A numerical assessment of strain rate effects on the critical state of crushable granular materials

The present study highlights the effects of strain rate on the critical state response of crushable granular materials. A set of drained triaxial tests is simulated using the discrete element method (DEM) to understand the rate effects on the stress-strain and volumetric behaviour of the granular sample. The DEM parameters are obtained by comparing the stress-strain and particle crushing behaviour of in-house experimental analysis on crushable coral sand under a slow strain rate. In DEM, the particles are subjected to varied strain rates under different initial confining pressures and initial densities to capture the rate effects on the macroscopic responses until the critical state. It is seen that crushing increases with increasing confining stress. However, a higher strain rate induces relatively lower crushing and higher strength in terms of both peak stress and residual stress. It is observed that in pressure-volume space, the critical state line alters with the increasing strain rate of the crushable samples, especially at high confining conditions, whereas strain rate effect on critical state seems to be negligible at low confining conditions due to the absence of particle crushing.

Режимы взаимодействия намагниченных стопок ВТСП-лент

On the basis of the critical state model, the interaction force of a pair of magnetic lines, which are sets of magnetized stacks of second-generation HTSC tapes, is calculated. The modes of magnetization of interacting rulers by an external magnetic field and the origin of the magnetization reversal of the stacks of tapes during multiple cycles of approaching - moving the rulers away from each other are considered. The force of interaction of the rulers is determined depending on the distance between them and the number of the cycle.

Радиационная обработка краев трещины сверхпроводящего слоя при реставрации ВТСП-ленты

Numerically, within the framework of the critical state model, the density of superconducting currents in a second-generation HTSC tape based on GdBa2Cu3O7-x is determined. It is shown that during the restoration of the transverse crack of the superconducting layer by shunting the crack with a piece of defect-free tape, the critical current of the restored area decreases by ~ 8%. It is shown that preliminary irradiation of the crack edges with ions of hydrogen, helium, neon, and oxygen makes it possible to restore the initial value of the critical current. The calculation of the effect of radiation on a superconducting tape was carried out using the SRIM software package

Grain roughness effect on the critical state line of crushable sands

A recently proposed DEM model for materials with rough crushable grains (Zhang et al. 2021; Ciantia et al. 2015; Otsubo et al. 2017) is here employed to examine the effect of contact roughness on the critical state line, a property of granular materials which is a) fundamental for the evaluation of liquefaction risk and liquefied responses and b) easily accessible through DEM simulation (Ciantia et al. 2019).

On the nature of self-field critical current in superconductors and its use as a probe of the superfluid density

<p>Superconductors are used in many applications where large electrical currents are needed. This is due to their ability to transport an electric current without resistance. There is however a limit to the magnitude of current that can be conducted before dissipation starts to occur. This is known as the critical current and is a topic of great interest in applied superconductivity. For type II superconductors, it is well known that vortex motion plays a role in the determination of the in-field critical current. This has led great effort in engineering the microstructure of these superconductors to hinder the motion of vortices and enhance their critical currents. However the self-field critical current (when there is no applied external field) generally does not see any enhancement due to efforts to pin vortex motion. The work here examines the behaviour of the self-field critical current in thin-film and cylindrical wire superconductors of many different superconductor types and sizes. It is found that a critical state is reached when the current density at the surface of the sample reaches the magnitude of Bc/μ₀λ for type I and Bc₁/μ₀λ for type II superconductors regardless of the size and material type. This finding shows that there is a fundamental limit to the self-field current density that cannot be enhanced by engineering the microstructure and is essentially of thermodynamic origin. The result also sets up the self-field critical current density as a probe of the superfluid density. This was explored in many different superconductor types by considering the temperature dependence of the self-field critical current. The ground-state magnetic penetration depth, groundstate energy gap and specific heat jump at the critical temperature were key thermodynamic parameters extracted from the critical current data. For a very large number of superconductors the extracted parameters in general matched well with literature values measured using conventional but much more complex techniques. A result inferred from the critical state was that the current distribution across the width of a rectangular superconductor would be uniform, contrary to expectations of the Meissner state. This was tested by measuring the perpendicular magnetic field resulting from a transport current in a superconducting tape as it reached the critical state. It was indeed found that the current distribution is uniform across the width. The self-field critical current was also measured in YBa₂Cu₃Oy samples with Zn impurities to measure the superfluid density and further test the self-field critical current as a measure of superfluid density and in particular explore whether it follows the canonical dependence on the transition temperature observed for superconductors with d-wave symmetry. Here the critical current was found to reduce as more impurities were added and indeed this matched its expected canonical reduction, following the superfluid density as Jc(sf) ∝p³/². These results taken together support the unexpected existence of a fundamental limit in the self-field critical current, which is thermodynamic in origin.</p>