AC loss calculation in high-temperature superconductor wires and windings with analytical and numerical models: Influence of Jc_{c}(B) dependence

Too high loss levels can severely limit the efficiency and the safe operation of several high-temperature superconductor (HTS) AC applications. A reliable estimation of AC losses, which includes the observed field-dependence of the superconductor\u27s critical current density on the magnetic field, is therefore paramount. In this contribution, we use numerical simulations to evaluate the AC losses of HTS coated conductors in a variety of working scenarios: individual wires carrying AC transport current and/or subjected to AC magnetic fields, and wire assemblies like stacks and arrays carrying AC transport current. Numerical results are compared to the corresponding analytical models and to experimental results. This work presents some general guidelines regarding the extent to which the dependence of the critical current density J$_{c}$ on the magnetic flux density B modifies the AC loss characteristics with respect to a constant-J$_{c}$ model based on the superconductor\u27s self-field critical current

Dynamic modeling of levitation of a superconducting bulk by coupled HH-magnetic field and Arbitrary Lagrangian-Eulerian formulations

Intrinsically stable magnetic levitation between superconductors and permanent magnets can be exploited in a variety of applications of great technical interest in the field of transportation (rail transportation), energy (flywheels) and industry. In this contribution, we present a new model for the calculation of levitation forces between superconducting bulks and permanent magnet, based on the $H$-formulation of Maxwell's equations coupled with an Arbitrary Lagrangian-Eulerian formulation. The model uses a moving mesh that adapts at each time step based on the time-change of the distance between a superconductor bulk and a permanent magnet. The model is validated against a fixed mesh model (recently in turn validated against experiments) that uses an analytical approach for calculating the magnetic field generated by the moving permanent magnet. Then, it is used to analyze the magnetic field dynamics both in field-cooled and zero-field-cooled conditions and successively used to test different configurations of permanent magnets and to compare them in terms of levitation forces. The easiness of implementation of this model and its flexibility in handling different geometries, material properties, and application scenarios make the model an attractive tool for the analysis and optimization of magnetic levitation-based applications

Nickel-Titanium peripheral stents: can fracture mechanics shed light on their fatigue failure?

The major concern about Nickel-Titanium (Ni-Ti) stents, which are the gold standard in the treatment of occlusive peripheral disease, is fatigue and the consequent fracture in vivo. Indeed, their failure might be responsible for severe drawbacks, among which is the re-occlusion of the treated artery. Although many phenomenological approaches have been proposed to study this topic, the current literature lacks extensive knowledge on the Ni-Ti local damage mechanisms produced by the cyclic loads that promote crack nucleation and lead to the failure of thin struts, such as those of stents. Moreover, due to the super-elastic property of the alloy, the standard approach for interpreting the fracture of metals might be not accurate for this case. This work aims at increasing awareness of fatigue failure in superelastic Ni-Ti thin struts, such as those of stents. To do so, multi-wire specimens, sharing the same dimensions and thermo-mechanical treatment of the stent struts, were fatigue tested under different strain levels and the number of cycles to failure was recorded for each sample. Numerical simulations corroborated the experimental results to gain information on the local stress and strain fields during the fatigue cycles. A fracture mechanics-based fatigue model adopting the cyclic J-integral was here proposed, giving promising results for the interpretation of such failures

Problematic internet use among high school students: Prevalence, associated factors and gender differences

This study aimed to measure the prevalence of Problematic Internet Use (PIU) among high school students and to identify factors associated with PIU underlining gender differences. The students filled a self-administered, anonymous questionnaire collecting information on demographic characteristics and patterns of Internet use. Multiple logistic regression analysis was performed to identify factors associated with PIU in the overall sample and by gender. Twenty-five schools and 2022 students participated in the survey. Prevalence of PIU was 14.2% among males and 10.1% among females. Males 15-year-olds and females 14-year-olds had the highest PIU prevalence that progressively lowered with age among females. Only 13.5% of pupils declared parents controlled their Internet use. The sensation of feeling lonely, the frequency of use, the number of hours of connection, and visiting pornographic websites were associated with the risk of PIU in both genders. Attending vocational schools, the activities of chatting and file downloading, and the location of use at Internet point among males, and younger age among females were associated with PIU, whilst information searching was protective among females. PIU could become a public health problem in the next years. The physical and mental health consequences should be studied