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F Kun

Publications and source records attributed to F Kun.

10 recordsLinked to original sources
PubMed2005

Breakup of shells under explosion and impact.

A theoretical and experimental study of the fragmentation of closed thin shells made of a disordered brittle material is presented. Experiments were performed on eggshells under two different loading conditions: fragmentation due to an impact with a hard wall and explosion by a combustion mixture giving rise to power law fragment size distributions. For the theoretical investigations a three-dimensional discrete element model of shells is constructed. Molecular dynamics simulations of the two loading cases resulted in power law fragment mass distributions in satisfactory agreement with experiments. Based on large scale simulations we give evidence that power law distributions arise due to an underlying phase transition which proved to be abrupt and continuous for explosion and impact, respectively. Our results demonstrate that the fragmentation of closed shells defines a universality class, different from that of two- and three-dimensional bulk systems.

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PubMed2004

Structure of magnetic noise in dynamic fracture.

We present an experimental study of magnetic emission spectra recorded during impact fracture of steel. Novel features of dynamic fracture are revealed, i.e., the distribution of the voltage signals of the spectra; furthermore, the areas and energies of voltage peaks exhibit a power law behavior. The value of the exponents of the distributions proved to be characteristic for the failure mode: ductile failure gives rise to exponents significantly higher than brittle failure. The results imply that magnetic crackling noise accompanying impact fracture has a scale invariant structure which reveals new aspects of the dynamics of the fracture process.

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PubMed2004

Fragmentation of shells.

We present a theoretical and experimental study of the fragmentation of closed thin shells made of a disordered brittle material. Experiments were performed on brown and white hen egg shells under two different loading conditions: impact with a hard wall and explosion by a combustible mixture. Both give rise to power law fragment size distributions. A three-dimensional discrete element model of shells is worked out. Based on simulations of the model, we give evidence that power law fragment mass distributions arise due to an underlying phase transition which proved to be abrupt for explosion and continuous for impact. We demonstrate that the fragmentation of closed shells defines a new universality class of fragmentation phenomena.

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PubMed2004

Structure formation in binary colloids.

A theoretical study of the structure formation observed very recently [W. D. Ristenpart, I. A. Aksay, and D. A. Saville, Phys. Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model solely the dipole-dipole interaction of the particles is considered, electrohydrodynamic effects are excluded. Based on molecular dynamics simulations and analytic calculations we show that the total concentration of the particles, the relative concentration, and the relative dipole moment of the components determine the structure of the colloid. At low concentrations the kinetic aggregation of particles results in fractal structures which show a crossover behavior when increasing the concentration. At high concentration various lattice structures are obtained in a good agreement with experiments.

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PubMed2002

Restructuring of force networks.

The compaction of granular packings or soils is a collective process which for higher densities becomes increasingly slower reaching glassy behaviour. We present a study of this problem from various points of view, in particular we will represent the evolving force network that percolates through the system by an inverse fiber rupture model.

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PubMed2001

Bursts in a fiber bundle model with continuous damage.

We study the constitutive behavior, the damage process, and the properties of bursts in the continuous damage fiber bundle model introduced recently. Depending on its two parameters, the model provides various types of constitutive behaviors including macroscopic plasticity. Analytical results are obtained to characterize the damage process along the plastic plateau under strain controlled loading; furthermore, for stress controlled experiments we develop a simulation technique, and numerically explore the distribution of bursts of fiber breaks assuming an infinite range of interaction. Simulations revealed that under certain conditions power law distribution of bursts arises with an exponent significantly different from the mean field exponent 5/2. A phase diagram of the model characterizing the possible burst distributions is constructed.

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PubMed2001

Breakup of dipolar rings under a perpendicular magnetic field.

An experimental and theoretical study of the breakup process of rings, formed by magnetic microspheres, under the application of an external magnetic field perpendicular to the plane of the ring is presented. We found experimentally that when the value of the external magnetic field falls below a lower critical field the dipoles rotate in the ring without any distortion of the ring structure. However, exceeding the upper critical field causes sudden breakup of the ring into short chains aligned with the field. Between the lower and upper critical fields the system is in a metastable state, and hence, it is very sensitive to external perturbations. The spiral opening was found experimentally to be the lowest energy transition from the ring to the chain conformation. We worked out an analytic approach and we performed computer simulations, the results of which are in good agreement with experiments.

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PubMed1999

Aggregation kinetics and stability of structures formed by magnetic microspheres.

Stable rings formed by magnetization-controllable microspheres at zero field are observed and reported in this Rapid Communication. The magnetic microspheres were made by plating glass beads with a critical thickness of Ni film. We found that the ring leading to magnetic flux closure is the most stable configuration. At high concentrations, all individual rings, chains, and clusters join together to form a netlike structure. A computer simulation based on the dipole-dipole interaction, without thermal noise, has been carried out, and the results are in good agreement with the experimental observations. Based on an analytic approach we give a simple explanation of the formation and stability of rings.

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