You-He Zhou

Crack-inclusion problem for a long rectangular slab of superconductor under an electromagnetic force

Abstract The interaction between a crack and an inclusion in a type-II superconductor is investigated in this paper. Using the finite element method, the crack–inclusion problem can be solved. Numerical results are presented to illustrate fracture behavior of superconductor under electromagnetic force. The magnetic behavior of the superconductor is described by the critical-state Bean model. The stress intensity factors at the crack tip are obtained and discussed for decreasing field after zero-field cooling. Numerical results show that the stress intensity factors at crack tip are always larger with an elastic inclusion than for a rigid inclusion. Because of the barrier effect of the rigid inclusion, the values of the stress intensity factors decrease when the crack approaches the inclusion. Relative to rigid inclusion and no inclusion cases, elastic inclusion leads to the largest value of the stress intensity factor at crack tip. Thus, the crack propagation is easier near an elastic inclusion and the rigid inclusion is helpful for crack arrest.

Real-time acoustic emission testing based on wavelet transform for the failure process of thermal barrier coatings

The fracture type differentiation, quantification, and source identification are desirable and yet intractable in the acoustic emission (AE) testing of a complex coating system. In this letter, a technique combining wavelet transform and conventional AE parameter analysis was developed to study the tensile failure process of thermal barrier coatings in real time. It is demonstrated that the failure of thermal barrier coatings originates from surface vertical cracking and follows interface cracking, and that the AE count increases with tensile load following a power law. The cracking source identified from AE signals agrees well with that observed by optical microscopy. This technique provides a powerful tool for the study of failure processes of a wide range of coatings and thin films.

Fracture behaviors induced by electromagnetic force in a long cylindrical superconductor

In the present work, the crack problem of a long cylindrical superconductor with a center crack is investigated. The Bean and Kim model of critical state are considered, and the stress intensity factors are calculated for zero-field cooling (ZFC) and field cooling (FC) magnetization processes, respectively. Based on the finite element method, the variation laws of the stress intensity factors for different magnetic field are obtained first. As the applied field decreases, the stress intensity factors increase quickly first, and then decrease gradually after they reach a peak value. Subsequently, for every length of the crack, the maximum of the stress intensity factor is calculated. Numerical results obtained show that, the FC activation process has more significant influence on the stress intensity factors than the ZFC activation process. Finally, the results obtained from Bean model and Kim model are compared. When the dimensionless parameter p is lesser, the results in two different critical state mode...

A nonlinear transient constitutive model with eddy current effects for giant magnetostrictive materials

In this article, the dynamic magnetostrictive effect of giant magnetostrictive materials is studied and a novel general nonlinear transient constitutive model is established, in which the eddy current effects are incorporated. The validity and reliability of the obtained nonlinear transient constitutive model are verified by comparing its predicted results with existing experimental data. The excellent agreements between predicted results and experimental data indicate that the nonlinear transient constitutive model with eddy current effects can accurately capture frequency-dependent hysteretic behavior exhibited by giant magnetostrictive materials under different applied magnetic field amplitudes and frequencies. Moreover, the obtained constitutive model can also effectively capture the nonlinear magnetic-elastic-thermal coupling effect of giant magnetostrictive materials, since the stress and temperature-dependencies are incorporated in the nonlinear transient constitutive model. In such a case, the inf...

An analytical investigation on singularity of current distribution around a crack in a long cylindrical superconductor

This paper presents an analytical investigation to display the distribution of critical current flow and trapped magnetic field around a through crack centrally located in a long cylindrical superconductor on the basis of the Bean model and the Kim model. After a simple conformal mapping is employed to the case of that the superconductor is fully penetrated, the current streamlines, the current density, and the trapped field around the crack in the superconductor without deformation are obtained. It is shown that the crack results in a long-range disturbance of J(r) on the scale much larger than the crack length 2a and a large stagnation region of magnetic flux near the crack except for the singularity of the critical current at the crack tip. Meanwhile, it is also found that the singularity feature is different from the conventional stress one at the crack tip in a deformable body.

Simulation of electric displacement hysteresis and strain butterfly loops in perovskite ferroelectric films

A model for the electric displacement hysteresis and strain butterfly loops of ferroelectric films under electrical loading is proposed based on an improved Preisach model for nonlinear remanent polarization. Our model shows improved displacement and strain versus electric field loops that agree reasonably well with the experimental data. Compared to the previous model, the current model, including the history-dependent electric field effect, which is always neglected in the conventional model, provides electric displacement and strain loops with a full and symmetric shape. In addition, both the loops of electric displacement and strain under intrinsic defects and injected charges have also been investigated by our model.

Analytical solutions of the Ginzburg–Landau equations for deformable superconductors in a weak magnetic field

The objective of this paper is to obtain the analytical solutions which satisfy the differential Ginzburg–Landau equations and some boundary conditions. Based on the linear deformationtheory, the effects of prestrain on the wave function and magnetic potential of deformablesuperconductors have been investigated in the presence of a weak magnetic field. The results show that for the superconductors with a linearly elastic deformation, the wave function in the materials should be considered for two different cases, i.e., type I and type II. The prestrain effect in deformablesuperconductors should not be neglected in determining the superconductivity in superconductors.

Effect of prestrain on coherence length and order parameter

In this paper, an analytical method is presented for determining the effective coherence length and order parameter in a superconductor. The interaction of the deformations of the crystal lattice with the superconducting condensate is considered. A simplified free energy has been used in the literature to estimate the effect of the prestrain on the coherence length and the order parameter. The exact solutions for three dimensional and plane strain problems are obtained. Based on the linear elastic theory, an extensive analysis is made during the positive and negative prestrain for the isotropic material. The results show that the effect of the prestrain on the superconductivity is appreciable. A comparison is made between three dimensional and two dimensional results, and there is not significant difference between the results for two conditions. These explicit solutions provide a convenient tool for evaluating the superconductivity when the material undergoes the deformation.