Shape sensitivity analysis of an elastic contact problem: Convergence of the Nitsche based finite element approximation

In a recent work, we introduced a finite element approximation for the shape optimization of an elastic structure in sliding contact with a rigid foundation where the contact condition (Signorini’s condition) is approximated by Nitsche’s method and the shape gradient is obtained via the adjoint state method. The motivation of this work is to propose an a priori convergence analysis of the numerical approximation of the variables of the shape gradient (displacement and adjoint state) and to show some numerical results in agreement with the theoretical ones. The main difficulty comes from the non-differentiability of the contact condition in the classical sense which requires the notion of conical differentiability.     

液滴对弹性梯度薄基变形的影响

液滴润湿现象在细胞的变形和软器件的设计和制作中具有潜在的指导意义.该文在考虑三相接触线处线张力的情况下,研究了液滴引起的梯度薄基变形问题.首先,利用积分变换法求解了基底变形的本构方程,给出了变形的法向位移表达式.其次,讨论了基底弹性模量非梯度、指数型梯度和幂型梯度变化时基底的变形情况.最后,给出了液滴大小、弹性模量、线张力及梯度指数变化时位移的变化情况.数值结果表明弹性模量逐渐减小和梯度指数逐渐增大时,湿润脊逐渐变高,变形也越大;线张力和特征深度越小,位移的峰值越高,变形也越大;液滴半径较小时,湿润脊的对称性会变得更好.     

Numerical instabilities in structural optimization – analogy between topology & shape design problems

The formulation of structural optimization problems on the basis of the finite–element–method often leads to numerical instabilities resulting in non–optimal designs, which turn out to be difficult to realize from the engineering point of view. In the case of topology optimization problems the formation of designs characterized by oscillating density distributions such as the well–known “checkerboard–patterns” can be observed, whereas the solution of shape optimization problems often results in unfavourable designs with non–smooth boundary shapes caused by high–frequency oscillations of the boundary shape functions. Furthermore a strong dependence of the obtained designs on the finite–element–mesh can be observed in both cases. In this context we have already shown, that the topology design problem can be regularized by penalizing spatial oscillations of the density function by means of a penalty–approach based on the density gradient. In the present paper we apply the idea of problem regularization by penalizing oscillations of the design variable to overcome the numerical difficulties related to the shape design problem, where an analogous approach restricting the boundary surface can be introduced. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimal Design of Brittle Composite Materials: a Nonsmooth Approach

Our goal is to design brittle composite materials yielding maximal energy dissipation for a given static load case. We focus on the effect of variation of fiber shapes on resulting crack paths and thus on the fracture energy. To this end, we formulate a shape optimization problem, in which the cost function is the fracture energy and the state problem consists in the determination of the potentially discontinuous displacement field in the two-dimensional domain. Thereby, the behavior at the crack surfaces is modeled by cohesive laws. We impose a nonpenetration condition to avoid interpenetration of opposite crack sides. Accordingly, the state problem is formulated as variational inequality. This leads to potential nondifferentiability of the shape-state mapping. For the numerical solution, we derive first-order information in the form of subgradients. We conclude the article by numerical results.     

Solution of a coupled problem of thermopiezoelectricity based on a geometrically exact shell element

Based on a 7-parameter shell model, a numerical algorithm has been developed for solving a coupled problem of thermoelectroelasticity for a laminated piezoelectric shell subjected to a thermoelectromechanical loading. As unknowns, six tangential and transverse displacements of outer surfaces and the transverse displacement of shell midsurface are chosen. This choice provides a possibility of utilizing the complete 3D constitutive equations of thermopiezoelectricity. A geometrically exact 3D hybrid piezoelectric shell element is formulated by using nonconventional analytical integration. With the help of this finite element, solutions of coupled problems of thermoelectroelasticity for laminated plates and shells with segmented and distributed piezoelectric sensors and actuators are obtained.     

Numerical stability for a dynamic Naghdi shell

We consider an elastic model for a shell incorporating shear, membrane, bending and dynamic effects. We make use of the theory proposed by Arnold and Brezzi [1] based on a locking free non-standard mixed variational formulation. This method is implemented in terms of the displacement and rotation variables as the minimization of an altered energy functional. We extend this theory to the shell vibrations problem and establish optimal error estimates independent of the thickness, thereby proving that shear and membrane locking is avoided. We study the numerical stability both in static and dynamic regimes. The approximation schemes are tested on specific examples and the numerical results confirm the estimates obtained from theory.     

弹性基上锥壳一般弯曲问题的精确解

本文应用Donnell的简化假定,从弹性基上锥壳位移型微分方程组出发,通过引入一个位移函数U(s,θ)(在极限情况下就退化成V.S.Vlasov对于圆柱壳所引的位移函数[5]),将基本微分方程组化成为一个八阶可解偏微分方程.这个方程的一般解用级数形式给出.对于在实际中有广泛应用价值的Winkler弹性基上锥壳的轴对称弯曲问题,本文给出了详细的数值结果,并求出了边缘荷载作用下的影响系数,这对计算弹性基上锥壳组合结构有着重要的意义.     

Asymptotic justification of the intrinsic equations of Koiter's model of a linearly elastic shell

We show that the intrinsic equations of Koiter's model of a linearly elastic shell can be derived from the intrinsic formulation of the three-dimensional equations of a linearly elastic shell, by using an appropriate a priori assumption regarding the three-dimensional strain tensor fields appearing in these equations. To this end, we recast in particular the Dirichlet boundary conditions satisfied by any admissible displacement field as boundary conditions satisfied by the covariant components of the corresponding strain tensor field expressed in the natural curvilinear coordinates of the shell. Then we show that, when restricted to strain tensor fields satisfying a specific a priori assumption, these new boundary conditions reduce to those of the intrinsic equations of Koiter's model of a linearly elastic shell.     

Calculation of composite structures subjected to follower loads by using a geometrically exact shell element

Based on a 7-parameter shell model, a numerical algorithm has been developed for solving the geometrically nonlinear problem of a multilayer composite shell subjected to a follower pressure and undergoing large displacements and rotations. As unknowns, six displacements of the outer surfaces and addition ally the transverse displacement of midsurface of the shell are chosen. This allows one to use the Green–Lagrange strain tensor, introduced earlier by the authors, which exactly represents arbitrarily large rigid-body displacements of the shell in curvilinear coordinates of a reference surface. A geometrically exact solid shell element is formulated, which permits one to solve the nonlinear deformation problem for thin-walled composite structures subjected to a follower pressure by using a very small number of load steps.     

跳扩散盈余过程的最优投资和最优再保险

站在保险人的立场上,研究了跳扩散盈余过程的最优投资和最优再保险问题.在方差保费原理下,以盈余终值的期望指数效用达到最大作为最优准则,给出了最优策略和值函数的近似表达式.同时也证明了投资总比不投资好的结论.最后,通过一些数例和图表来进一步说明所获得的结论.     

Shape Methods for the Transmission Problem with a Single Measurement

In the current work, we consider the inverse conductivity problem of recovering inclusion with one measurement. First, we use conformal mapping techniques for determining the location of the anomaly and estimating its size. We then get a good initial guess for quasi-Newton type method. The inverse problem is treated from the shape optimization point of view. We give a rigorous proof for the existence of the derivative of the state function and of shape functionals. We consider both least squares fitting and Kohn and Vogelius functionals. For the numerical implementation, we use a parameterization of shapes coupled with a boundary element method. Several numerical examples indicate the superiority of the Kohn and Vogelius functional over least squares fitting.     

Vector spherical spline interpolation—basic theory and computational aspects

Vector spherical interpolation is discussed from both the theoretical and computational points of view. The theory of vector spherical harmonics is an essential tool. An estimate is given for the absolute error of the interpolation process; an efficient algorithm is developed for the computation of a vector spherical interpoiant. The displacement boundary value problem of determining the elastic field from a finite number of discretely given displacement vectors is solved by the use of vector splines.     

The infinitesimal rigid displacement lemma in Lipschitz co‐ordinates and application to shells with minimal regularity

We establish a version of the infinitesimal rigid displacement lemma in curvilinear Lipschitz co‐ordinates. We give an application to linearly elastic shells whose midsurface and normal vector are both Lipschitz. Copyright © 2004 John Wiley Sons, Ltd.     

On a model of a flexural prestressed shell

E. Sanchez‐Palencia We derive a linearized prestressed elastic shell model from a nonlinear Kirchhoff model of elastic plates. The model is given in terms of displacement and micro‐rotation of the cross‐sections. In addition to the standard membrane, transverse shear, and flexural terms, the model also contains a nonstandard prestress term. The prestress is of the same order as flexural effects; hence, the model is appropriate when flexural effects dominate over membrane ones. We prove the existence and uniqueness of the solutions by Lax–Milgram theorem and compare solution with the solution of the standard shell model via numerical examples. Copyright © 2015 John Wiley & Sons, Ltd.     

Comments on Membrane Locking

Membrane locking is a severe issue frequently neglected in the context of low-order (four node isoparametric) shell elements. From a theoretical point of view, the present contribution illustrates the underlying mechanism by means of shallow shell theory. From a numerical point of view, different element formulations based on standard enhanced assumed membrane strains and reduced integration methods have been compared to each other applying standard as well as modified benchmark examples. A modified integration scheme is proposed. According to preliminary results it behaves virtually optimal within the limits of finite element perfectibility. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impact indentation of a rigid body into an elastic layer. Axisymmetric problem

An axisymmetric contact-impact problem is considered for an elastic layer subjected to normal indentation of a rigid body. An exact analytical solution is obtained in the case of a blunt shape of the indenter having a given velocity, and the stress pattern under multiple reflections is analyzed depending on the layer thickness. A numerical solution of the problem with arbitrary indenter shape is obtained on the basis of the simplified model of the theory of elasticity having a single displacement coincident with the impact direction. The explicit finite difference algorithm is designed on the basis of the mesh dispersion minimization technique. A parametric analysis is presented of the stress pattern developed with time with respect to variations of irregular shapes of the indenter and its masses.     

Numerical treatment of classic scattering from bounded objects

Classic scattering from objects of arbitrary shape must generally be treated by numerical methods. It has proven very difficult to describe scattering from general bounded objects without resorting to frequency-limiting approximations. The starting point of many numerical methods is the Helmholtz integral representation of a given wavefield. From that point several departures are possible for constructing computationally feasible approximate schemes. To date, attempts at direct solutions have been rare.One method (originated by P. Waterman) that attacks the exact numerical solution for a very broad class of problems begins with the Helmholtz integral representations for a point exterior and interior to the target in a partial wave expansion. After truncating the partial wave space, one arrives at a set of matrix equations useful in describing the field. This method is often referred to as the T-matrix method, null-field, or extended integral equation method. It leads to a unique solution of the exterior boundary integral equation by incorporating the interior solution (extinction theorem) as a constraint. In principle, there are no theoretical limitations on frequency, although numerical complications can arise and must be appropriately dealt with for the method to be computationally reliable.For submerged objects the formalism will be outlined for acoustical scattering from targets that are rigid; sound-soft and penetrable; elastic solids; elastic shells; and layered elastic objects. Finally, illustrations of several numerical examples for the above will be presented to emphasize specific response features peculiar to a variety of targets.     

The dynamics of a bar in the presence of obstacles

The dynamic evolution of visco-elastic and purely elastic bars hitting rigid and elastic obstacles are studied, from either the theoretical, numerical or computational point of view.     

Cavity identification in linear elasticity and thermoelasticity

The aim of this paper is an analysis of geometric inverse problems in linear elasticity and thermoelasticity related to the identification of cavities in two and three spatial dimensions. The overdetermined boundary data used for the reconstruction are the displacement and temperature on a part of the boundary. We derive identifiability results and directional stability estimates, the latter using the concept of shape derivatives, whose form is known in elasticity and newly derived for thermoelasticity. For numerical reconstructions we use a least‐squares formulation and a geometric gradient descent based on the associated shape derivatives. The directional stability estimates guarantee the stability of the gradient descent approach, so that an iterative regularization is obtained. This iterative scheme is then regularized by a level set approach allowing the reconstruction of multiply connected shapes. Copyright © 2007 John Wiley & Sons, Ltd.     

Shape-Aware Matching of Implicit Surfaces Based on Thin Shell Energies

A shape sensitive, variational approach for the matching of surfaces considered as thin elastic shells is investigated. The elasticity functional to be minimized takes into account two different types of nonlinear energies: a membrane energy measuring the rate of tangential distortion when deforming the reference shell into the template shell, and a bending energy measuring the bending under the deformation in terms of the change of the shape operators from the undeformed into the deformed configuration. The variational method applies to surfaces described as level sets. It is mathematically well-posed, and an existence proof of an optimal matching deformation is given. The variational model is implemented using a finite element discretization combined with a narrow band approach on an efficient hierarchical grid structure. For the optimization, a regularized nonlinear conjugate gradient scheme and a cascadic multilevel strategy are used. The features of the proposed approach are studied for synthetic test cases and a collection of geometry processing examples.