Issue 41

E. Tolmacheva (Lyapunova) et alii, Frattura ed Integrità Strutturale, 41 (2017) 552-561; DOI: 10.3221/IGF-ESIS.41.65 560 Figure 11 : Porosity in different parts of the samples deformed at different maximum loads. 1 – comminuted area, 2 – cracked area, 3 – undisturbed area of the sample. C ONCLUSIONS n this paper we discussed the results of experiments on dynamic indentation of aluminum ceramic samples, which were performed on the original setup based on the split Hopkinson bar technique. The regularities of structure evolution caused by indenter penetration were studied using the computer tomography data on samples subjected to different loads. The analysis of CT data made with the use of the ImageJ free software package revealed the existence of dense area of pressurized material (comminuted area) in the vicinity of the indenter and the formation of multiple cracks in the underlying area. It was found that the higher is the magnitude of the applied indentation load, the bigger is the diameter and height of the comminuted area. Furthermore, with the growth of the load the crack pattern becomes denser and the size of the cracks around the comminuted area increases. The porosity of the comminuted area was found to be practically the same for all values of the applied loads, whereas the porosity in the cracked region is an order of magnitude higher and continues to grow with increasing magnitude of the applied load. Porosity of the undisturbed region of the material was 4 - 5 times higher than in the comminuted area, which makes different areas with different porosity values well-distinguishable. It is important that crack formation begins not at the indenter hole as might be expected, but at the comminuted area lying below. This circumstance can be explained by the indenter spherical tip shape. Indeed, the shape of the indenter is known to have considerable influence on the deformation (and fracture) process and the measured hardness as well. A sharp indenter produces a nominally constant plastic strain, while a spherical tip generates an increasingly growing contact stress with increase of the depth of penetration. This allows one to investigate the transition from the elastic to plastic response and to determine the contact stress-strain property of materials [14]. The obtained fracture behavior of alumina ceramics, namely, the existence of the comminuted area, which gives rise to numerous cracks, has much in common with the fracture behavior of such natural composite as dentin and enamel [15, 16]. The regularities of dynamic indentation of such biocomposite will be the subject of our future investigations. A CKNOWLEDGMENT he experimental study of alumina samples was supported by the Russian Science Foundation grant No. 15-19- 10007. I T