Explicit Dynamics Simulations Research Articles

Cell Model Approaches for Predicting the Swelling and Mechanical Properties of Polyelectrolyte Gels

We present two successive mean-field approximations for describing the mechanical properties and the swelling equilibrium of polyelectrolyte gels in contact with a salt solution. The first mean-field approximation reduces the many-chain problem of a gel to a corresponding single chain problem. The second mean-field step integrates out the degrees of freedom of the flexible chain and the ions. It replaces the particle-based description of the polyelectrolyte with suitable charge distributions and an effective elasticity term. These simplifications result in a computationally very efficient Poisson-Boltzmann cell-gel description. Despite their simplicity, the single chain cell-gel model shows excellent and the PB model very good agreement with explicit molecular dynamics simulations of the reference periodic monodisperse network model for varying chain length, polymer charge fraction, and external reservoir salt concentrations. Comparisons of our models to the Katchalsky model reveal that our approach is superior for strongly charged chains and can also predict the bulk moduli more accurately. We further discuss chain length polydispersity effects, investigate changes in the solvent permittivity, and demonstrate the robustness of our approach to parameter variations coming from several modeling assumptions.

A new challenge in the DEM/FEM simulation of the shot peening process: The residual stress field at a sharp edge

Shot peening (SP) is used to enhance the fatigue life of metallic components. The knowledge of the resulting residual stress (RS) field in the vicinity of geometrical features working as stress raisers is essential to fully integrate SP into the design stage, but is of very difficult achievement, both from a numerical and experimental standpoint. Finite elements (FE) proved to be a powerful tool for tuning the SP process and predicting the RSs, and Discrete element (DE) simulations can be used to study the dynamics of peening particles, especially in the vicinity of geometrical features. A synergistic approach (DEM-FEM) can be effective to study the effects of SP near edges or notches. This paper explores the potentiality of DEM-FEM to simulate SP on an edge. The micro-SP treatment with ceramic beads on the aluminum alloy Al-7075-T651 is considered. DE simulations are devised to study the steady state of the SP nozzle and to predict the impact velocity of the shots when the nozzle approaches and turns around the edge. These data are used to set up explicit dynamic FE simulations of the SP in the immediate vicinity of the edge, keeping into account the stochastic nature of the SP process. The resulting RS field is compared to the RS field computed using thermal fields in a FEM simulation, calibrated with experimental data, from XRD analysis.

The in-plane dynamic crushing of quadrilateral chiral honeycomb coatings

For the anti-impact performance of hyperelastic coatings covered on the ship hull, the in-plane dynamic crushing behaviors of quadrilateral chiral honeycomb coatings (including chiral and anti-chiral) were numerically studied by explicit dynamic finite element simulations. The deformation mode, energy absorption and supporting reaction force under different impact speeds have been analyzed. The results show that: (1) If the impact speed is low, the chiral honeycomb coatings display the rotational deformation; (2) With the increase of impact speed, the deformation at the top of coating is remarkable; (3) The response of energy absorption and supporting reaction force corresponds to the macroscopic deformation mode of chiral honeycomb coatings.

Simulation of Structural Applications and Sheet Metal Forming Processes Based on Quadratic Solid–Shell Elements with Explicit Dynamic Formulation

In this work, nonlinear dynamic analysis of thin structures is investigated using quadratic solid–shell (SHB-EXP) elements. The proposed SHB-EXP elements are based on a fully three-dimensional formulation using an in-plane reduced-integration scheme along with the assumed-strain method in order to alleviate most locking phenomena. These developments consist of a 20-node hexahedral element, denoted SHB20-EXP, and its 15-node prismatic counterpart, denoted SHB15-EXP. The formulation of these elements is combined with fully three-dimensional behavior models, including elastic behavior as well as anisotropic plastic behavior for metallic materials. The resulting formulations are implemented into the ABAQUS explicit/dynamic software package in the framework of large displacements and rotations. First, to assess the performance of the SHB-EXP elements, four representative nonlinear dynamic benchmark tests have been conducted. Then, impact/crash problem and deep drawing of cylindrical cup have been performed to demonstrate the capabilities of the SHB-EXP elements in handling various types of nonlinearities (large strains, anisotropic plasticity, and double-sided contact). Comparisons with results obtained by ABAQUS elements as well as with reference solutions taken from the literature show the good capabilities of the developed quadratic SHB-EXP elements for the explicit dynamic simulation of thin structures.

Explicit Dynamics Simulation of High-Speed Railway Bearing Based On ANSYS/LS-DYNA

A solid model of a high-speed train axle box bearing was built by using 3D modeling software. On the platform of ANSYS/LS-DYNA, the combined dynamic and finite element analysis method is used to apply the combined radial and axial loads to the high-speed rail bearing. In order to demonstrate the dynamic contact characteristics of the bearing during operation, two operating conditions were designed. One is running at a uniform speed in a straight line, the other is running at an assumed speed with a minimum turning radius. The results show that the stress distribution, velocity and acceleration of bearing components under two different working conditions are obviously different. The research can provide reference for the design, selection and optimization of domestic high-speed rail bearings.

Design and Testing of an Arc Resistant Power Transformer Tank

This paper presents the design and testing of a full-scale 330-kV, 210-MVA transformer tank designed to withstand the pressure effects of a 20 MJ, three-cycle low-impedance fault in oil, and fail at its cover in a safe and controlled manner above that amount of energy. The transformer tank, equipped with a mockup active part, was designed for arc resistance using nonlinear static finite-element analysis and a specific rupture criterion. It was then tested using an experimental methodology based on injecting pressurized air into the water-filled tank. To adequately reproduce the effects of an arc in oil, the test parameters were determined using explicit dynamic simulations. The transformer tank performed as predicted by the finite-element and explicit dynamic simulations. The energy levels used in the tests here are, to our knowledge, the largest ever reported. This study demonstrates that it is feasible to design a power transformer to safely withstand high-energy low-impedance faults of short duration.

FEM explicit dynamics simulation and NIR hyperspectral reflectance imaging for determination of impact bruises of Lycium barbarum L.

A 3D scanner was used to reverse engineer a model to assess impact bruising of fresh Goji (Lycium barbarum L.) fruit. Based on the results of physical compression tests, an elastic-plastic model was selected as the material mechanics model for the fruit. A finite element method explicit dynamics simulation was performed to predict impact bruising using Abaqus software. Simulation results showed that fruit were not damaged at a drop height of 0.2-0.5 m, an impact angle of 10–30 °, and with an impact material of wood board, foam board, and nylon board. Using response surface methodology, the effects of drop height, impact material, and impact angle on the bruise rate of fruit were determined. The optimized parameters were a drop height of 0.24 m, foam board as the impact material, and an impact angle of 13.11°. Near infrared hyperspectral reflectance imaging was also applied to this problem, and visualization models were established in Matlab. The field experiment showed a bruise rate was 9.8%. These results provide a theoretical basis for future studies of harvesting, transporting, sorting, grading, and packaging machines designed for use with L. barbarum.

Mechanical collision simulation of potato tubers

AbstractThis paper presents the results of an investigation on internal stress progression and the explicit dynamics simulation of the bruising behavior of potato tubers under dynamic mechanical collision. Physical measurements, mechanical tests, advanced solid modeling, and engineering simulation techniques were utilized in the study. The tuber samples used in the simulation were reverse engineered and finite element analysis (FEA) was set up to simulate the collision‐based bruising behavior of the potato tubers. The total number of identical tuber models used in the simulation was 17. The numerical data of the FEA results revealed useful stress distribution and mechanical behavior visuals. These results are presented in a frame that can be used to describe bruise susceptibility value on potato‐like agricultural crops. The modulus of elasticity was calculated from compression test data as 3.12 MPa. Structural stresses of 1.40 and 3.13 MPa on the impacting (hitting) and impacted (hit) tubers (respectively) were obtained. These stress values indicate that bruising is likely to occur on the tubers. This research paper provides a useful how‐to‐do strategy to further research on complicated bruising investigations of solid‐like agricultural products through advanced engineering simulation techniques.Practical applicationsThis research aims to simulate realistic dynamic deformation of potato tubers during mechanical collision, which is very hard to achieve through physical or analytical expressions. This is attractive because related food processing industries have shown their interest in determining the physical properties and bruising behavior of food/agricultural products using experimental, numerical, and engineering simulation methods so that it can be used in their food processing technology. Very limited data have been found available in the literature about the subject of FEM‐based explicit dynamics simulation of solid‐like agricultural crops such as the self‐collision case of potato tubers (which is very important for indoor or outdoor potato processing). Comparative investigations on determination of modulus of elasticity are very limited as well. Most of the research focused on single calculation theory and linear static loading assumption‐based FEM simulation solutions. Here, we report a “how‐to‐do” case study for dynamic self‐collision simulation of potato tubers.

Methionine Oxidation Changes the Mechanism of A\u03b2 Peptide Binding to the DMPC Bilayer

Using all-atom explicit solvent replica exchange molecular dynamics simulations with solute tempering, we study the effect of methionine oxidation on Aβ10–40 peptide binding to the zwitterionic DMPC bilayer. By comparing oxidized and reduced peptides, we identified changes in the binding mechanism caused by this modification. First, Met35 oxidation unravels C-terminal helix in the bound peptides. Second, oxidation destabilizes intrapeptide interactions and expands bound peptides. We explain these outcomes by the loss of amphiphilic character of the C-terminal helix due to oxidation. Third, oxidation “polarizes” Aβ binding to the DMPC bilayer by strengthening the interactions of the C-terminus with lipids while largely releasing the rest of the peptide from bilayer. Fourth, in contrast to the wild-type peptide, oxidized Aβ induces significantly smaller bilayer thinning and drop in lipid density within the binding footprint. These observations are the consequence of mixing oxidized peptide amino acids with lipids promoted by enhanced Aβ conformational fluctuations. Fifth, methionine oxidation reduces the affinity of Aβ binding to the DMPC bilayer by disrupting favorable intrapeptide interactions upon binding, which offset the gains from better hydration. Reduced binding affinity of the oxidized Aβ may represent the molecular basis for its reduced cytotoxicity.

Structural polymorphism in actin filaments modulates gelsolin binding

The conformational states of actin filaments determine their biological functions and numerous actin binding proteins tightly control filament structure. Gelsolin is an actin regulatory protein that remodels actin cytoskeleton by severing and capping the barbed‐end of an actin filament. A recent study demonstrates that actin filaments exist in multiple conformations with varying tilted states, suggesting structural polymorphism in the filament. Yet, it is unknown how polymorphic nature of actin filament structure affects gelsolin binding. Here we investigated the interactions between the segment 1 domain of gelsolin and filaments with distinct structural states identified in the recent experimental study, utilizing molecular docking and the explicit molecular dynamics (MD) simulations. Our simulation results show that the filaments with different tilted states influence the binding modes of gelsolin segment‐1 domain. Analysis on buried area and the number of hydrogen bonds in the actin‐binding interface indicate that distinct filament structure results in different binding affinities of gelsolin. Distributions of salt‐bridge interactions between actin subunit and gelsolin may explain molecular basis of the filament structure‐dependent gelsolin binding affinities. This study suggests that structural polymorphism in actin filaments may play an important role in modulating interactions between actin filaments and binding proteins.Support or Funding InformationThis study was supported by the UCF start‐up fund and In‐House grant for Hyeran Kang. We acknowledge the computational time from UCF stokes cluster.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Molecular Crowding Modulates Actin Filament Mechanics and Structure

The cellular environment is crowded with high concentrations of macromolecules that significantly reduce the accessible volume limiting biomolecule interactions. Reductions in cellular volume can generate depletion forces that affect protein assembly and stability. The mechanical and structural properties of actin filaments play critical roles in various cellular functions including structural support, cell movement, division, and intracellular transport. Although the effects of molecular crowding on actin polymerization have been shown, how crowded environments affect filament conformations, dynamics, and mechanical properties is unknown. In this study, we investigate the effects of solution crowding on the modulations of filament mechanics and structure both in vitro and in silico. Direct visualization of filaments in the presence of crowding agents is achieved by fluorescence microscopy imaging, allowing for the quantification of filament thermal bending dynamics and mechanics. Biophysical analysis indicate that molecular crowding modulates thermal fluctuations, enhances filament's effective bending stiffness, and reduces average filament lengths. Utilizing the explicit molecular dynamics simulations, we demonstrate that molecular crowding alters filament conformations and structural properties indicating overall compaction and stabilization that are directly coupled to filament mechanics. Taken together, our study suggests the interplay between excluded volume effects and non‐specific interactions raised from molecular crowding may modulate actin filament mechanics and structure.Support or Funding InformationThis study was supported by the UCF start‐up fund and In‐House grant for H.K. The authors would like to thank the National Science Foundation for support of this work through REU site EEC 1560007. We acknowledge the computational time from UCF stokes cluster.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Elastic Wave Propagation in Open-Cell Foams

This work examines elastic wave propagation phenomena in open-cell foams with the use of the Bloch wave method and finite element analysis. Random foam topologies are generated with the Surface Evolver and subsequently meshed with Timoshenko beam elements, creating open-cell foam models. Convergence studies on band diagrams of different domain sizes indicate that a representative volume element (RVE) consists of at least 83 cells. Wave directionality and energy flow features are investigated by extracting phase and group velocity plots. Explicit dynamic simulations are performed on finite size domains of the considered foam structure to validate the RVE results. The effect of topological disorder is studied in detail, and excellent agreement is found between the wave behavior of the random foam and that of both the regular and perturbed Kelvin foams in the low-frequency regime. In higher modes and frequencies, however, as the wavelengths become smaller, disorder has a significant effect and the deviation between regular and random foam increases significantly.

Simplified FEM modelling for the collapse assessment of a masonry vault

This study is motivated from the collapse of an old masonry building in the Southern Italy. FEM analyses are carried out focusing on the influence of the contrasting wall on the stability of the vault. In the analyses, the structure is subjected to a damage scenario on the contrasting wall due to a demolition project, and the consequence of the damage is evaluated using the explicit dynamic simulation made by Ls-Dyna®. A micro modelling technique (discrete FEM model) is adopted to model the masonry: the mortar is modelled by contact surfaces between the masonry units, which are explicitly modelled by blocks of meshes. This modelling technique is proven to be effective to predict the collapse behavior of the structure.

Mesh Convergence Behavior and the Effect of Element Integration of a HumanHead Injury Model.

Numerous head injury models exist that vary in mesh density by orders of magnitude. A careful study of the mesh convergence behavior is necessary, especially in terms of strain most relevant to brain injury. To this end, as well as to investigate the effect of element integration scheme on simulated strains, we re-meshed the Worcester Head Injury Model at five mesh densities (~ 7.2-1000k high-quality hexahedral elements of the brain). Results from explicit dynamic simulations of three cadaveric impacts and an in vivo head rotation were compared. First, scalar metrics of the whole brain only considering magnitude were used, including peak maximum principal strain and population-based median strain. They were further extended to deep white matter regions and the entire brain elements, respectively, to form two "response vectors" to account for both magnitude and distribution. Using benchmark enhanced full-integration elements (C3D8I), a minimum of 202.8k brain elements werenecessary to converge for response vectors of the deep white matterregions. This model was further used to simulate with reduced integration (C3D8R). We found that hourglass energy higher than the commonrule of thumb (e.g., up to 44.38% vs. < 10% of internal energy) was necessary to maintain comparable strain relative to C3D8I. Based on these results, it is recommended that a humanhead injury model should have a minimum number of 202.8 k elements, oran average element size of no larger than 1.8mm, for the brain. C3D8R formulation with relax stiffness hourglass control using a high scaling factor is also recommended to achieve sufficient accuracy without substantial computational cost.

Structure and solvation thermodynamics of asymmetric poly (acrylic acid)-b-polystyrene polyelectrolyte block copolymer micelle in water: Effect of charge density and chemical composition

Molecular structure and thermodynamic behavior of micelles formed by polyelectrolyte-neutral block copolymer chains, in particular polystyrene-b-poly(acrylic acid) (PS-b-PAA), in salt-free aqueous solution was studied via detailed explicit atomistic molecular dynamics simulations as a function of the copolymer composition (XPS) and the fractional charge (f) of ionizable PAA blocks. Micelle formation occurs via self-aggregation of PS-b-PAA copolymer chains with formation of PS core and PAA corona. The size of the PS core shows power-law dependence with respect to number of PS units per chain with an exponent 0.56 in agreement with scaling relations and experimental observations. The overall size of the micelle increases linearly with XPS. Increase in XPS results in a linear increase in micelle size and change in shape from spherical to prolate. The number of hydrogen bonds, pair correlation functions and various interatomic pair-wise energetic contributions to solvation enthalpy indicate favorable copolymer-water interaction. Micelle solvation is significantly influenced by hydrogen bonding interactions of carboxylic hydrogen atoms with water oxygens. Atom density profiles, solvation enthalpy and correlation functions of copolymer-Na+ ion pairs confirm the existence of micelle in osmotic regime, in agreement with mean-field theory.

Explicit dynamic fracture simulation of two-phase materials using a novel meso-structure modelling approach

In order to reduce the experimental cost and enable the numerical modelling better approaches the actual meso-structures of two-phase materials, a novel meso-scale modelling approach, combining the image-based and the parameterized modelling approaches, has been presented. In this approach, only limited samples of the studied two-phase material need to be provided for establishing an aggregate library, through which finite element models with different spatial distributions and volume fractions of aggregate can be arbitrarily generated for the use of virtual test. In the present virtual test, fracture failures were simulated in the framework of explicit dynamics. A rate-dependent cohesive zone model (CZM) was implemented through the user subroutines in ABAQUS to characterize the dynamic damage and fracture of the meso-structures. At the same time, an algorithm of embedding cohesive elements automatically into the potential damage zones was put forward. Finally, two typical numerical examples were given to verify these proposed methods and models, meanwhile the influences of spatial distribution and volume fraction of aggregate on the mechanical performance of the meso-structure were investigated. It can be validated that the present developed methods and models can effectively and efficiently characterize the dynamic fracture behavior of two-phase materials.

Determination of time dependent stress distribution on a potato tuber during drop case

AbstractRealistic representation of time‐dependent internal stress progression and deformation behavior of a potato tuber during a sample drop case has been studied in this article. A reverse engineering approach, compressive tests, slow motion camera records and finite element analysis (FEA) were employed to analyze the drop case deformation behavior of a sample potato tuber. Simulation results provided useful numerical data and stress distribution visuals. The numerical results are presented in a format that can be used for the determination of bruise susceptibility magnitude on solid‐like agricultural products during drop case. The visual observations revealed that slow motion camera images and simulation printouts were in good correlation. The modulus of elasticity of the potato specimens was calculated from experimental data to be 3.12 MPa and simulation results showed that the maximum equivalent stress was 0.526 MPa on the tuber. This value for stress indicates that bruising is not likely on the tuber under a pre‐defined drop height. In order to test the simulation accuracy, empirical, and simulation‐based estimates for total energy in this drop case were compared. The relative difference between empirical and simulation results was 1.27%. This study provide a good “how to do” guide to further research on the utilization of (FEM)‐based time‐dependent simulation approach in complex mechanical impact based damaging analyses and industry focused applications related to solid‐like agricultural products such as potato.Practical applicationsThe engineering simulation based “how to do” pathway presented in this study is a scientific novelty because the explicit dynamics simulation technique for potato tuber damage under drop case and its visual verification has been limitedly introduced in the literature. This study present deeper analysis on material model description, slow motion camera records, time dependent non‐linear stress analysis and FEM based Explicit Dynamics Simulation procedures. This study aims to represent a realistic non‐linear deformation case of the tuber which is very complicated to obtain through physical and/or empirical expressions. As a further step from other literature studies, this research has presented a novel realistic time‐dependent non‐linear drop test simulation based on physical compressive material test data. The findings have been prepared in a form which may be used as input parameters in design studies for solid‐like agricultural products (such as potato tubers) processing machinery systems used in food/agricultural industries.

Monte Carlo Potential Energy Sampling for Molecular Entropy in Zeolites

Zeolites are widely applied as molecular sieves and porous host materials for active sites in heterogeneous catalysis. Adsorption and reaction kinetics depend critically on the molecular entropy in the zeolite. In this work, we introduce a method to calculate the entropy of molecules in zeolites using Monte Carlo integration of the semiclassical partition function. The method is demonstrated for N2 and CH4 in chabazite and MFI silicalites. We find that the molecular entropy is lowered by a factor between 1/3 and 1/2 with respect to the gas-phase value. The results are corroborated by explicit molecular dynamics simulations revealing the active molecular degrees of freedom. The possibility of accurate entropy estimations opens up for an improved description of catalytic reactions and sorption phenomena in zeolites.

Sampling rare events in stochastic reaction-diffusion systems within trajectory looping.

In bistable reaction-diffusion systems, transitions between stable states typically occur on timescales orders of magnitude longer than the chemical equilibration time. Estimation of transition rates within explicit Brownian dynamics simulations is computationally prohibitively costly. We present a method that exploits a single trajectory, generated by a prior simulation of diffusive motions of molecules, to sample chemical kinetic processes on timescales several orders of magnitude longer than the duration of the diffusive trajectory. In this approach, we "loop" the diffusive trajectory by transferring chemical states of the molecules from the last to the first time step of the trajectory. Trajectory looping can be applied to enhance sampling of rare events in biochemical systems in which the number of reacting molecules is constant, as in cellular signal transduction pathways. As an example, we consider a bistable system of autophosphorylating kinases, for which we calculate state-to-state transition rates and traveling wave velocities. We provide an open-source implementation of the method.

Nonequilibrium Statistical Mechanics of Two-Dimensional Vortices.

It has been observed empirically that two-dimensional vortices tend to cluster, forming a giant vortex. To account for this observation, Onsager introduced the concept of negative absolute temperature in equilibrium statistical mechanics. In this Letter, we show that in the thermodynamic limit a system of interacting vortices does not relax to the thermodynamic equilibrium but becomes trapped in a nonequilibrium stationary state. We show that the vortex distribution in this nonequilibrium stationary state has a characteristic core-halo structure, which can be predicted apriori. All the theoretical results are compared with explicit molecular dynamics simulations.