A Magneto-Mechanically Coupled Phase-Field Model at Large Deformation

The aggregate magneto-mechanical behavior of magneto-rheological elastomers (MREs) stems from the magnetic properties of the ferromagnetic inclusion and the mechanical properties of the matrix material. We propose a large deformation micro-magnetic theory, to predict the behavior and interaction of ferromagnetic particles inside an elastomeric matrix. A rate-type variational principle, with the magnetization as the order parameter is proposed. A large deformation Landau-Lifshitz-Gilbert equation for the time evolution of the magnetization, is obtained directly from the proposed variational principle. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling and Simulation of Rate-Dependent Magneto-Active Polymers

In the present work, the magneto-viscoelastic behavior of MAPs is studied by a thermodynamically consistent constitutive model. A finite deformation based framework of nonlinear magneto-viscoelastic coupling is introduced with a multiplicative decomposition of the deformation gradient. The viscosity is captured by evolution equations of the internal variables introduced. We propose energy functions for pure magnetic and magneto-mechanical coupling such that saturation behavior of the magnetostriction and magnetization is captured. After having established the general framework, the model is studied for homogeneous deformations for the purpose of a least-square-based parameter identification from experimental data. The model predictions of non-linear magneto-mechanical responses with strong rate and field dependency are presented. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micromechanics of Electro- and Magneto-active Soft Composites

We study the coupled behavior in soft active microstructured materials undergoing large deformations in the presence of an external electric or magnetic field. We focus on the role of the microstructures on the coupled behavior, and examine the phenomenon in the composites with (a) periodic composites with rectangular and hexagonal periodic unit cells, and (b) in composites with the random distributions of active particles embedded in a soft matrix. We show that for these similar microstructures exhibit very different responses in terms of the actuation, and the coupling phenomenon. Next, we consider the macroscopic and microscopic instabilities in the active composites. We show that the external field has a significant influence of the instability phenomena, and can stabilize or destabilize the composites depending on the direction relative to composite geometry. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computational Homogenization in Micro-Magneto-Elasticity

The overall macroscopic response of magneto-mechanically coupled materials stems from complex magnetization evolution and corresponding domain wall motion occurring on a lower length scale. In order to account for such effects we propose a computational homogenization approach that incorporates a ferromagnetic phase-field formulation into a macroscopic Boltzmann continuum. This scale-bridging is obtained by rigorous definition of rate-type and incremental variational principles. An extended version of the classical Hill-Mandel macro-homogeneity condition is obtained as a consequence. In order to satisfy the unity constraint of the magnetization on the micro-scale, an efficient operator-split method is proposed. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variational Homogenization of Micro-Electro-Elasticity

Understanding of micromechanical mechanisms in functional materials with electro-mechanical coupling is a highly demanding area of simulation technology and increasing interest has been shown in the last decades. Smart materials are characterized by microstructural properties, which can be changed by external stress and electric field stimuli, and hence find use as the active components in sensors and actuators. In this context, a key challenge is to combine models for microscopic electric domain evolution with variational principles of homogenization. We outline a variational-based micro-electro-elastic model for the micro-structural evolution of electric domains in ferroelectric ceramics. The micro-to-macro transition is performed on the basis of variational principles, extending purely mechanical formulations to coupled electro-mechanics. We focus on an electro-mechanical Boltzmann continuum on the macro-scale with mechanical displacement and electric potential as primary variables. The material model on the micro-scale is described by a gradient-extended continuum formulation taking into account the polarization vector field and its gradient, see Landis [1] and Schrade et al. [2] for conceptually similar approaches. A crucial aspect of the proposed homogenization analysis is the derivation of appropriate boundary conditions on the surface of the representative volume element. In this work we derive stiff Dirichlet-type, soft Neumann-type, and periodic boundary constraints starting from a generalized Hill-Mandel macrohomogeneity condition. Furthermore, we propose techniques to incorporate these boundary conditions in the variational principles of homogenization by means of Lagrange multiplier methods. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micro-Electro-Elastic Modeling of Electric Field- and Stress-Driven Domain Wall Motion

Recently, increasing interest in functional materials such as piezoceramics has been shown. Such materials are characterized by properties, which can be significantly changed by external stimuli, such as stress, electric or magnetic fields. We outline a micro-electro-elastic model for the evolution of electrically and mechanically poled domains incorporating the surrounding free space. To this end, recently developed incremental variational principles (Miehe & Rosato [1]) for local dissipative response need to be extended to gradient-type phase-field models, including an embedding into the free space. The variational setting serves as a natural starting point for a compact and symmetric finite element implementation, considering the mechanical displacement, the electric polarization treated as an order parameter, and the electric potential induced by the polarization as the primary variables. The latter is defined on both the solid domain as well as the surrounding free space. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical and asymptotic study of non‐axisymmetric magnetohydrodynamic boundary layer stagnation‐point flows

Both numerical and asymptotic analyses are performed to study the similarity solutions of three‐dimensional boundary‐layer viscous stagnation point flow in the presence of a uniform magnetic field. The three‐dimensional boundary‐layer is analyzed in a non‐axisymmetric stagnation point flow, in which the flow is developed because of influence of both applied magnetic field and external mainstream flow. Two approaches for the governing equations are employed: the Keller‐box numerical simulations solving full nonlinear coupled system and a corresponding linearized system that is obtained under a far‐field behavior and in the limit of large shear‐to‐strain‐rate parameter (λ). From these two approaches, the flow phenomena reveals a rich structure of new family of solutions for various values of the magnetic number and λ. The various results for the wall stresses and the displacement thicknesses are presented along with some velocity profiles in both directions. The analysis discovered that the flow separation occurs in the secondary flow direction in the absence of magnetic field, and the flow separation disappears when the applied magnetic field is increased. The flow field is divided into a near‐field (due to viscous forces) and far‐field (due to mainstream flows), and the velocity profiles form because of an interaction between two regions. The magnetic field plays an important role in reducing the thickness of the boundary‐layer. A physical explanation for all observed phenomena is discussed. Copyright © 2017 John Wiley & Sons, Ltd.     

On Configurational-Force-Driven Crack Propagation and Adaptive Mesh Refinement in Finite Inelasticity

We introduce a consistent variational framework for inelasticity at finite strains, yielding dual balances in physical and material space as the Euler equations. The formulation is employed for the simultaneous usage of configurational forces as both driving forces for crack propagation as well as h-adaptive mesh refinement. The theoretical basis builds upon a global balance of internal and external power, where the mechanical response is exclusively governed by two scalar functions, the free energy function and a dissipation potential. The resulting variational structure is exploited in the context of fracture mechanics and yields evolution equations for internal variables. In the discrete setting, we present a geometry model fully separated from the finite element mesh structure that represents structural changes of the material configuration due to crack propagation. Advanced meshing algorithms provide an optimal discretization at the crack tip. Local and global criteria are obtained via error estimators based on configurational forces being interpreted as indicators of an energetic misfit due to an insufficient discretization. The numerical handling is decomposed into a staggered algorithm scheme for the dual set of equilibrium equations in material and physical space and efficient mesh generation tools. Exemplary numerical examples are considered to illustrate the method and to underline the effects of inelastic material behaviour in the presented context. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generic behavior of the density of states in random matrix theory and equilibrium problems in the presence of real analytic external fields

The equilibrium measure in the presence of an external field plays a role in a number of areas in analysis, for example, in random matrix theory: The limiting mean density of eigenvalues is precisely the density of the equilibrium measure. Typical behavior for the equilibrium measure is:

• 1 it is positive on the interior of a finite number of intervals,
• 2 it vanishes like a square root at endpoints, and
• 3 outside the support, there is strict inequality in the Euler‐Lagrange variational conditions.

If these conditions hold, then the limiting local eigenvalue statistics is loosely described by a “bulk,” in which there is universal behavior involving the sine kernel, and “edge effects,” in which there is a universal behavior involving the Airy kernel. Through techniques from potential theory and integrable systems, we show that this “regular” behavior is generic for equilibrium measures associated with real analytic external fields. In particular, we show that for any one‐parameter family of external fields V/c, the equilibrium measure exhibits this regular behavior except for an at most countable number of values of c. We discuss applications of our results to random matrices, orthogonal polynomials, and integrable systems. © 2000 John Wiley & Sons, Inc.     

Strong stability of nonlinear semigroups with weak dissipation and non-compact resolvent–applications to structural acoustics

We consider asymptotic stability, in the strong topology, of a nonlinear coupled system of partial differential equations (PDEs) arising in structural–acoustic interactions. The coupling involves parabolic and hyperbolic dynamics with interaction on an interface–a manifold of lower dimension. The distinctive feature of the model is that the resolvent associated with the generator governing the evolution is not compact and the dissipation considered is ‘weak’. Thus, strong stability is not to be generally expected. In linear problems this difficulty is circumvented by the use of Taubrien theorems and spectral analysis [W. Arendt and C.J.K. Batty, Tauberian theorems and stability of one-parameter semi-groups, Trans. Amer. Math. Soc. 306(8) (1988), pp. 837–852, Y.I. Lyubich and V.Q. Phong, Asymptotic stability of linear differential equations ain Banach spaces, Studia Math., LXXXXVII, (1988), pp. 37–42, G.M. Sklyar, On the maximal asymptotica for linear equations in Banach spaces, 2009]. However these methods are not applicable to nonlinear dynamics.

In this article, we present an approach to strong stability that is applicable to nonlinear semigroups governed by multivalued generators with non-compact resolvents. The method relies on a suitable relaxation of Lasalle invariance principle [J.P. LaSalle, Stability theory and invariance principles, in Dynamical Systems, Vol. 1, L. Cerasir, J.K. Hale, J.P. LaSalle, eds., Academic Press, New York, 1976, pp. 211–222] which then requires appropriate unique continuation theorems along with a string of a-priori PDE estimates specific to parabolic–hyperbolic coupled systems.     

A finite-element analysis of critical-state models for type-II superconductivity in 3D

^** Email: c.m.elliott{at}sussex.ac.uk^*** Corresponding author. Email: y.kashima{at}sussex.ac.uk We consider the numerical analysis of evolution variationalinequalities which are derived from Maxwell's equations coupledwith a nonlinear constitutive relation between the electricfield and the current density and governing the magnetic fieldaround a type-II bulk superconductor located in 3D space. Thenonlinear Ohm's law is formulated using the subdifferentialof a convex energy so the theory is applied to the Bean critical-statemodel, a power law model and an extended Bean critical-statemodel. The magnetic field in the nonconducting region is expressedas a gradient of a magnetic scalar potential in order to handlethe curl-free constraint. The variational inequalities are discretizedin time implicitly and in space by Nédélec's curl-conformingfinite element of lowest order. The nonsmooth energies are smoothedwith a regularization parameter so that the fully discrete problemis a system of nonlinear algebraic equations at each time step.We prove various convergence results. Some numerical simulationsunder a uniform external magnetic field are presented.     

Large‐time behavior of a two‐scale semilinear reaction–diffusion system for concrete sulfatation

We study the large‐time behavior of (weak) solutions to a two‐scale reaction–diffusion system coupled with a nonlinear ordinary differential equations modeling the partly dissipative corrosion of concrete (or cement)‐based materials with sulfates. We prove that as t → ∞ , the solution to the original two‐scale system converges to the corresponding two‐scale stationary system. To obtain the main result, we make use essentially of the theory of evolution equations governed by subdifferential operators of time‐dependent convex functions developed combined with a series of two‐scale energy‐like time‐independent estimates. Copyright © 2014 John Wiley & Sons, Ltd.     

A Geometrically Exact Incremental Variational Formulation for Phase Field Models in Micromagnetics

Magnetic materials have been finding increasingly wide areas of application. We focus here on the continuum modeling of such materials and present an incremental variational principle for a dissipative micro-magneto-elastic model. It describes the quasi-static evolution of both magnetically as well as mechanically driven magnetic domains, which also incorporates the surrounding free space. Furthermore, the algorithmic preservation of the geometrical nature of the variables is an important challenge from the numerical perspective and to this end we present a novel FE discretization whereby the geometric property of the magnetization director is pointwise exactly preserved by nonlinear rotational updates at the nodes. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A variational formulation of nonlocalmaterials withmicrostructure based on dual macro- and micro-balances

We investigate a variational setting of nonlocal materials with microstructure and outline aspects of its numerical implementation. Thereby, the current state of the evolving microstructure is described by independent global degrees in addition to the macroscopic displacement field, so-called order parameters. Focussing on standard-dissipative materials, the constitutive response is governed by two fundamental functions for the energy storage and the dissipation. Based on these functions, a global dissipation postulate is introduced. Its exploitation constitutes a global variation formulation of nonlocal materials, which can be related to a minimization principle. Following this methodology, we end up with coupled macro- and microscopic field equations and corresponding boundary conditions. On the numerical side, we consider the weak counterpart of these coupled field equations and obtain after linearization a fully coupled system for increments of the displacement and the order parameters. Due to the underlying variational structure, this system of equations is symmetric. In order to show the capability of the proposed setting, we specify the above outlined scenario to a model problem of isotropic damage mechanics. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrodynamic limit for ∇φ interface model on a wall

 We consider random evolution of an interface on a hard wall under periodic boundary conditions. The dynamics are governed by a system of stochastic differential equations of Skorohod type, which is Langevin equation associated with massless Hamiltonian added a strong repelling force for the interface to stay over the wall. We study its macroscopic behavior under a suitable large scale space-time limit and derive a nonlinear partial differential equation, which describes the mean curvature motion except for some anisotropy effects, with reflection at the wall. Such equation is characterized by an evolutionary variational inequality. Received: 10 January 2002 / Revised version: 18 August 2002 / Published online: 15 April 2003 Mathematics Subject Classification (2000): 60K35, 82C24, 35K55, 35K85 Key words or phrases: Hydrodynamic limit – Effective interfaces – Hard wall – Skorohod's stochastic differential equation – Evolutionary variational inequality     

Rank-One Convexification Approach for the Modeling of Magnetic Shape Memory Response

We present an incremental energy minimization model for magnetic shape memory alloys (MSMAs) whose derivation departs from the constrained theory of magnetoelasticity [1], but additionally accounts for elastic deformations, magnetization rotation, and dissipative mechanisms. The minimization of the proposed incremental energy yields the evolution of the internal state variables. In this sense, the presented modeling concept clearly distinguishes itself from standard phenomenological approaches to MSMA modeling [4]. The extended model is applied to simulate the response of single crystalline Ni₂MnGa. It is shown to accurately capture the nonlinear, anisotropic, hysteretic, and highly stress level-dependent features of MSMA behavior, based on just a few fundamental material parameters, which is validated by comparison to experimental data. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure of principal eigenvectors and genetic diversity

The main concern of this paper is long-term genotypic diversity. Genotypes are represented as finite sequences (s₁,s₂,…,s_n), where the entries {s_i} are drawn from a finite alphabet. The mutation matrix is given in terms of Hamming distances. It is proved that the long time behavior of solutions for a class of genotype evolution models is governed by the principal eigenvectors of the sum of the mutation and fitness matrices. It is proved that the components of principal eigenvectors are symmetric and monotonely decreasing in terms of Hamming distances whenever the fitness matrix has those properties.     

Partially Classical Limit of the Nelson Model

We consider the Nelson model which describes a quantum system of nonrelativistic identical particles coupled to a possibly massless scalar Bose field through a Yukawa type interaction. We study the limiting behaviour of that model in a situation where the number of Bose excitations becomes infinite while the coupling constant tends to zero. In that limit the appropriately rescaled Bose field converges in a suitable sense to a classical solution of the free wave or Klein-Gordon equation depending on whether the mass of the field is zero or not, the quantum fluctuations around that solution satisfy the wave or Klein-Gordon equation and the evolution of the nonrelativistic particles is governed by a quantum dynamics with an external potential given by the previous classical solution. Communicated by Vincent Rivasseau submitted 20/01/05, accepted 23/01/05