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Physics
Compressive fracture of brittle materials under shock loading
G.I. Kanel, S.V. Razorenov, A.S. Savinykh, E.B. Zaretsky
Institute for High Energy Densities, RAS
Institute of Problems of Chemical Physics, RAS
Ben-Gurion University of the Negev, Beer-Sheva, Israel
The report reviews methods of diagnosing the brittle behavior of materials under shock compression.
The failure waves present an example of catastrophic fracture of brittle materials under compression. The failure wave is a network of cracks that are nucleated on the surface and propagate into the stressed body. When the stressed state ahead of the failure wave is supported, the process is self-supported like the combustion or detonation waves. The propagation velocity of the failure wave is less than the sound speed, it is not directly related to the compressibility but is determined by the crack growth speed. In accordance with conservation lows, compatible increase of normal stress and the material density occur in the failure wave, whereas the deviatoric stresses relax. Behind the failure wave the material completely or almost completely loses its resistance to tension. The failure waves are formed when compressive stresses exceed some threshold value, which can be identified as a failure threshold, but remain below the elastic limit of the glass. Ductility suppresses cracking of the material. The failure waves were not observed in experiments with hard ceramics (alumina and boron carbide) and single crystals (quartz and silicon).
It is known that the elastic limit under compression of ceramics and rocks strongly depends on the lateral pressure in the region of brittle response, whereas at enough high pressure this dependence becomes much weaker or disappears with beginning of ductility. Basing on this fact, we have suggested a method of determining of the mode (ductile vs brittle behavior) of shock compression of hard brittle ceramics. Experiments with alumina and boron carbide ceramics have been carried out. The samples were precisely cut disks of very certain diameter. The controlled lateral pressure Gpa in the samples was created by means of hot pressing the samples into steel rings of smaller inner diameter. At shock compression, the lateral pressure should result in small increase of the recorded stress behind the elastic precursor front in the case of ductile response of the material and three times larger increase in the case of brittle response. The results of experiments unambiguously demonstrate different response of alumina and boron carbide ceramics: alumina behaves as a ductile material, whereas brittle compressive fracture certainly occurs in the boron carbide.
A method of measurements in quasi-spherical divergent shock waves has been developed for determining the conditions and criteria of a brittle-ductile transition for hard ceramics. Explosive generators of convex shock waves have been worked out, the corresponding wave dynamics has been analyzed and preliminary measurements have been performed for the alumina and boron carbide ceramics.
Note. Abstracts are published in author's edition
Last update: 06-Jul-2012 (11:52:45)