Issue 43

M. Davydova et alii, Frattura ed Integrità Strutturale, 43 (2018) 106-112; DOI: 10.3221/IGF-ESIS.43.08 112 Processing of CT images of non-deformed samples using the ImageJ code allowed us to conclude, that mentioned features of ceramics fragmentation with different porosity should be associated with a large cluster, which is formed in the process of preparation of ceramic samples with porosity of 30% (from the initial powder compact with 60% porosity) and combines up to 98% of all sample pores. Initial fracture stage, which involves multiple initiation and growth of porous defects plays the key role in fragmentation of sample with low porosity 2% (initial 20%). But for the samples with 30% porosity (initial 60%), this stage is practically absent due to the presence of pore cluster, and the failure is caused by the loss of stability of the partitions between the pores. This hypothesis is confirmed by the following facts:  stress-strain curves for high porosity samples have two maxima, the first of which corresponds to the pore collapse and the second - to the deformation of compacted material;  in the initial stage of fracture, the intervals between the fractoluminescence impulses for the sample with 2% porosity are two or three times longer than for the sample with 30% porosity. The increase in porosity from 2% to 30% leads to an 18-fold growth of the number of fractoluminescence impulses, which corresponds to the scale-invariant power law distribution of time intervals (straight line in Fig. 2(c)). The scatter of power law exponent (Fig. 2(b)) for the sample with 30% porosity is also due to the formation of a cluster, which provides a wide range of possible fracture variants. The scale invariant law associated with the increase of porosity is also the fundamental subject of research, because it characterizes the material ability to demonstrate the criticality of damage-failure transition due to the involvement of total spectra of the space-time scales into the mechanisms of the defect-induced structural relaxation. The established characteristic features of fragmentation may be crucial for studying the absorption mechanisms [6,7], which can exhibit scaling (structural wave fronts [8]) in a wide range of load intensity. R EFERENCES [1] Buyakova, S.P., Maslovskii, V.L., Nikitin, D.S., Kulkov, S.N., Mechanical instability of a porous material, Technical Physics Letters, 27(12) (2001) 981-983. DOI: 10.1134/1.1432322 [2] Kulkov, S.N., Maslovskii, V.L., Buyakova, S.P., Nikitin, D.S., The non-Hooke’s behavior of porous zirconia subjected to high-rate compressive deformation, Technical Physics. The Russian Journal of Applied Physics., 47(3) (2002) 320- 324. DOI: https://doi.org/10.1134/1.1463121 [3] Buyakova, S. P., Kulkov, S. N., Effect of mechanical processing of ultrafine ZRO 2 + 3 wt % MgO powder on the microstructure of ceramics produced from it, Inorganic Materials, 46 (2010) 1155–1158. DOI:10.1134/S0020168510100249 [4] Kalatur, E.S., Kozlova, A.V., Buyakova, S.P., Kulkov, S.N., Deformation behavior of zirconia-based porous ceramics, IOP Conf. Series: Materials Science and Engineering, 47 (2013) P.012004. DOI: 10.1088/1757-899X/47/1/012004. [5] Davydova, M.M., Uvarov, S.V., Naimark, O.B., Space-time scale invariance under dynamic fragmentation of quasi- brittle materials, Phys. Mesomechanics, 19(1) (2016) 86-92. DOI: 10.1134/S1029959916010094. [6] Naimark, O.B., Some regularities of scaling in plasticity, fracture, and turbulence, Phys. Mesomechanics, 19(3) (2016) 307-318. DOI: 10.1134/S1029959916030097. [7] Naimark, O.B., Defect induced transitions as mechanisms of plasticity and failure in multifield continua in: G. Capriz, P. M. Mariano (Eds.), Advances in Multifield Theories of Continua with Substructure, Birkhäuser, Boston, (2004) 75- 114. DOI: https://doi.org/10.1007/978-0-8176-8158-6. [8] Grady, D., Structured shock waves and the fourth-power law, J. of Applied Physics, 107 (2010) 013506. DOI: http://dx.doi.org/10.1063/1.3269720.