Documents published in Scipedia

• A thermo-coupled constitutive model for semi-crystalline polymers at finite strains: Application to varying degrees of crystallinity and temperatures

  M. Reuvers, B. Boes, S. Felder, T. Brepols, S. Reese

  eccomas2022.

  
  

  Abstract

  Thermoplastic materials are widely used for thermoforming and injection moulding processes, since their low density in combination with a high strength to mass ratio are interesting for various industrial applications. Semi-crystalline polymers make up a subcategory of thermoplastics, which partly crystallize after cool-down from the molten state. During the thermoforming process, residual stresses can arise, due to complex material behavior under different temperatures and strain rates. Therefore, computational models are needed to predict the material response and minimize production errors. This work presents a thermomechanically consistent phenomenological material formulation at finite strains, based on [1]. In order to account for the highly nonlinear material behavior, elasto-plastic and visco-elastic contributions are combined in the model formulation. To account for the crystalline regions, a hyperelastic-plastic framework is chosen, based on [2, 3]. Kinematic hardening of Arruda-Boyce form is incorporated in the formulation, as well as associated plastic flow. The material parameters depend on both, the temperature as well as the degree of crystallinity. A comparison to experiments with varying degrees of crystallinity and temperatures is presented, where a special blending technique ensures stable crystallinity conditions.

  Abstract
  Thermoplastic materials are widely used for thermoforming and injection moulding processes, since their low density in combination with a high strength to mass ratio are interesting [...]

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• Finite element analyses of shear yielding and crazing in glassy polymers under cyclic mode I loading

  T. Laschuetza, T. Seelig

  eccomas2022.

  
  

  Abstract

  A simple and yet physically motivated continuum-micromechanical model for crazing is developed, focussing on cyclic loading. The model features fibril drawing and fibril creep deformation, loose hanging fibrils upon unloading and the morphology change fibrils undergo between craze initiation up to a fully developed craze. The crazing model is implemented in a user material subroutine in the commercial finite element programme ABAQUS. The performance is investigated on a mode I crack growth boundary value problem under cyclic loading. Experimentally measured craze/crack opening profiles from the literature are reasonably-well captured by the model. The results exhibit further interesting model characteristics, such as a variation of the craze length in the course of a load cycle.

  Abstract
  A simple and yet physically motivated continuum-micromechanical model for crazing is developed, focussing on cyclic loading. The model features fibril drawing and fibril creep [...]

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• Immersed Boundary Analysis of Models with Internal State Variables: Applications to Hydrogels

  J. Barrera Cruz, K. Maute

  eccomas2022.

  
  

  Abstract

  Research on Soft Active Materials (SAMs) has flourished in recent years driven mainly by potential applications to actuation systems, tissue engineering, and soft robotics. These applications benefit from the unique properties of SAMs such as large deformations, a wide range of stimulants, and high motion complexities. Hydrogels are among the dominant members of SAMs. Their highly nonlinear chemo-mechanical transient behavior is described by equations that include rates of internal state variables representing the local swelling state of the gel. Hence, the simulation of hydrogels requires intricate numerical approaches with stabilization schemes. This paper presents an immersed boundary analysis technique to simulate models with internal state variables. A hydrogel model is used as an example to describe the components of the proposed technique. Level sets define the material layout on a fixed background mesh and a generalized version of the extended finite element method predicts the response. The influence of the internal state variables on the stability of the physical analysis is examined. While focusing on an XFEM approach for hydrogels, the presented theory can be extrapolated to similar applications using models with internal state variables (e.g., shape memory polymers) and other immersed boundary analysis technique (e.g., CutFEM).

  Abstract
  Research on Soft Active Materials (SAMs) has flourished in recent years driven mainly by potential applications to actuation systems, tissue engineering, and soft robotics. [...]

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• Simple modeling for stiffness evaluation of bolted joints using interfacial element

  Y. Kishimoto, Y. Kobayashi

  eccomas2022.

  
  

  Abstract

  This study has developed a novel finite element named interfacial element which simulates the contact between microscale asperities at contact surfaces of bolted joints. In this element, the contact is assumed to be the Hertzian contact of elastic asperities whose peak heights obey the Gaussian distribution. Based on this assumption, the stiffness of the interfacial element is derived from the compressive stress and the surface texture of the interfaces. On the other hand, it is necessary for large-scale simulations that target the entire vehicle body to reduce the number of nodes and elements in the finite element models. This study has further proposed simple modeling for stiffness evaluation of bolted joints using the interfacial element. Finite element simulations by simplified models in which heads, axes and holes of bolts were ignored were conducted and compared with detailed models and hammering tests. The results revealed that the mean value of the natural frequency of the simplified models had good agreement with that of the detailed models and the hammering tests though the calculation accuracy of the simplified models were lower than the detailed models. The bolt heads and the nuts could be ignored by increasing the density of the bolt axes to be equal to the total weight.

  Abstract
  This study has developed a novel finite element named interfacial element which simulates the contact between microscale asperities at contact surfaces of bolted joints. In [...]

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• Development of a FE² Multiscale Model of Chloride Ions Transport in Recycled Aggregates Concrete

  A. Fanara, L. Courard, F. Collin

  eccomas2022.

  
  

  Abstract

  Decreasing CO2 emissions and preserving natural resources are necessary to the well-being of our civilisations. In the construction industry, recycling old concrete members could be part of the solution to reach theses objectives. Recycled Concrete Aggregates (RCA), obtained by crushing of demolished concrete structures, can substitute the Natural Aggregates (NA) inside the so-called Recycled Aggregates Concrete (RAC). The durability of RAC is not guaranteed in the current state of research. RCA are indeed composed of natural aggregates partially embedded in an adherent mortar paste, increasing the porosity and water absorption of RAC. This research aims to better predict the influence of RCA on chloride ions ingress inside concrete. It started with an experimental phase where multiple experiments have been performed to determine the transfer properties and the chloride ions diffusion coefficients of a mortar paste and concretes made from NA or 100% RCA. In this context, the microstructure of the RCA influences deeply the permeability, water content distribution and chloride ingress. Therefore, these properties must be included into a numerical model that integrates the microstructural information in a proper way. A numerical homogenization technique, based on the Finite Element square (FE2 ) method [5, 13], is implemented into a coupled multiscale model of water flows and advection/diffusion of chlorides in saturated concrete, in order to model the complex flow behaviour encountered.

  Abstract
  Decreasing CO2 emissions and preserving natural resources are necessary to the well-being of our civilisations. In the construction industry, recycling old concrete members [...]

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• Probabilistic modeling of LCF failure times using a epidemiological crack percolation model

  M. Harder, P. Lion, L. Mäde, T. Beck, H. Gottschalk

  eccomas2022.

  
  

  Abstract

  The analysis of standardized low cycle fatigue (LCF) experiments shows that the failure times widely scatter. Furthermore, mechanical components often fail before the deterministic failure time is reached. A possibility to overcome these problems is to consider probabilistic failure times. Our approach for probabilistic life prediction is based on the microstructure of the metal. Since we focus on nickel-base alloys we consider a coarse grained microstructure, with random oriented FCC grains. This leads to random distributed Schmid factors and different anisotropic stress in each grain. To gain crack initiation times, we use Coffin-MansonBasquin and Ramberg-Osgood equation on stresses corrected with probabilistic Schmid factors. Using these single grain crack initiation times, we have developed an epidemiological crack growth model over multiple grains. In this mesoscopic crack percolation model, cracked grains induce a stress increase in neighboring grains. This stress increase is realized using a machine learning model trained on data generated from finite element simulations. The resulting crack clusters are evaluated with a failure criterion based on a multimodal stress intensity factor. From the generated failure times, we calculate surface dependent hazard rates using a Monte Carlo framework. We compare the obtained failure time distributions to data from LCF experiments and find good coincidence of predicted and measured scatter bands.

  Abstract
  The analysis of standardized low cycle fatigue (LCF) experiments shows that the failure times widely scatter. Furthermore, mechanical components often fail before the deterministic [...]

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• Modeling imperfect interfaces in layered beams through multi- and single-variable zigzag kinematics

  R. Massabò, I. Monetto

  eccomas2022.

  
  

  Abstract

  Multiscale structural models based on the coupling of a zigzag kinematics and a cohesive crack approach have been recently formulated to analyze the response of shear deformable layered structures with imperfect interfaces and describe progressive delamination fracture (Massabò, in Handbook of Damage Mechanics, Springer, 2022, pp.665-698). The zigzag kinematics accounts for zigzag effects associated to the elastic mismatch of the layers and displacement jumps due to interfacial imperfections, using a limited number of variables, which is independent of the number of layers. The effects of imperfect interfaces on the response of structures subjected to thermo-mechanical loading and on wave propagation and dispersion have been studied analytically and the advantages of this approach over discrete layer models and layerwise theories have been highlighted and discussed. In the presentation we review and discuss these models and present preliminary results on novel single-variable formulations, which have been inspired by a technique developed for homogeneous Timoshenko beams in (Kiendl et al.

  Abstract
  Multiscale structural models based on the coupling of a zigzag kinematics and a cohesive crack approach have been recently formulated to analyze the response of shear deformable [...]

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• Application of an adaptive stable GFEM for fracture propagation in plain concrete

  A. El-Tohfa, F. Mukhtar

  eccomas2022.

  
  

  Abstract

  Generalized finite element method (GFEM) has proven itself as a tool of choice over the conventional FEM in fracture analysis due to enhanced computational efficiency as well as allowing cracks to propagate independently of the domain mesh ­ Thanks to the use of enrichments chosen based on the a priori knowledge of the solution behavior. With the many versions of the method's formulations in the literature, their stability issues, compared to the standard FEM, are often unresolved. This paper presents the use of an adaptive stable GFEM to plain concrete fracture propagation. Having verified the formulation's accuracy and stability based on the Linear Elastic Fracture Mechanics in previous studies and its two-scale (global-local) version on concrete fracture, the present work seeks to verify its capabilities in capturing the size effect behavior in concrete. A set of fracture simulations in geometrically similar experimental concrete beams, under a 3-point bending regime, is presented based on a bilinear cohesive model. In addition to the GFEM's agreement with the experimental load-displacement response and the effect of the initial notch-to-depth ratio, the simulation successfully captures the size effect behavior when presented on the popular Type II Size Effect plot ­ the so-called Bazant's law.

  Abstract
  Generalized finite element method (GFEM) has proven itself as a tool of choice over the conventional FEM in fracture analysis due to enhanced computational efficiency as well [...]

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• Inverse Dynamics of Geometrically Exact Beams

  T. Ströhle, P. Betsch

  eccomas2022.

  
  

  Abstract

  This paper is concerned with the inverse dynamics of flexible mechanical systems whose motion is governed by quasi-linear hyperbolic partial differential equations. Problems that appear by applying classical solution strategies to the problem at hand, e.g. integrating the problem at hand sequentially in space and time will be adressed in this work. Motivated by the hyperbolic structure of the underlying initial boundary value problem, two methods that are based on a simultaneous space-time integration will be presented. Special emphasize will be given to the phenomena of wave propagation within geometrically exact beams and its relevance regarding the inverse dynamics problem.

  Abstract
  This paper is concerned with the inverse dynamics of flexible mechanical systems whose motion is governed by quasi-linear hyperbolic partial differential equations. Problems [...]

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• Influence of Cooling Lubricants on the Interaction Between Indenter and Material Surface During Scratch Tests

  F. Kästner, K. de Payrebrune

  eccomas2022.

  
  

  Abstract

  Striving for the optimization and the increase of efficiency of various systems demands further developments of the classic manufacturing methods. Especially grinding processes, which are characterized by undefined cutting-edge geometries, reveal many fields where there still is a lack of understanding. In particular, the processes at and their effects on the individual abrasive grit are insufficiently researched and, therefore, do not allow sufficiently accurate behavior predictions. In order to optimize grinding processes and, ultimately, the resulting quality of the workpiece surface, it is necessary to look at the entire process in a holistic way. Due to the large number of influences to which the grinding process is subject, it is initially advisable to break down the process as far as possible into individual scratch tests and then gradually return to the overall process. One approach is the development and expansion of an FEM-based physical force model, which allows for the simulation and prediction of a scratch tests and, subsequently, also the entire grinding process with all relevant influencing factors. One of these influencing factors, which are essential but mostly unconsidered, are cooling lubricants, especially their tribologically favorable influence on the interaction between workpiece and indenter. Therefore, it is important to identify and investigate the different aspects, such as the friction phenomena of scratch tests that are influenced by the use of cooling lubricants. In addition to temperature and force characteristics, which have been found to differ with and without cooling lubricant, differences in the scratch geometry on the material surface have also been observed in recent tests. Based on these findings, this work examines the relationship between scratch geometry and cooling lubricant. It turned out that scratch tests conducted with cooling lubricants have an influence on the topography of the scratch on the workpiece surface in addition to the influence on the tangential and normal forces. The ratio of scratch width to scratch depth is used for evaluation. A reduction of this ratio is observerd in the scratches with cooling lubricants and is, therefore, interpreted as a reduction of the scratch width as a result of the use of cooling lubricants.

  Abstract
  Striving for the optimization and the increase of efficiency of various systems demands further developments of the classic manufacturing methods. Especially grinding processes, [...]

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