David Bourne, Sergio Conti and Stefan Müller Folding patterns in partially delaminated thin films preprint 2015 http://arxiv.org/abs/1512.06320

Abstract: Michael Ortiz and Gustavo Gioia showed in the 90s that the complex patterns arising in compressed elastic films can be analyzed within the context of the calculus of variations. Their initial work focused on films partially debonded from the substrate, subject to isotropic compression arising from the difference in thermal expansion coefficients between film and substrate. In the following two decades different geometries have been studied, as for example anisotropic compression. We review recent mathematical progress in this area, focusing on the rich phase diagram of partially debonded films with a lateral boundary condition.

Anton Bovier and Martina Baar From stochastic, individual-based models to the canonical equation of adaptive dynamics -- in one step 2015 http://arxiv.org/abs/1505.02421

Anton Bovier and Lisa B. Hartung Variable speed branching Brownian motion 1. Extremal processes in the weak correlation regime Lat. Am. J. Probab. Math. Stat., 12(1): 261-291 2015 http://alea.impa.br/articles/v12/12-11.pdf

Abstract: We prove the convergence of the extremal processes for variable speed
branching Brownian motions where the ”speed functions”, that describe the timeinhomogeneous
variance, lie strictly below their concave hull and satisfy a certain
weak regularity condition. These limiting objects are universal in the sense that
they only depend on the slope of the speed function at 0 and the final time t.
The proof is based on previous results for two-speed BBM obtained in Bovier and
Hartung (2014) and uses Gaussian comparison arguments to extend these to the
general case.

Anton Bovier and Hannah Mayer A conditional strong large deviation result and a functional central limit theorem for the rate function ALEA Lat. Am. J. Probab. Math. Stat., 12(1): 533--550 2015 http://alea.impa.br/articles/v12/12-21.pdf

Abstract: In this paper we will prove the existence of weak solutions to the Korteweg-de Vries initial value problem on the real line with H^{-1} initial data; moreover, we will study the problem of orbital and asymptotic H^{s} stability of solitons for integers s≥ -1; finally, we will also prove new a priori H^{-1} bounds for solutions to the Korteweg-de Vries equation. The paper will utilise the Miura transformation to link the Korteweg-de Vries equation to the modified Korteweg-de Vries equation.

Annegret Y. Burtscher and Roland Donninger Hyperboloidal evolution and global dynamics for the focusing cubic wave equation 2015 http://arxiv.org/abs/1511.08600

Sunil Chhita and Patrik L. Ferrari A combinatorial identity for the speed of growth in an anisotropic KPZ model arXiv e-prints 2015 http://arxiv.org/abs/1508.01665

Abstract: The speed of growth for a particular stochastic growth model introduced by Borodin and Ferrari in [Comm. Math. Phys. 325 (2014), 603-684], which belongs to the KPZ anisotropic universality class, was computed using multi-time correlations. The model was recently generalized by Toninelli in [arXiv:1503.05339] and for this generalization the stationary measure is known but the time correlations are unknown. In this note, we obtain algebraic and combinatorial proofs for the expression of the speed of growth from the prescribed dynamics.

Sergio Conti, Johannes Diermeier and Barbara Zwicknagl Deformation concentration for martensitic microstructures in the limit of low volume fraction Preprint 2015 http://arxiv.org/abs/1512.07023

Abstract: We consider a singularly-perturbed nonconvex energy functional which arises in the study of microstructures in shape memory alloys. The scaling law for the minimal energy predicts a transition from a parameter regime in which uniform structures are favored, to a regime in which the formation of fine patterns is expected. We focus on the transition regime and derive the reduced model in the sense of $Γ$-convergence. The limit functional turns out to be similar to the Mumford-Shah functional with additional constraints on the jump set of admissible functions. One key ingredient in the proof is an approximation result for $SBV^p$ functions whose jump sets have a prescribed orientation.

Sergio Conti, Heiner Olbermann and Ian Tobasco Symmetry breaking in indented elastic cones Preprint 2015 http://arxiv.org/abs/1512.07029

Abstract: Motivated by simulations of carbon nanocones (see Jordan and Crespi, Phys. Rev. Lett., 2004), we consider a variational plate model for an elastic cone under compression in the direction of the cone symmetry axis. Assuming radial symmetry, and modeling the compression by suitable Dirichlet boundary conditions at the center and the boundary of the sheet, we identify the energy scaling law in the von-Kármán plate model. Specifically, we find that three different regimes arise with increasing indentation $δ$: initially the energetic cost of the logarithmic singularity dominates, then there is a linear response corresponding to a moderate deformation close to the boundary of the cone, and for larger $δ$ a localized inversion takes place in the central region. Then we show that for large enough indentations minimizers of the elastic energy cannot be radially symmetric. We do so by an explicit construction that achieves lower elastic energy than the minimum amount possible for radially symmetric deformations.

Sergio Conti, Matteo Focardi and Flaviana Iurlano Integral representation for functionals defined on $SBD^p$ in dimension two ArXiv Preprint 2015 http://arxiv.org/abs/1510.00145

Sergio Conti, Adriana Garroni and Stefan Müller Dislocation microstructures and strain-gradient plasticity with one active slip plane Preprint arXiv 1512.03076 2015 http://arxiv.org/abs/1512.03076

Sergio Conti and Barbara Zwicknagl Low volume-fraction microstructures in martensites and crystal plasticity preprint 2015 http://arxiv.org/abs/1507.04521

Abstract: We study microstructure formation in two nonconvex singularly-perturbed variational problems from materials science, one modeling austenite-martensite interfaces in shape-memory alloys, the other one slip structures in the plastic deformation of crystals. For both functionals we determine the scaling of the optimal energy in terms of the parameters of the problem, leading to a characterization of the mesoscopic phase diagram. Our results identify the presence of a new phase, which is intermediate between the classical laminar microstructures and branching patterns. The new phase, characterized by partial branching, appears for both problems in the limit of small volume fraction, that is, if one of the variants (or of the slip systems) dominates the picture and the volume fraction of the other one is small.

Sergio Conti, Felix Otto and Sylvia Serfaty Branched Microstructures in the Ginzburg-Landau Model of Type-I Superconductors preprint 2015 http://arxiv.org/abs/1507.00836

Sergio Conti, Janusz Ginster and Martin Rumpf A $BV$ Functional and its Relaxation for Joint Motion Estimation and Image Sequence Recovery ESAIM: Mathematical Modelling and Numerical Analysis, 49(5): 1463-1487 2015 http://dx.doi.org/10.1051/m2an/2015036

Abstract: The estimation of motion in an image sequence is a fundamental task in image processing. Frequently, the image sequence is corrupted by noise and one simultaneously asks for the underlying motion field and a restored sequence. In smoothly shaded regions of the restored image sequence the brightness constancy assumption along motion paths leads to a pointwise differential condition on the motion field. At object boundaries which are edge discontinuities both for the image intensity and for the motion field this condition is no longer well defined. In this paper a total-variation type functional is discussed for joint image restoration and motion estimation. This functional turns out not to be lower semicontinuous, and in particular fine-scale oscillations may appear around edges. By the general theory of vector valued $BV$ functionals its relaxation leads to the appearance of a singular part of the energy density, which can be determined by the solution of a local minimization problem at edges. Based on bounds for the singular part of the energy and under appropriate assumptions on the local intensity variation one can exclude the existence of microstructures and obtain a model well-suited for simultaneous image restoration and motion estimation. Indeed, the relaxed model incorporates a generalized variational formulation of the brightness constancy assumption. The analytical findings are related to ambiguity problems in motion estimation such as the proper distinction between foreground and background motion at object edges.

Sergio Conti, Adriana Garroni and Michael Ortiz The line-tension approximation as the dilute limit of linear-elastic dislocations Arch. Ration. Mech. Anal., 218(2): 699-755 2015 http://dx.doi.org/10.1007/s00205-015-0869-7

Abstract: We prove that the classical line-tension approximation for dislocations in crystals, i.e., the approximation that neglects interactions at a distance between dislocation segments and accords dislocations energy in proportion to their length, follows as the $\Gamma$-limit of regularized linear-elasticity as the lattice parameter becomes increasingly small or, equivalently, as the dislocation measure becomes increasingly dilute. We consider two regularizations of the theory of linear-elastic dislocations: a core-cutoff and a mollification of the dislocation measure. We show that both regularizations give the same energy in the limit, namely, an energy defined on matrix-valued divergence-free measures concentrated on lines. The corresponding self-energy per unit length $\psi(b,t)$, which depends on the local Burgers vector and orientation of the dislocation, does not, however, necessarily coincide with the self-energy per unit length $\psi_0(b,t)$ obtained from the classical theory of the prelogarithmic factor of linear-elastic straight dislocations. Indeed, microstructure can occur at small scales resulting in a further relaxation the classical energy down to its $\calH^1$-elliptic envelope.

Abstract: Dislocations in crystals can be studied by a Peierls-Nabarro type model, which couples linear elasticity with a nonconvex term modeling plastic slip. In the limit of small lattice spacing, and for dislocations restricted to planes, we show that it reduces to a line-tension model, with an energy depending on the local orientation and Burgers vector of the dislocation. This model predicts, for specific geometries, spontaneous formation of microstructure, in the sense that straight dislocations are unstable towards a zig-zag pattern. Coupling between dislocations in different planes can lead to microstructures over several length scales.

Sergio Conti, Matteo Focardi and Flaviana Iurlano Which special functions of bounded deformation have bounded variation preprint 2015 http://arxiv.org/abs/1502.07464

Abstract: Functions of bounded deformation (BD) arise naturally in the study of fracture and damage in a geometrically linear context. They are related to functions of bounded variation (BV), but are less well understood. We discuss here the relation to BV under additional regularity assumptions, which may require the regular part of the strain to have higher integrability or the jump set to have finite area or the Cantor part to vanish. On the positive side, we prove that BD functions which are piecewise affine on a Caccioppoli partition are in GSBV, and we prove that $SBD^p$ functions are approximately continuous $H^n-1$-a.e. away from the jump set. On the negative side, we construct a function which is $BD$ but not in BV and has distributional strain consisting only of a jump part, and one which has a distributional strain consisting of only a Cantor part.

Sergio Conti, Adriana Garroni and Annalisa Massaccesi Modeling of dislocations and relaxation of functionals on 1-currents with discrete multiplicity Calc. Var. PDE, 54(2): 1847-1874 2015 http://dx.doi.org/10.1007/s00526-015-0846-x

Abstract: In the modeling of dislocations one is lead naturally to energies concentrated on lines, where the integrand depends on the orientation and on the Burgers vector of the dislocation, which belongs to a discrete lattice. The dislocations may be identified with divergence-free matrix-valued measures supported on curves or with 1-currents with multiplicity in a lattice. In this paper we develop the theory of relaxation for these energies and provide one physically motivated example in which the relaxation for some Burgers vectors is nontrivial and can be determined explicitly. From a technical viewpoint the key ingredients are an approximation and a structure theorem for 1-currents with multiplicity in a lattice.

Sergio Conti and Georg Dolzmann On the theory of relaxation in nonlinear elasticity with constraints on the determinant Arch. Rat. Mech. Anal., 217(2): 413-437 2015 http://dx.doi.org/10.1007/s00205-014-0835-9

Abstract: We consider vectorial variational problems in nonlinear elasticity of the form I[u]=∫W(Du)dx, where W is continuous on matrices with a positive determinant and diverges to infinity along sequences of matrices whose determinant is positive and tends to zero. We show that, under suitable growth assumptions, the functional ∫Wqc(Du)dx is an upper bound on the relaxation of I, and coincides with the relaxation if the quasiconvex envelope W qc of W is polyconvex and has p-growth from below with p≧n. This includes several physically relevant examples. We also show how a constraint of incompressibility can be incorporated in our results.

Carlota M. Cuesta, Maria Calle and Juan J. L. Velázquez Interfaces determined by capillarity and gravity in a two-dimensional porous medium 2015 http://arxiv.org/abs/1505.03676