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P Gumbsch

Publications and source records attributed to P Gumbsch.

13 recordsLinked to original sources
PubMed2005

Understanding of the phase transformation from fullerite to amorphous carbon at the microscopic level.

We study the shock-induced phase transformation from fullerite to a dense amorphous carbon phase by tight-binding molecular dynamics. For increasing hydrostatic pressures P, the C60 cages are found to polymerize at P<10 GPa, to break at P approximately 40 GPa, and to slowly collapse further at P>40 GPa. By contrast, in the presence of additional shear stresses, the cages are destroyed at much lower pressures (P<30 GPa). We explain this fact in terms of a continuum model, the snap-through instability of a spherical shell. Surprisingly, the relaxed high-density structures display no intermediate-range order.

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PubMed2002

Colour-coded duplex sonography of the testes of dogs.

Colour-coded duplex sonography was used to examine the testes of 42 dogs whose testes were normal on clinical examination. With colour Doppler, the blood flow in the supratesticular region could be displayed clearly in 70 per cent of the cases, independent of the dog's age, bodyweight, pulse rate and the volume of its testes. The marginal artery could be displayed clearly in 56 per cent of the cases, and the clarity was correlated with bodyweight (P<0.001), testicular volume (P<0.001), and pulse rate (P<0.001). The intratesticular blood flow could not be discerned in 42 per cent of the cases. With pulsed Doppler, the mean resistive index (RI) in the supratesticular region was 0.57, and the mean pulsatility index (PI) was 1.00. In the marginal artery, the mean RI was 0.49, and mean PI was 0.78. In the intratesticular arteries, the mean RI was 0.47, and the mean PI was 0.75. At all three sites both indices were independent of age, bodyweight, pulse rate and testicular volume.

Animals↗
PubMed2001

Plasticity and an inverse brittle-to-ductile transition in strontium titanate.

The use of ceramic materials is often restricted by a transition from ductile behavior to brittle fracture with decreasing temperature. For example, strontium titanate ( SrTiO3) is known to be extremely fragile and brittle below 1300 K. It is therefore surprising to find that SrTiO3 single crystals can be deformed in compression below 1050 K again. Extensive plastic deformation up to 7% strain at low yield stresses of the order of only 120 MPa is possible at room temperature. Low temperature plasticity is carried by the same [110] [110] dislocations as the high temperature deformation along the [001] axis. From this we conclude that these dislocations must exist in two different core configurations.

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PubMed2001

Melting mechanisms at the limit of superheating.

The atomic-scale details during melting of a surface-free Lennard-Jones crystal were monitored using molecular dynamics simulations. Melting occurs when the superheated crystal spontaneously generates a sufficiently large number of spatially correlated destabilized particles that simultaneously satisfy the Lindemann and Born instability criteria. The accumulation and coalescence of these internal local lattice instabilities constitute the primary mechanism for homogeneous melt nucleation inside the crystal, in lieu of surface nucleation for equilibrium melting. The vibrational and elastic lattice instability criteria as well as the homogeneous nucleation theory all coincide in determining the superheating limit.

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PubMed2001

Alloying effects on electromigration mass transport.

Small amounts of alloying elements can significantly retard electromigration in conductor lines. This phenomenon is experimentally well established but is still lacking a fundamental explanation. An atomic-level mechanism for this behavior is proposed here which is based on a kinetic analysis of diffusion in crystalline interfaces. It predicts a reduction or reversal of the flux of host atoms for physically reasonable parameters and can account for the observed effect of copper on electromigration in aluminum conductor lines.

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PubMed2000

Energy dissipation and path instabilities in dynamic fracture of silicon single crystals

Brittle fracture usually proceeds at crack driving forces which are larger than those needed to create the new fracture surfaces. This surplus can lead to faster crack propagation or to the onset of additional dissipation mechanisms. Dynamic fracture experiments on silicon single crystals reported here show several distinct transitions between different dissipation mechanisms. Cleavage fracture is followed by the propagation of a faceted crack front, which is finally followed by a path instability and the propagation of multiple cracks. The fracture surface qualitatively corresponds to the mirror, mist, and hackle morphology of amorphous materials. However, the corresponding fracture mechanisms, which remain largely unknown in the amorphous materials, can clearly be identified here.

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PubMed2000

Directional anisotropy in the cleavage fracture of silicon

Total-energy pseudopotential calculations are used to study the cleavage anisotropy in silicon. It is shown that cracks propagate easily on 111 and 110 planes provided crack propagation proceeds in the <1;10> direction. In contrast, if the crack is driven in a <001> direction on a 110 plane the bond breaking process is discontinuous and associated with pronounced relaxations of the surrounding atoms, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the experimentally observed cleavage anisotropy in silicon single crystals.

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PubMed1999

Dislocations faster than the speed of sound

It is thought that dislocations cannot surpass the sound barrier at the shear wave velocity because the energy spent in radiation has a singularity there. Atomistic simulations show that dislocations can move faster than the speed of sound if they are created as supersonic dislocations at a strong stress concentration and are subjected to high shear stresses. This behavior is important for the understanding of low-temperature deformation processes such as mechanical twinning and may be relevant for the dynamics of tectonic faults. The motion of the dislocations at a speed of 2 times the shear wave velocity can be understood from a linear elastic analysis, but many of the peculiarities of the supersonic dislocations are dominated by nonlinear effects that require a realistic atomistic description.

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PubMed1998

Controlling factors for the brittle-to-ductile transition in tungsten single crystals

Materials performance in structural applications is often restricted by a transition from ductile response to brittle fracture with decreasing temperature. This transition is currently viewed as being controlled either by dislocation mobility or by the nucleation of dislocations. Fracture experiments on tungsten single crystals reported here provide evidence for the importance of dislocation nucleation for the fracture toughness in the semibrittle regime. However, it is shown that the transition itself, in general, is controlled by dislocation mobility rather than by nucleation.

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