higher than those in tension. CeramTec Alotec 98% alumina's indirect tensile strength and uniaxial compression showed strain rate sensitivity whose compression was greater than in tension. Moreover, no strain-rate-dependency or stress-state-dependency were observed for Young's modulus.

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