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Impact Comminution of Concrete and Rock Due to Kinetic Energy of High-Rate Shear

Release time:January 12, 2015 /

Speaker:Professor Zdeněk P. Bažant

Date:Jan.13,2015

Time: 9:00

Venue:the 2nd floor, NewMain Building Conference Center

Abstract:

Fragmentation, crushing and pulverization of solids, briefly called comminution, has long been a problem of interest for mining, tunneling, explosions, meteorite impact, missile impact and penetration, ground shock and various kinds of industrial processes. While many semi-empirical models for impact analysis abound in the literature, and whereas the fragmentation in the so-called `Mescall' zones of impacted or shocked solids has been explained by branching of dynamically propagating cracks, a viable comminution model appears for macroscopic dynamic finite element analysis of large structures is unavailable and is proposed in this lecture. By contrast to static fracture, in which the driving force is the release of strain energy, here the central idea is that the driving force of comminution under compression at strain rates >10/s is the release of the kinetic energy of shear strain rate of forming particles, whose density can exceed the maximum possible strain energy density by several orders of magnitude. It is shown that the particle size or crack spacing should be proportional to the -2/3 power of the shear strain rate, that the comminution by high-rate shear is mathematically equivalent to an apparent shear viscosity proportional to the -1/3 power of the shear strain rate, and that the drop of the density of kinetic energy of shearing of forming particle is proportional to the 2/3 power of that rate. The proposed theory is inspired by analogy with turbulence, in which the kinetic energy of shear strain rate is analogous to the kinetic energy of rotating eddies and the fracture energy dissipation at interfaces of forming particles is analogous to the viscous energy dissipation between adjacent eddies. A dimensionless characteristic analogous to the Reynolds number, delineating classical and kinetic energy fractures, is formulated. In combination with the microplane model, the new theory greatly improves predictions of the exit velocity or penetration depth of missiles impacting concrete walls while remaining anchored in quasistatic laboratory tests of damage in concrete.

Introduction of the speaker:

Professor Zdeněk P.Bažant, who comes from Northwestern University, enjoys high reputation in mechanics and Civil Engineering as a scientist around the world. At present, he has been the academician of the National Academy of Sciences, American Academy of Arts and Sciences and National Academy of Engineering. He is also the academician of Italian, Austrian, Spanish and Czech National Academy of Sciences, and the academician of Academia Europaea and European Academy of Sciences and Arts. As the Honorary member of ASCE, ASME and ACI, he owns 7 Honorary Doctoral Degrees.