Strain-rate-dependent tensile response of an alumina ceramic: Experiments and modeling

The strain-rate-dependent tensile response of a commercial alumina ceramic (CeramTec 98% alumina) is investigated by experimental and modeling methods. The experiments at different loading rates are carried out on a standard MTS machine and a modified split-Hopkinson pressure bar system with flattened Brazilian disk specimens. High-speed imaging coupled with digital image correlation (DIC) is used to measure the strain fields, and this enables us to capture the fracture process and the corresponding stress field based on theoretical considerations. In the dynamic tests, it is verified that multiple cracks appear simultaneously around the locations of maximum tensile stress and strain. Next, a matching approach based on theoretical models (i.e., the uniform and sinusoidal load models) is proposed to synchronize the stress and strain history curves in time, and the matching results show the tensile cracks are often generated prior to the peak stress as visualized in ultra-high-speed camera images. This peak stress corresponds to the failure of the sample structure, which is different from the material tensile strength as an inherent material property. In this study, we use the term "overloading" to describe the structural failure of the material. The difference between the peak stress and material tensile strength is associated with the time it takes for the crack to propagate, interact, and span the structure during the loading process, which is termed as "time-dependent structural failure". The strain-rate-dependent tensile strength of the alumina ceramics is computed with a correction method, and the tensile strength is defined as the tensile stress when the central crack first appears in the ultra-high-speed camera images. Then, the fracture surfaces of the alumina fragments are examined by Scanning electron microscopy to explore the fracture mechanism in the failure process. Finally, a strain-rate-dependent tensile strength model is proposed to describe the tensile strength of the CeramTec 98% alumina and other alumina ceramics in the literature.