Issue 41

E. Tolmacheva (Lyapunova) et alii, Frattura ed Integrità Strutturale, 41 (2017) 552-561; DOI: 10.3221/IGF-ESIS.41.65 553 Accurate experiments on dynamic loading of ceramics can provide valuable information about the rate-dependent indentation properties of structural ceramics. However, studying the fracture pattern formed under dynamic indentation of bulk ceramic samples and estimation of the damage zone size is non-trivial, because it is hidden inside the solid. Therefore a frequently used approach is the use of alternative sample geometry (planar) or experimental schemes, which can distort the results. In this work, we employ X-ray computer tomography to reveal the inner structure of the bulk alumina ceramic samples in the damage region. It allows us to compare the regularities of the mechanical behavior of the material with the fracture pattern caused by loading. M ATERIALS AND METHODS lumina ceramics was produced by plasma spraying of alumina powder with average particle size of about 50 µm (Intech, Yekaterinburg). Preparation of samples includes mechanical polishing of their surface and controlling its flatness. The standard mechanical properties of the material were investigated on the Shimadzu AGX-50kN testing machine (Institute of natural science and mathematics, Ural Federal University, Yekaterinburg, Russia) during uniaxial compression experiments at a loading rate of 100 µm/min. Microhardness was measured by the standard Vickers microhardness testing machine (Institute of natural science and mathematics, Ural Federal University, Yekaterinburg, Russia) with the test load of 200 g. The static and dynamic indentation experiments were carried out with standard conical indenters commonly employed in microhardness measurement equipment (base angle of 120±12º and curvature radius 0.200±0.005 mm). The static indentation experiments at a loading rate of 50...100 μm/min were carried out on the Testometric testing machine with original holder for indenter. The dynamic indentation test was performed on the modified experimental setup based on the split Hopkinson pressure bar technique [6-10]. The static and dynamic indentation experiments were conducted in the Institute of Continuous Media Mechanics, UB of RAS, Perm, Russia. The inner structure of samples and fracture pattern formed after dynamic indentation were studied by X-ray computer tomography ( Nikon Metrology XT H 225+160 LC ) at Perm State University, Perm, Russia. The obtained resolution of tomography images was 6.1-6.3 μm/pixel for samples 9 mm in diameter and 8.3 µm/pixel for 14 mm diameter sample. ImageJ free software package [11,12] was used for numerical analysis of tomography images. U NIAXIAL STATIC COMPRESSION EXPERIMENT he uniaxial static compression σ-ε curves for alumina samples are shown in Fig. 1. Material toughness and elastic modulus were found to be 710±50 MPa and 6.0±0.1 GPa, respectively. Microhardness measured by standard technique with Vickers indenter was 4.0±1.2 GPa for applied load of 200 g. Figure 1 : Uniaxial compression of alumina samples. A T