Issue 43

M. Davydova et alii, Frattura ed Integrità Strutturale, 43 (2018) 106-112; DOI: 10.3221/IGF-ESIS.43.08 107 compacting within the steel molds with a hydraulic punch under compacting pressure of 70 MPa. The obtained compacts were sintered in air at 1550°С and then subjected to isothermal exposure within an hour. The samples had the shape of cylinders of diameter from 8.5 to 13 mm, length from 6.8 to 12.2 mm and weight from 2.2 to 6.3g. Porosity of the initial powder before compacting varied from 10% to 60%. Figure 1 : a) The scheme of experimental set-up (1-sample, 2-input bar, 3-output bar, 4-photomultiplier tube (PMT), 5- teflon ring, 6- impedance matched WC insert, 7- plastic cylinder); b) stress-strain curve for ceramic samples with porosity of 10%, 20%, 45%, 60%. Dynamic tests were carried out using a split Hopkinson pressure bar, (Fig.1(a)), which consists of the input (3 m in length and 25 mm in diameter) and output (1 m in length and 25 mm in diameter) bars to provide single-pulse loading conditions. The bars were made of high-strength steel (  ~1900 B GPa ). A sample was sandwiched between the bars and separated from them by the impedance–matched tungsten carbide (WC) inserts, which prevented samples from being indented into the bars, the hardness of which was less than the hardness of the examined ceramics. The position of the bars was carefully adjusted before each loading to ensure a uniform distribution of the force applied to the end faces of the samples. In order to eliminate the effect of dispersion in the bars and to provide the dynamic stress equilibrium conditions (the equality of forces applied to the specimen ends) during tests, we used a brass pulse shaper, which was a 7x7 mm plate of thickness 1.4 mm. By varying the striker velocity, we managed to increase the deformation rate from 400 to 3000 s -1 . The study of fragmentation statistics (fragment size distribution and distribution of time intervals between the fractoluminiscence impulses) required modification of the traditional scheme of the split Hopkinson bar. The samples and the ends of the bars adjacent to the samples were placed into plastic cylinders, which allowed the fragments of ceramics to remain confined within the cylinder and provided dimming necessary for registration of the fractoluminiscence impulses. The formation of fracture surfaces during sample fragmentation initiated light emission, which was recorded by the two a) b)