Documents published in Scipedia

• Two-scale asymptotic homogenization in a MEMS auxetic structure for over etch identification

  D. Faraci, A. Nastro, V. Zega, C. Comi

  eccomas2022.

  
  

  Abstract

  The development and optimization of Micro electro-mechanical systems (MEMS) devices, due to their small size scale, require testing and precise characterization. As an example, over etch, which is the deviation between the designed masks and the effective dimensions of the suspended parts, strongly influences the performances of MEMS; therefore, to predict the correct functioning of the device its actual value must be carefully identified. In this work, we propose an efficient, time-saving tool to identify fabrication imperfections in MEMS devices. In particular, we replace the complex geometry of a MEMS mechanical filter with an equivalent homogeneous medium, whose linear-elastic effective properties are evaluated employing twoscale asymptotic homogenization and we identify the over etch by minimizing the relative error between experimental data and corresponding predictions obtained for different combinations of over etch.

  Abstract
  The development and optimization of Micro electro-mechanical systems (MEMS) devices, due to their small size scale, require testing and precise characterization. As an example, [...]

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• Development of a 3D visco-elastic model for wood under large deformations

  R. Lazo-Molina, C. Guzmán, J. Pina, E. Saavedra Flores, S. Yanez

  eccomas2022.

  
  

  Abstract

  Some of the most frequently observed phenomena in structural materials are creep and relaxation. Both are associated with time-dependent behavior and dissipative rheological variables. In the case of wood, long-term creep can produce excessive deformation and instability problems by magnifying short-term deflections. Also, wood presents changes in its mechanical properties due to its hygroscopy, so that moisture appears as an important parameter to be considered. A priori, it is known that the moisture content in wood cells and the angle of microfibrils are parameters that directly affect the overall cell stiffness. With this information, material physics hypotheses can be elaborated to develop a constitutive model in large deformations to predict phenomena such as creep and relaxation in the medium and/or long term. The microstructure of the cell wall can be represented through a model of fiber-reinforced composite material originally developed for biomaterials such as arteries and fibrous tissues. Where the anisotropic character is conferred by the distribution of the fibers within the isotropic matrix of the material. This work aims to adapt these models to represent the mechanical behavior of the wood cell with faithful representation of its microstructure. FEniCS is used for the numerical implementation of the material. In this paper the validations, current status, conclusions, and perspectives of this research are presented.

  Abstract
  Some of the most frequently observed phenomena in structural materials are creep and relaxation. Both are associated with time-dependent behavior and dissipative rheological [...]

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• Simulation of soft robots with nonlinear material behavior using the cosserat rod theory

  M. Grube, R. Seifried

  eccomas2022.

  
  

  Abstract

  For the simulation and model-based control of soft robots accurate models are required. In this contribution the simulation of a simple soft robot with the cosserat rod theory is examined for both linear and nonlinear material models. The soft robot is fabricated out of silicone. Thereby stiffness and damping properties are investigated. In addition to the achievable accuracy, the computation time is also examined.

  Abstract
  For the simulation and model-based control of soft robots accurate models are required. In this contribution the simulation of a simple soft robot with the cosserat rod theory [...]

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• Architectured and additively manufactured double-negative index metamaterials

  C. Almeida, J. Cardoso, P. Coelho, A. Velhinho, J. Xavier, J. Borges

  eccomas2022.

  
  

  Abstract

  Developing a new generation of multifunctional metamaterials with unusual thermoelastic properties enables a wide range of industrial applications, particularly in the aerospace industry. However, obtaining metamaterials with target properties by the systematic design of their microstructure and architecture remains a major challenge to this day. Topology Optimization (TO) is a powerful tool that can be used to develop the so-called anepectic metamaterials that combine both negative Poisson's Ratio (NPR) and negative thermal expansion (NTE). Here, an overview of the existing contributions in the literature regarding such metamaterials is presented. A Finite Element (FE) model for an anepectic microstructure is presented here for the purpose of simulating in silico the experimental results obtained in previous works. It is noted that scarce contributions resort to TO to design such metamaterials and even fewer present experimental validation. The present work presents a state of the art of anepectic metamaterials and emphasizes thus the importance of the engineering-cycle completion, i.e., starting with the systematic and optimal design of metamaterials and ending up in prototype fabrication and its verification.

  Abstract
  Developing a new generation of multifunctional metamaterials with unusual thermoelastic properties enables a wide range of industrial applications, particularly in the aerospace [...]

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• A gradient-extended anisotropic damage-plasticity model in the logarithmic strain space

  H. Holthusen, T. Brepols, J. Simon, S. Reese

  eccomas2022.

  
  

  Abstract

  Within this contribution, we discuss additional theoretical as well as numerical aspects of the material model developed in [1, 2], where a `two-surface' damage-plasticity model is proposed accounting for induced damage anisotropy by means of a second order damage tensor. The constitutive framework is stated in terms of logarithmic strain measures, while the total strain is additively decomposed into elastic and plastic parts. Moreover, a novel gradientextension based on the damage tensor's invariants is presented using the micromorphic approach introduced in [3]. Finally, going beyond the numerical examples presented in [1, 2], we study the model's ability to cure mesh-dependency in a three-dimensional setup.

  Abstract
  Within this contribution, we discuss additional theoretical as well as numerical aspects of the material model developed in [1, 2], where a `two-surface' damage-plasticity [...]

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• Dynamic Response of Offshore Wind Turbines under Nonlinear Irregular Ocean Waves

  H. Pezeshki, D. Pavlou, S. C. Siriwardane

  eccomas2022.

  
  

  Abstract

  Offshore wind turbines (OWT) are exposed to different categories of ocean waves during their lifetime. Most ocean waves are categorized in the second-order nonlinear theory in normal and severe sea states, and their spectra combine different heights and frequencies. In the present study, a model for the dynamic response of OWTs to the wave load obtained irregular second-order nonlinear ocean waves is proposed. The foundation-wave-structure interaction and the effect of the nacelle-rotor assembly are simulated. Numerical results are provided and discussed.

  Abstract
  Offshore wind turbines (OWT) are exposed to different categories of ocean waves during their lifetime. Most ocean waves are categorized in the second-order nonlinear [...]

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• Wave Based Method for 2D Unsaturated Elastodynamic Soil under Harmonic Loading

  M. Lainer, G. Müller

  eccomas2022.

  
  

  Abstract

  The Wave Based Method (WBM), which describes field variables of a boundary value problem with weighted wave functions, is used to model a layered halfspace under harmonic loading. This 2D soil structure is investigated for partially saturated layers. According to the Berryman-Thigpen-Chin model (BTC model) the capillary effects between the water and air phases enclosed by pores are negligibly small for elastic waves in the low frequency range. The mixture of water and air is treated as one fluid, for which the material parameters used in Biot's theory for fully saturated poroelastic structures have to be computed. Within this work, the BTC model is applied to describe soil layers within a halfspace, which are not perfectly dry or fully saturated. It is presented how the degree of saturation within a partially saturated soil layer affects the system's response.

  Abstract
  The Wave Based Method (WBM), which describes field variables of a boundary value problem with weighted wave functions, is used to model a layered halfspace under harmonic [...]

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• S-N-based Fatigue Damage Modelling of Offshore Structures

  F. Bjørheim, S. C. Siriwardane, D. Pavlou

  eccomas2022.

  
  

  Abstract

  Recently proposed damage models are presented and compared. In addition, a deterministic algorithm for nonlinear fatigue damage monitoring is presented and discussed. Furthermore, the commonly adopted functional forms for damage modelling is proposed and the adequacy of their functional form is directly investigated by the help of experimental data. Thereafter, the accuracy and whether the presented models give conservative estimates in comparison to Miner’s rule are checked with experimental data. It is found that the proposed models generally perform better for various materials commonly subjected to fatigue. Finally, it is discussed how both the theory and deterministic algorithms now exist for both adopting nonlinear functions in design if the expected loading sequence can be determined, whereas it can always be adopted for fatigue based structural health monitoring.

  Abstract
  Recently proposed damage models are presented and compared. In addition, a deterministic algorithm for nonlinear fatigue damage monitoring is presented and discussed. Furthermore, [...]

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• Influences of the geometrical nonlinearity on the complex band structures of periodic lattice frame structures

  M. Mellmann, B. Ankay, E. Perras, C. Zhang

  eccomas2022.

  
  

  Abstract

  In this study, the complex band structures of geometrically nonlinear periodic frame structures are calculated by using the Spectral Element Method (SEM). For this purpose, the spectral element matrix for a geometrically nonlinear beam element is derived. By solving the inverse eigenvalue problem for computing the complex dispersion curves k() instead of the conventional eigenvalue problem for calculating the real dispersion curves (k), the complex wave vector can be obtained, whose imaginary parts describes the evanescent behavior of the Bloch waves. Subsequently, the geometrically nonlinear effects on the evanescent behavior of the Bloch waves are investigated by evaluating the dispersion curves and the transmission spectra.

  Abstract
  In this study, the complex band structures of geometrically nonlinear periodic frame structures are calculated by using the Spectral Element Method (SEM). For this purpose, [...]

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• A variational-based mixed finite element formulation for liquid crystal elastomers

  M. Gross, J. Dietzsch, F. Concas

  eccomas2022.

  
  

  Abstract

  Liquid crystal elastomers (LCEs) are soft materials, which are capable of large deformations induced by temperature changes and ultraviolet irradiation [1]. Since many years, these materials are under investigation in experimental researches as actuator materials. LCEs arise from a nematic polymer melt, consisting of long and flexible polymer chains as well as oriented and rigid rod-like molecules, the so-called mesogens, by crosslinking. In order to numerically simulate LCE materials by using the finite element method, a continuum model is necessary, including in a thermo-viscoelastic material formulation of the polymer chains the orientation effects of the mesogens. This can be performed by introducing a normalized direction vector as an independent field, and deriving from additional (orientational) balance laws independent differential equations [2]. These differential equations describe the independent rotation of the rigid mesogens connected with the flexible polymer chains. The orientation-dependent stress law of LCEs arises from an anisotropic free energy, comparable with fibre-reinforced materials. But, in contrast to fibre-reinforced materials, the direction vector of a LCE model has to be independent. In contrast to [2], we apply a variational principle for deriving a new mixed finite element formulation, which is based on drilling degrees of freedom for describing the mesogens rotation [3]. This principle leads to an extended set of balance laws.

  Abstract
  Liquid crystal elastomers (LCEs) are soft materials, which are capable of large deformations induced by temperature changes and ultraviolet irradiation [1]. Since many years, [...]

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