Vademecumbased approach to multiscale topological material design
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Abstract
The work deals on computational design of structural materials by resorting to computational homogenization and topological optimization techniques. The goal is then to minimize the structural (macroscale) compliance by appropriately designing the material distribution (microstructure) at a lower scale (microscale), which, in turn, rules the mechanical properties of the material. The specific features of the proposed approach are: (1) The cost function to be optimized (structural stiffness) is defined at the macroscale, whereas the design variables defining the microstructural topology lie on the low scale. Therefore a coupled, twoscale (macro/micro), optimization problem is solved unlike the classical, singlescale, topological optimization problems. (2) To overcome the exorbitant computational cost stemming from the multiplicative character of the aforementioned multiscale approach, a specific strategy, based on the consultation of a discrete material catalog of microscale optimized topologies (Computational Vademecum) is used. The Computational Vademecum is computed in an offline process, which is performed only once for every constitutivematerial, and it can be subsequently consulted as many times as desired in the online design process. This results into a large diminution of the resulting computational costs, which make affordable the proposed methodology for multiscale computational material design. Some representative examples assess the performance of the considered approach.
Keywords
Topological optimization Multiscale material design Computational Vademecum Topological derivative Finite element modellingBackground
In the last decades, topological structural optimization has gained considerable importance in the Computational Mechanics field. Besides the increasing interest of the scientific community, practical applications have been accomplished in the aeronautical [1], automotive and civil engineering industry. In fact, topology optimization tools can be found in more than thirty commercial software packages [2], e.g. Abaqus [3], Altair HyperWorks [4].
After the groundbreaking contribution in [5], a densitylike approach, termed the solid isotropic material with penalization (SIMP) method, has been successfully developed during the last years. Unfortunately, some numerical problems may occur, like checkerboard and meshdependency. Furthermore, artificial parameters must be provided.
Alternatively, other approaches, such as evolutionary structural optimization (ESO/BESO), have been proposed [6]. Even though a pseudodensity does not need to be defined, heuristic criteria is used to find possible suboptimal solutions.
More recently, shape sensitivity has become a successful topological optimization tool. Initially, in [7], the socalled shape optimization theory was introduced by Allaire. Within this theory, the topology optimization problem is solved by means of a level set algorithm and a Hamilton–Jacobi evolution equation. Nevertheless, in order to remove material, a topology with holes must be provided as an initial guess. Such conditioning leads to specific optimal topologies. In addition, from the work in [8], the topological derivative mathematical concept has been recently used for structural topology optimization. Both shape optimization and asymptotic expansion theories are essential for the closedform expression of the topological derivative. Its success is due to the analytical expressions that measure an specific shape functional sensitivity when an inclusion is introduced into a fixed domain. Later studies have been consolidated in the reference book [9]. Additionally, in [10], a level set algorithm is proposed taking advantage of the topological derivative theory as a gradient of steepest descent algorithm. It is worth noting that this last approach is exempt from artificial parameters.
Bearing this in mind, the same methodology can be used for material design [11] besides structural optimization. Computational multiscale approaches provide an appropriate framework to achieve it. Either asymptotic multiscale [12] or variational multiscale theories [13] have been extensively used in the recent years. Although the former is based on analytical expansion theories, the latter seems to fit more naturally into the FEM context, specially due to its variational framework. In practice, they lead to a \(\text {FE}{}^{2}\) problem where standard FEM discretizations of the elastic equations are considered at both the macro and microscales.
Apart from [11], other works (cf. [14, 15, 16]) have recently studied topological optimization in a multiscale context, the main drawback being the expensive computational multiscale problem to be faced. In consequence, and in order to keep bounded the computational cost, either the microstructure topology is kept constant over the whole domain or very coarse discretizations of this microstructure can only be considered.
In this work, an alternate directions algorithm is proposed as in [12]. Then, inspired in the Computational Vademecum concept [17], the computationally expensive construction of a catalog of optimal material designs is made ofline, and only a trivial selection of the optimal microstructure is done in the online process. As in PGD [17], or POD strategies [18, 19], a considerable reduction on the computational cost is obtained. To validate this approach, some numerical examples are shown.
The paper is organized as follows: first, in “Multiscale topology optimization framework” section, the variational multiscale theory is presented and its main features are summarized. Additionally, the topological derivative concept on the microscale is introduced and the useful final expression is shown. A levelset based algorithm is described and some examples are presented. In “Multiscale topology optimization problem” section, the formulation of the target problem is stated and conveniently reformulated. A preliminary study of the problem complexity is also presented. In “Vademecumbased computational cost reduction” section, the Computational Vademecum is computed and some examples are shown. In addition, the homogenized constitutive tensor distribution is represented. In “Numerical algorithm” section, an alternate directions algorithm is conceptually described and some numerical examples are solved in “Numerical examples” section. Conclusions and future perspectives are summarized in “Concluding remarks” section.
Multiscale topology optimization framework
Multiscale problem
To overcome this difficulty, multiscale techniques, based on homogenization theories, aim at representing heterogeneities of the small length scale \(l_{\mu }\) by taking mean values of the microscale material properties. Each homogeneous region of the continuum macroscopic domain, represented by the corresponding integration/sampling point, retrieves the constitutive law from the average response of a representative microscopic domain, i.e. the representative volume element (RVE) (cf. Fig. 1).
Different theories have been developed in order to set up the corresponding mathematical framework [12, 13]. Asymptotic expansion and variational multiscale may be nowadays the most successful approaches in the computational mechanics community. Even though asymptotic expansion is a rigorous mathematical theory and has been used for a long time, variational multiscale theory seems to be easier to generalize and extend to nonlinear problems. Furthermore, variational approaches usually fit more naturally in the context of finite element methods. In our study, this last approach will be used. For more information, the reader is referred to the works in [13].
Multiscale variational framework
Hill Mandel principle
 First, \(\delta \tilde{\epsilon }_{\mu }=0\Rightarrow \delta \epsilon _{\mu }=\delta \epsilon \) and, as a result, (7) takes the following form,$$\begin{aligned} \sigma :\delta \epsilon =\frac{1}{\Omega _{\mu }}\int _{\Omega _{\mu }} \sigma _{\mu }:\delta \epsilon \forall \delta \epsilon \,{\in }\,\mathbb {V}_{\epsilon }. \end{aligned}$$(9)
 Second, \(\delta \epsilon =0\Rightarrow \delta \epsilon _{\mu }=\delta \tilde{\epsilon }_{\mu }\), therefore (7) becomes$$\begin{aligned} \sigma :\delta \epsilon =\frac{1}{\Omega _{\mu }}\int _{\Omega _{\mu }} \sigma _{\mu }:\delta \tilde{\epsilon }_{\mu }=0 \forall \delta \tilde{\epsilon }_{\mu }\,{\in }\, \mathbb {V}_{\tilde{\epsilon }_{\mu }}. \end{aligned}$$(11)
Equilibrium equation and microcell boundary conditions
The homogenized constitutive tensor
Topological derivative at the microscale
Recently, topological derivative has emerged as a useful tool for structural topology optimization [10, 12, 20, 21] and material design [11, 22]. Sokolowski and Zochowski, in [8], set up the foundation of the topological derivative theory, and some years later, such theory consolidates in the reference book [9].
In the topological derivative theory context, a closed formula is obtained as the sensitivity of a shape functional when an inclusion on the domain is introduced. Its derivation is based primarily on the concepts of shape optimization and topological asymptotic expansion as follows.
Topological derivative of the homogenized constitutive tensor
Subsequently, the elastic problem on the perturbed domain is presented. First, a circular hole \(B_{\hat{\epsilon }}\) of radius \({ \hat{\epsilon }}\) and center \(\hat{y}\) is set in the domain \({\Omega _\mu }\), and then an inclusion of the same shape but with different material properties is introduced, see Fig. 3 . For the case of an actual void created in the domain \(\Omega \), the reader is referred to the work in [9].
Level set algorithm
An important implication of the topological derivative concept is that the homogenized constitutive tensor sensibility to the introduction of an inclusion may be determined. This allows using this sensibility as the drivingforce in a topological optimization problems (cf. [10]).
Representative examples

Uniaxial horizontal stressstate: \({\sigma }=\left[ \begin{array}{c} 1\\ 0\\ 0 \end{array}\right] \)

Shear stressstate: \({ \sigma }=\left[ \begin{array}{c} 0\\ 0\\ 1 \end{array}\right] \)

Bulk stressstate: \({ \sigma }=\frac{1}{\sqrt{2}}\left[ \begin{array}{c} 1\\ 1\\ 0 \end{array}\right] \)
Multiscale topology optimization problem
Once the computational multiscale homogenization framework has been introduced and the microstructure topology optimization method has been described, next step is to use these ingredients for material design purposes. In other words, to determine the optimal microstructure topology at every point of the macrostructure domain in order to achieve a functional goal at the macroscale level as it is sketched in Fig. 6.
Formulation
The classical topological optimization problem in structural analysis refers to the minimum compliance (or maximum stiffness) design. Here the classical singlescale problem of determining the optimum distribution of a certain material mass at the macroscale (or structural scale), to achieve the minimum compliance of the resulting structure [5], is reformulated as a twoscale problem in the following sense: the goal is the optimal distribution of a given material mass, but now at the microscale level for every structural point (given the shape and topology at the structural scale).
 1.
The objective function to be minimized is highly nonlinear and defined at the macroscale level.
 2.
The design variables (the values of the characteristic function \({{\chi }}_{\mu }\)) are defined at the microscale
 3.
The equilibrium equation couples both macro and micro levels since, although the stresses are defined macroscopically, the constitutive Eq. (29) depends on the microstructural topology.
Algorithmic separability
As pointedout above, the addressed minimization problem in Eq. (47) entails multiscale coupling and nonlinearities. Due to its nonlinear character different solutions, corresponding to different local minima, might be expected. Due to the multiscale coupling character, the computational robustness of the minimization scheme can be seriously affected.
Algorithmic complexity
Once problem (49) is stated, its algorithmic complexity (here understood as the number of operations to be performed) is tackled .
Considering a typical FEM discretization involving, say, n finite elements at every scale (47) Open image in new window design variables ( Open image in new window values of the characteristic function, \(\chi _{\mu }\), at the RVE times Open image in new window macroscopic sampling points) are involved. Hence, one optimization problem with a grand total of Open image in new window design variables has to be solved.
In contrast, problem (49), may be seen as a Open image in new window local designvariable optimization problem, solved Open image in new window times (one for every macroscopic sampling point). This makes an enormous difference in terms of the problem complexity and the corresponding computational cost.
The local optimization problem is then sought, as iteratively solving the global equilibrium equation, starting with a given initial microstructure topology distribution, and modifying it by the algorithm (40) leading to a new RVE topology (in an uncoupled way from the other RVE’s), until convergence. A similar approach is presented in [14].
In addition, a natural strategy to tackle the problem in (49) consists of solving the local optimization problem at all the Open image in new window macroscopic sampling points in an way uncoupled from each other, which makes the corresponding algorithm highly parallelizable.
Vademecumbased computational cost reduction
Despite resorting to parallel computation, problem (49) still exhibits high complexity and it becomes computationally unaffordable for reallife problems. In this scenario, a more efficient approach is proposed here. The main idea consists of optimizing “a priori” a very large discreteset of microstructures, in the set of possible macrostresses acting on the RVE, leading to the socalled “Material Catalogue” or “Computational Vademecum” [17]. Then, when a certain optimal microstructure topology is required in the global (multiscale) design problem a certain optimal microstructure topology is requested, for a given stressstate at the macroscale sampling point, the Vademecum is consulted and the closest optimal solution is extracted.
More precisely: given the mechanical properties of the basematerial, the expensive computations requested for the Vademecum construction are done onceforall, in an offline process, and the Vademecum outputs (typically the tangent constitutive operator, \(\mathbb {C}_{h}\), solution of Eq. (42) and the compliance are stored in a database for, a sufficiently large, discrete set of entries \({ \sigma }.\)
The actual multiscale material design problem is then performed “online”, and it only involves a recursive equilibrium analysis at the macroscale combined with consultations to the Vademecum. This translates into an impressive reduction of the computational cost of the online material design process. It is highlighted that the Vademecum remains the same for a given basematerial, disregard the kind of macroscale structural problem aimed at being optimized.
Parametric domain
The success of the proposed Vademecumbased strategy crucially relies on the appropriate determination of the Vademecum entries so that a good balance of the error/computationalcost is achieved. Indeed, since the error is produced by the closestentry strategy, a higher number of entries lowers the resulting error but increases the computational cost related to the construction of the Vademecum.
Hence, the parametric domain is represented by the unit radius sphere. Each point of the sphere can be seen as a microstructure optimization case, which returns some homogenized elastic properties associated to an optimal topology (see Fig. 7).
For the subsequent examples the sphere has been discretized by means of a structured mesh of 16386 points. However, the bottom half points need not to be computed because they have an homologous point at the top half of the sphere. This is because \(\sigma \) or \(\sigma \), plugged in problem (51), result in the same objective function. Besides, only a quarter part of the top half of the sphere must be computed due to symmetries and mirroring. Consequently, an eighth of the whole sphere has been actually considered for the computations, resulting into 2145 points to be actually computed.
As for the involved computational cost: it is remarked that the Vademecum computation is fully parallelizable for the different entries (every case is fully uncoupled), and, therefore, the algorithm is “embarrassingly parallel” in clusters of distributed memory architecture.
Some examples of this optimal RVE topologies can be seen in Fig. 8. They have been computed for the solid volume fraction at the RVE \(V_{\mu }=0.6\). Note that no length scale control scheme is imposed, thus, thin and non manufacturable topologies might appear. Perimeter penalization, see [25], or filtering techniques might be applied as potential remedies.
It is worth mentioning that construction of such Vademecum requires a very robust methodology for the RVE topological design, so that none of the desired entry points fails to be computed. In this sense it has to be remarked that the use of the topological derivative concept and its application in the algorithm (40), as proposed in this work, fulfills this requirement. All cases converged for a constant value of the penalty, with a convergence tolerance \({ \theta <1^{o}}\) and a tolerance on the volume constraint \(\text {TOL}<0.001\).
It is also remarkable from Fig. 8, that, for many cases, the obtained optimal topologies are far from being intuitive.
Associated to the optimal topologies, and based on problem (51), the corresponding RVE compliances for every point of the parametric space are computed, and they are displayed in Fig. 9. It has to be emphasized that, from the theoretical point of view, there is no guarantee that a local minimum has been achieved in all cases, although numerical experiences have evidenced that the obtained compliances are very close to the global minima.
As explained before, the homogenized constitutive operator, \(C_{h}\), and the compliance (cost function), constitute the relevant “outputs” of the Vademecum. Indeed, only the compliance is strictly needed in the iterative algorithm. However, the homogenized constitutive operator is also stored in the Vademecun in order to be used for postprocessing purposes, when requested, at the end of the analysis.
Accordingly, in Fig. 10, the Vademecum outputs for the optimal homogenized components of \(\mathbb {C}_{h}\) are presented.
Numerical algorithm
In the following the numerical algorithm, implemented in a Matlab code, for solving the minimization problem (49) is presented.
The described algorithm stems naturally from Eq. (49). It is based on the alternate directions scheme, extensively used in the literature (see for example [12]). Since linear elasticity is considered in the mechanical stage, the stresses are obtained by solving a linear problem, using the topologyupdated stiffnessmatrix. Then, in the subsequent optimization stage, by using the closest entry to the obtained stresses, the optimal mechanical properties, and the corresponding topology, are obtained by consultation of the material catalog (Vademecum). Moreover, the nonlinear character of the optimization problem is already accounted for in the Computational Vademecum construction (offline).
As for the global coupled problem, the nonlinearity is accounted for by the alternate directions (fixed point) algorithm, and, therefore, only a linear convergence ratio is expected. In spite of this, “Numerical examples” section, it is shown that only few iterations are required for solving this nonlinearity.
Numerical examples
In order to assess the proposed approach some numerical examples are presented next. In all cases the solid volume fraction at the RVE is \(V_{\mu }=0.6\), thus satisfying \(\int _{\Omega _{\mu }}\chi _{\mu }=V_{\mu }=0.6\). As start point in the iterative procedure, a microstructure with a centered circular void fulfilling this condition is taken at all RVE’s (see Fig. 11a).
Cantilever beam
The microstructure topological design of the cantilever beam in Fig. 11 is studied. The dimensions are 2 m length \(\times \) 1 m height, and plane stress conditions are assumed. The beam is loaded by a unit vertical point force, at the right end center, and it is clamped at the left end.
In Fig. 12, the evolution of the global cost function (structural compliance) and of the residue of the alternate directions algorithm is depicted. As it can be observed in Fig. 12, four iterations suffice to achieve full convergence with a 30 % reduction of the original compliance. The convergence ratio of the iterative process is linear, as expected from the used alternate directions algorithm.
Bending beam
The evolving microstructure distribution during the iterative design process is depicted in Fig. 13. In Fig. 14 it can be observed that a fast convergence is achieved also in this case (six iterations) leading to almost 40 % reduction of the structural compliance.
Concluding remarks
In this work a new approach to computational material design has been presented and applied to multiscale topological design of structural materials. A computational homogenization scheme for two scales (FE2) has been considered to account for the impact of the lower scale material distribution on the global structural properties. This is done through the resulting pointwise homogenized elastic properties.
In this context, a throughscale optimization problem has been introduced, where the cost function (compliance) is defined at the structural scale, whereas the design variables (the values of the characteristic function at the RVE) are defined at the microscale. This is a challenge which is faced in an innovative way in this work.
To overcome the exorbitant cost typically encountered in brute force FE2 homogenization methods, and taking advantage of the macro/micro separability properties of the stated problem, a Vademecumbased strategy has been devised. It consists of the, offline, construction of a discrete material catalog, containing an appropriate large set of optimal RVE designs, each one corresponding to a single macroscopic stress state. The topological derivative mathematical tool, jointly with a level set algorithm, based on “slerp” interpolations and a specific line search method, have proved robust enough to provide the optimal RVE designs necessary for the Vademecum construction (more than 2000 for the presented examples). Once this Vademecum is available, it can be repeatedly consulted for the optimal material design of any structure made of the same base material.
For the examples presented in this work, a prescribed volume fraction, has been considered in the construction of the Computational Vademecum. Naturally, the Vademecum computation could also be extended to account for a range of volume fractions, this not implying any conceptual change in the computations, other than the extension of the Vademecum parametric domain with an additional dimension (the volume fraction).
However, the use of this extended Vademecum in a context where the volume fraction may vary from point to point (e.g. when optimizing the weight of the macroscopic structure) is not trivial and it is left for future works.
In this framework a global optimization algorithm, based on consultation of the Vademecum in combination with the equilibrium solution of the macroscopic structural problem, results into a highly nonlinear problem that, however, can be solved by means of a fixed point algorithm in remarkably robust fashion. by means of a fixed point (or alternate directions) algorithm.
The resulting strategy becomes both robust and computationally efficient for tackling the microstructure computational design problem in structural materials. Some representative examples illustrate the proposed methodology, and show that large reductions of the structural compliance (up to 40 %) can be achieved by using the proposed “local” topology design strategy, in front of the alternative “homogeneous” material distribution.
It is worth mentioning that, although a smooth transition of material distribution is often seen by the authors, some incompatibilities could appear. Additional constraints must be imposed in order to avoid manufacturability limitations. In our current ongoing work, this issue is addressed and an improved methodology will be proposed.
The issue of the topological continuity/compatibility at microscale points has also to be commented. In fact, it has to be remarked that a pointtopoint varying topology, as considered here, is an idealized situation, which is not manufacturable in practice and, therefore, it has little industrial interest.
In realistic cases, a piecewise constant topological design, so that the same topology is kept constant in specific macroscopic parts (or components), has much more interest. The extension of the here proposed approach to these cases is going to be presented in a forthcoming work.
Some other important issues remain open in the setting of computational material design. Typically: (1) manufacturability issues, related to piecewise design of the macrostructure in homogeneous components, and (2) simultaneous topological design, at the macro and micro scales. They are object of ongoing research, and they will be presented by the authors in future works.
Notes
Author's contributions
AF contributed to the theoretical developments and was in charge of the numerical implementation and validation of the results. JO conceived the work and was in charge of the theoretical formulation JC participated in the design and coordination of the work and in the assessment of the numerical results OL contributed to the general assessment of the work and to draft the manuscript. All authors read and approved the final manuscript.
Acknowledgements
The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/20072013) / ERC Grant Agreement n. 320815 (ERC Advanced Grant Project “Advanced tools for computational design of engineering materials” COMPDESMAT). Oriol LloberasValls gratefully acknowledges the funding received from the Spanish Ministry of Economy and Competitiveness through the “Juan de la Cierva” Postdoctoral Junior Grant: JCI201213782 and the National Research Plan 2014: MAT201460919R.
Competing interests
The authors declare that they have no competing interests.
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