Biomedical subjects

Ferenc Kun

Publications and source records attributed to Ferenc Kun.

8 recordsLinked to original sources

PubMed2006

Local load sharing fiber bundles with a lower cutoff of strength disorder.

We study the failure properties of fiber bundles with a finite lower cutoff of the strength disorder varying the range of interaction between the limiting cases of completely global and completely local load sharing. Computer simulations revealed that at any range of load redistribution there exists a critical cutoff strength where the macroscopic response of the bundle becomes perfectly brittle, i.e., linearly elastic behavior is obtained up to global failure, which occurs catastrophically after the breaking of a small number of fibers. As an extension of recent mean field studies [Phys. Rev. Lett. 95, 125501 (2005)], we demonstrate that approaching the critical cutoff, the size distribution of bursts of breaking fibers shows a crossover to a universal power law form with an exponent 3/2 independent of the range of interaction.

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PubMed2006

Failure process of a bundle of plastic fibers.

We present an extension of fiber bundle models considering that failed fibers still carry a fraction 0 < or = alpha < or = 1 of their failure load. The value of alpha interpolates between the perfectly brittle failure (alpha = 0) and perfectly plastic behavior (alpha = 1) of fibers. We show that the finite load bearing capacity of broken fibers has a substantial effect on the failure process of the bundle. In the case of global load sharing it is found that for alpha --> 1 the macroscopic response of the bundle becomes perfectly plastic with a yield stress equal to the average fiber strength. On the microlevel, the size distribution of avalanches has a crossover from a power law of exponent approximately 2.5 to a faster exponential decay. For localized load sharing, computer simulations revealed a sharp transition at a well-defined value alpha(c) from a phase where macroscopic failure occurs due to localization as a consequence of local stress enhancements, to another one where the disordered fiber strength dominates the damage process. Analyzing the microstructure of damage, the transition proved to be analogous to percolation. At the critical point alpha(c), the spanning cluster of damage is found to be compact with a fractal boundary. The distribution of bursts of fiber breakings shows a power-law behavior with a universal exponent approximately 1.5 equal to the mean-field exponent of fiber bundles of critical strength distributions. The model can be relevant to understand the shear failure of glued interfaces where failed regions can still transmit load by remaining in contact.

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PubMed2005

Attraction-limited cluster-cluster aggregation of Ising dipolar particles.

The attraction-limited cluster-cluster aggregation of two-dimensional Ising dipolar particles with or without particle-size dispersity is studied. The fast decrease of the number of even-sized clusters for relatively smaller clusters is observed. The threshold concentration is also determined, under which the dynamics of aggregation is explained using the well-known dynamic scaling theory of Vicsek and Family. Above the threshold concentration, the dynamics depends on particle-size dispersity. Furthermore, it is suggested that, even in the dilute limit, the dynamic exponents are affected by the screening of the surrounding clusters on collision between two clusters.

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PubMed2003

Time evolution of damage under variable ranges of load transfer.

We study the time evolution of damage in a fiber bundle model in which the range of interaction of fibers varies through an adjustable stress transfer function recently introduced. We find that the lifetime of the material exhibits a crossover from mean-field to short-range behavior as in the static case. Numerical calculations showed that the value at which the transition takes place depends on the system's disorder. Finally, we have performed a microscopic analysis of the failure process. Our results confirm that the growth dynamics of the largest crack is radically different in the two limiting regimes of load transfer during the first stages of breaking.

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PubMed2003

Scaling laws of creep rupture of fiber bundles.

We study the creep rupture of fiber composites in the framework of fiber bundle models. Two fiber bundle models are introduced based on different microscopic mechanisms responsible for the macroscopic creep behavior. Analytical and numerical calculations show that above a critical load the deformation of the creeping system monotonically increases in time resulting in global failure at a finite time t(f), while below the critical load the system suffers only partial failure and the deformation tends to a constant value giving rise to an infinite lifetime. It is found that approaching the critical load from below and above the creeping system is characterized by universal power laws when the fibers have long-range interaction. The lifetime of the composite above the critical point has a universal dependence on the system size.

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PubMed2002

Evolution of percolating force chains in compressed granular media.

The evolution of effective force chains percolating through a compressed granular system is investigated. We compressed an ensemble of spherical particles monitoring the macroscopic constitutive behavior and the acoustic signals emitted by microscopic rearrangements of particles. We applied the continuous damage model of fiber bundles to describe the evolution of the array of force chains. The model provides a nonlinear constitutive behavior in good quantitative agreement with the experimental results. The amplitude distribution of acoustic signals was found experimentally to follow a power law of an exponent delta=1.15+/-0.05, which is in good agreement with the analytic solution of the model.

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PubMed2002

Fracture model with variable range of interaction.

We introduce a fiber bundle model where the interaction among fibers is modeled by an adjustable stress-transfer function that can interpolate between the two limiting cases of load redistribution, i.e., the global and the local load sharing schemes. By varying the range of interaction, several features of the model are numerically studied and a crossover from mean-field to short-range behavior is obtained. The properties of the two regimes and the emergence of the crossover in between are explored by numerically studying the dependence of the ultimate strength of the material on the system size, the distribution of avalanches of breakings, and of the cluster sizes of broken fibers. Finally, we analyze the moments of the cluster size distributions to accurately determine the value at which the crossover is observed.

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PubMed2002

Creep rupture of viscoelastic fiber bundles.

We study the creep rupture of bundles of viscoelastic fibers occurring under uniaxial constant tensile loading. A fiber bundle model is introduced that combines the viscoelastic constitutive behavior and the strain controlled breaking of fibers. Analytical and numerical calculations showed that above a critical external load the deformation of the system monotonically increases in time resulting in global failure at a finite time t(f), while below the critical load the deformation tends to a constant value giving rise to an infinite lifetime. Our studies revealed that the nature of the transition between the two regimes, i.e., the behavior of t(f) at the critical load sigma(c), strongly depends on the range of load sharing: for global load sharing t(f) has a power law divergence at sigma(c) with a universal exponent of 0.5, however, for local load sharing the transition becomes abrupt: at the critical load t(f) jumps to a finite value, analogous to second- and first-order phase transitions, respectively. The acoustic response of the bundle during creep is also studied.

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