Documents published in Scipedia

• 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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• Laminar Flow Control along the Attachment Line of a Swept Wing

  J. Methel, F. Méry, M. Forte, O. Vermeersch

  eccomas2022.

  
  

  Abstract

  Reducing aircraft fuel consumption by maximising the extent of laminar flow on wings assumes that the initial flow, along the wing's attachment line, is laminar. However, if the wing is attached to a solid wall, the wing's attachment line can be contaminated by the turbulent boundary layer developing over the solid wall for flow conditions summarised in a critical Reynolds number (R) greater than 250. Since typical R values encountered in flight can be well above 400, techniques, such as wall suction along the wing's leading edge were developed to further delay the threshold R at which contamination occurs. The present paper presents the results from an experimental investigation performed on the ONERA DTP-A model fitted with leading edge suction capabilities. The experiment was performed in the ONERA F2 wind tunnel in the framework of the EU-funded Clean Sky 2 HLFC-WIN project (LPA-IADP platform), while the suction panels were manufactured by Aernnova, an aero-component manufacturing company. Hot film measurements and infra-red thermography showed that attachment line contamination could effectively be delayed up to threshold R values of 1000 for large suction flow rates. Although panels from different manufacturing processes and with different geometric characteristics were tested, no significant difference from these parameters were observed.

  Abstract
  Reducing aircraft fuel consumption by maximising the extent of laminar flow on wings assumes that the initial flow, along the wing's attachment line, is laminar. However, [...]

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• Investigation of performance metrics in regression analysis and machine learning-based prediction models

  V. Plevris, G. Solorzano, N. Bakas, M. Ben Seghier

  eccomas2022.

  
  

  Abstract

  Performance metrics (Evaluation metrics or error metrics) are crucial components of regression analysis and machine learning-based prediction models. A performance metric can be defined as a logical and mathematical construct designed to measure how close the predicted outcome is to the actual result. A variety of performance metrics have been described and proposed in the literature. Knowledge about the metrics' properties needs to be systematized to simplify their design and use. In this work, we examine various regression related metrics (14 in total) for continuous variables, including the most widely used ones, such as the (root) mean squared error, the mean absolute error, the Pearson correlation coefficient, and the coefficient of determination, among many others. We provide their mathematical formulations, as well as a discussion on their use, their characteristics, advantages, disadvantages, and limitations, through theoretical analysis and a detailed numerical example. The 10 unitless metrics are further investigated through a numerical analysis with Monte Carlo Simulation based on (i) random guessing and (ii) the addition of random noise with various noise ratios to the predicted values. Some of the metrics show a poor or inconsistent performance, while others exhibit good performance as evaluation measures of the 'goodness of fit'. We highlight the importance of the usage of the right metrics to obtain good predictions in machine learning and regression models in general.

  Abstract
  Performance metrics (Evaluation metrics or error metrics) are crucial components of regression analysis and machine learning-based prediction models. A performance metric [...]

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• ANN-based surrogate model for predicting the lateral load capacity of RC shear walls

  G. Solorzano, V. Plevris

  eccomas2022.

  
  

  Abstract

  Reinforced concrete (RC) shear walls are often used as the main lateral-resisting component in the seismic design of buildings. They provide a large percentage of the lateral stiffness of the structure, and therefore, they may experience large shear stresses at some point under earthquake loading. Consequentially, to accurately predict their behavior, it is recommended to use detailed finite element (FE) modeling with appropriate non-linear constitutive models for concrete and steel. However, such types of simulations are challenging and could significantly increase the computational time required to obtain the analysis results. In this paper, we study the viability of creating an artificial neural-network-based surrogate model of the RC shear wall that is able to capture its nonlinear behavior and predict the results obtained with a detailed FE model, offering a much lower computational effort. For this purpose, we develop a detailed parametric non-linear FE model based on well-established practices and validated studies using the OpenSees finite element software framework. The FE model consists of multi-layer shell elements with the vertical and transverse reinforcement included as smeared rebar layers. The concrete layers implement a damage mechanism with smeared crack constitutive model, whereas the rebar layers consider a uniaxial plasticity material law. The parametric FE model is used to build a large database of RC walls of different sizes and characteristics, with their corresponding lateral load capacity that is obtained through the detailed non-linear pushover analysis. Finally, the obtained database is used to train and validate the ANN-surrogate model. The developed model is able to accurately predict the lateral load capacity of RC shear walls without the need of detailed FE modeling, thus drastically reducing the complexity and the computational time required for the numerical solution and providing a reliable and robust analysis alternative, with only small compromise of accuracy.

  Abstract
  Reinforced concrete (RC) shear walls are often used as the main lateral-resisting component in the seismic design of buildings. They provide a large percentage of the lateral [...]

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• Stochastic Simulation of the Head Impact on Windscreens

  C. Brokmann, C. Alter, S. Kolling

  eccomas2022.

  
  

  Abstract

  In accidents involving cars with pedestrians, the impact of the head on structural parts of the vehicle takes a significant risk of injury. If the head hits the windscreen, the injury is highly influenced by glass fracture. In pedestrian protection tests, a head impactor is shot on the windscreen while the resultant acceleration at the COG of the head is measured. To assess the risk of fatal or serious injury, a head injury criterion (HIC) as an explicit function of the measured acceleration can be determined. The braking strength of glass which has a major impact on the head acceleration, however, is not deterministic but depends on production-related micro-cracks on the glass surface as well as on the loading rate. The aim of the present paper is to show a pragmatic method, how to include the stochastic failure of glass in crash and impact simulations. The methodology includes a fracture mechanical model for the strain rate-dependent failure of glass, the experimental determination of the glass strength for the different areas of a windscreen (surface, edge, and screen-printing area), the statistical evaluation of the experimental data and the computation of a HIC probability distribution by stochastic simulation.

  Abstract
  In accidents involving cars with pedestrians, the impact of the head on structural parts of the vehicle takes a significant risk of injury. If the head hits the windscreen, [...]

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