Nonlinear Explicit Finite Element Code Research Articles

Behaviour of cellular materials under impact loading

AbstractThe paper describes experimental and computational testing of regular open‐cell cellular structures behaviour under impact loading. Open‐cell cellular specimens made of aluminium alloy and polymer were experimentally tested under quasi‐static and dynamic compressive loading in order to evaluate the failure conditions and the strain rate sensitivity. Additionally, specimens with viscous fillers have been tested to determine the increase of the energy absorption due to filler effects. The tests have shown that brittle behaviour of the cellular structure due to sudden rupture of intercellular walls observed in quasi‐static and dynamic tests is reduced by introduction of viscous filler, while at the same time the energy absorption is increased.The influence of fluid filler on open‐cell cellular material behaviour under impact loading was further investigated with parametric computational simulations, where fully coupled interaction between the base material and the pore filler was considered. The explicit nonlinear finite element code LS‐DYNA was used for this purpose. Different failure criteria were evaluated to simulate the collapsing of intercellular walls and the failure mechanism of cellular structures in general.The new computational models and presented procedures enable determination of the optimal geometric and material parameters of cellular materials with viscous fillers for individual application demands. For example, the cellular structure stiffness and impact energy absorption through controlled deformation can be easily adapted to improve the efficiency of crash absorbers.

Optimum design for energy absorption of bitubal hexagonal columns with honeycomb core

This paper is concerned with the maximisation problem of the crushing energy absorption of bitubal hexagonal aluminium columns with honeycomb core under dynamic axial crushing. The honeycomb core is made up of 18 internal cell walls with the same thickness. The mass of the whole crush element and the outer dimension of the section are kept constant, while the side length of inner profile and the thickness of the inner and outer wall are used as design variables. The nonlinear explicit finite element code LS-DYNA is adopted to simulate the axial crushing behaviour of bitubal columns and the response surface method is applied to obtain the optimum design in the design space. The results show that the specific energy absorption of bitubal hexagonal columns with honeycomb core can be significantly improved by optimisation and allow us to present useful proposals for the crashworthy design of similar structures.

Behaviour of Cellular Structures under Impact Loading a Computational Study

New multiphysical computational models for simulation of regular open and closed-cell cellular structures behaviour under compressive impact loading are presented. The behaviour of cellular structures with fluid fillers under uniaxial impact loading and large deformations has been analyzed with the explicit nonlinear finite element code LS-DYNA. The behaviour of closed-cell cellular structure has been evaluated with the use of the representative volume element, where the influence of residual gas inside the closed pores has been studied. Open-cell cellular structure was modelled as a whole to properly account for considered fluid flow through the cells, which significantly influences macroscopic behaviour of cellular structure. The fluid has been modelled by applying a Smoothed Particle Hydrodynamics (SPH) method. Computational simulations showed that the base material has the highest influence on the behaviour of cellular structures under impact conditions. The increase of the relative density and strain rate results in increase of the cellular structure stiffness. Parametrical numerical simulations have also confirmed that filler influences the macroscopic behaviour of the cellular structures which depends on the loading type and the size of the cellular structure. In open-cell cellular structures with higher filler viscosity and higher relative density, increased impact energy absorption has been observed.

Energy absorption of axially compressed thin-walled square tubes with patterns

The paper suggests the introduction of patterns to the surface of conventional thin-walled square tubes to improve the energy absorption capacity under axial compressive loads. A quasi-static axial crushing analysis has been conducted numerically by the nonlinear explicit finite element code LS-DYNA. Two types of patterns constructed using the basic pyramid elements were introduced. Type A pattern was aimed at triggering the extensional mode for relatively thin square tubes whereas type B pattern was intended to develop new collapse mode capable of absorbing more energy during collapse. A total of 30 tubes with a length of 120 mm, thickness 1.2 mm and widths of 40 or 60 mm were simulated. Numerical results showed that all tubes with type A patterns developed the extensional collapse mode instead of the symmetric collapse mode and absorbed about 15–32.5% more energy than conventional thin-walled square tubes with a mass increase less than 5%. Meanwhile, a new collapse mode named octagonal collapse mode was observed for tubes with type B pattern and the energy absorption of tubes developing this mode increased by 54–93% compared with the conventional tube. The influence of various configurations of the patterns on the deformation and energy absorption of the tubes was also discussed. The paper opens up a new avenue in design of high energy absorption components.

Behaviour of cellular structures with fluid fillers under impact loading

The paper describes experimental and computational testing of regular open-cell cellular structures behaviour under impact loading. Open-cell cellular specimens made of aluminium alloy and polymer were experimentally tested under quasi-static and dynamic compressive loading in order to evaluate the failure conditions and the strain rate sensitivity. Additionally, specimens with viscous fillers have been tested to determine the increase of the energy absorption due to filler effects. The tests have shown that brittle behaviour of the cellular structure due to sudden rupture of intercellular walls observed in quasi-static and dynamic tests is reduced by introduction of viscous filler, while at the same time the energy absorption is increased. The influence of fluid filler on open-cell cellular material behaviour under impact loading was further investigated with parametric computational simulations, where fully coupled interaction between the base material and the pore filler was considered. The explicit nonlinear finite element code ls-dyna was used for this purpose. Different failure criteria were evaluated to simulate the collapsing of intercellular walls and the failure mechanism of cellular structures in general. The new computational models and presented procedures enable determination of the optimal geometric and material parameters of cellular materials with viscous fillers for individual application demands. For example, the cellular structure stiffness and impact energy absorption through controlled deformation can be easily adapted to improve the efficiency of crash absorbers

Design automation of a laminated armor for best impact performance using approximate optimization method

Values of characteristic parameters such as thickness have big influence on the impact behavior of a laminated armor. Determination of those values for an optimum laminated armor usually requires evaluation of many computationally costly impact analyses. In this paper, automated, efficient and effective determination of design parameters in a laminated armor for best impact performance is investigated using an approximate optimization method. The approximate optimization method is generated by coupling a parametric preprocessor, finite element analysis software, response surface approximations and a numerical optimization algorithm. The whole coupling is achieved through a commercial code, ANSYS Design Optimization Module. The laminated armor of interest consists of three layers. The frontal layer is alumina ceramic, and it is supported by a 4340 steel mid-layer and a 2024-T3 aluminum rear layer. The armor is impacted by a projectile with velocities of 1000 and 2000 m/s. The surface normal of the armor has an oblique angle with the projectile's moving direction. The 3-D impact analysis of the armor is conducted using non-linear explicit dynamic finite element code ANSYS/LS-DYNA. Optimization of the armor is performed to find the best thickness values of layers and oblique (orientation) angle towards the least penetration of the projectile.

Fiber Reorientation in Laminated and Woven Composites for Finite Element Simulations

A simple technique for fiber reorientation in laminated and woven composite material is presented in this paper. This technique is suitable for nonlinear finite element simulations of composite structure failure. The developed technique is implemented in the nonlinear explicit finite element code DYNA3D to demonstrate the effect of the fiber reorientation on the behavior of laminated composite materials in crash simulations. The same technique is added to a woven fabric composite material micromechanical model to account for the change of the model geometry. Using an incremental approach programmed in the MATLAB software, the stiffness change of the woven composite micromechanical model as a function of the yarn tow reorientation is investigated. Significant change in the elastic properties is observed as a result of the scissoring effect and fiber reorientation in woven fabric composites.

Development of the DYNA3D simulation code with automated fracture procedure for brick elements

AbstractNumerical simulation of cracked structures is an important aspect in structural safety assessment. In recent years, there has been an increasing rate of development of numerical codes for modelling fracture procedure. The subject of this investigation is implementing automated fracture models in the DYNA3D non‐linear explicit finite element code to simulate pseudo 3D crack growth procedure. The implemented models have the capabilities of simulating automatic crack propagation without user intervention. The implementation is carried on solid elements. The methodology of implementing fracture models is described. An element deletion‐and‐replacement remeshing procedure is proposed for updating the explicit geometric description of evolving cracks. Fracture parameters such as stress intensity factors, energy release rates and crack tip opening angle are evaluated. The maximum circumferential stress criterion is used to predict the direction of crack advancement. Seven crack problems are presented to verify the effectiveness of the methodology. Mesh sensitivity and loading rate effects are studied in the validation of the presented procedure. Copyright © 2003 John Wiley & Sons, Ltd.

Parametric Study of Effects of Brain-Skull Boundary Conditions and Brain Material Properties on Responses of Simplified Finite Element Brain Model under Angular Acceleration Impulse in Sagittal Plane

In the present study, the effects of brain-skull boundary conditions on the responses of a simplified three-dimensional finite element model of a thin sagittal slice of the human head were investigated. The model was excited using a time-dependent angular velocity with a maximum of 16 rad/s (maximum angular acceleration, around 5000 rad/s2). The present study was conducted using the non-linear explicit finite element code LS-DYNA. Three methods for simulation of the brain-skull boundary conditions were investigated: 1) with brain rigidly attached to the skull; 2) using frictionless sliding contact allowing no separation between the brain and skull; and 3) direct simulation of cerebrospinal fluid CSF using a layer of eight-noded solid elements with fluid-like properties. Varying the method for simulation of the brain-skull boundary conditions appreciably affected the brain responses. However, varying parameters of a given method, such as viscosity of cerebrospinal fluid CSF and CSF-skull friction coefficient, exerted only minor effects on these responses. The present results suggest that accurate simulation of brain-skull boundary conditions requires direct representation of the subarachnoidal space/CSF as a fluid-like medium.

Finite element analysis of impacts on water and its application to helicopter water landing and occupant safety

The study of hydrodynamic impact between a body in motion and a free water surface finds applications, in aeronautical fields, in splashdown and ditching problems. The effect of this impact is often prominent in the design phase of the project and, therefore, the importance of studying the event with more accuracy than in the past is imperative. Usually the study of the phenomenon is dealt with experiments, empirical laws, and lately, with finite element simulations. These simulations are performed by means of special codes that allow the fluid-structure coupling; these codes have their origin in Lagrangian finite element programs developed for crash analysis improved with possibility of interfacing with Eulerian spatial description, typical of fluids. Critical points in this type of modelling are the fluid-structure interaction algorithms, constitutive modelling of the fluid and time efficiency of the computation. This study describes an effort that focuses on the development of a crash modelling and simulation approach utilizing a non-linear explicit finite-element code (LSDYNA 960) to demonstrate the potential for helicopter water impact analysis in the development of crash design criteria and concepts. Initially, the water model shall be developed and validated using default Lagrangian techniques. Subsequently, more accurate Arbitrary Lagrangian Eulerian analyses will be conducted to obtain finer results for the ball impact scenario and helicopter impact. Finally, the response of an occupant for the above helicopter crash test shall be analyzed using the MADYMO code, utilizing accelerations obtained from the LSDYNA output. Lumbar load, the most crucial mode of injury in these types of crashes will be investigated and discussed.

Micromechanics Based Composite Material Model for Crashworthiness Explicit Finite Element Simulation

A micromechanical model is developed for laminated composite materials and implemented in the explicit finite element method. The objective of this study is to get an accurate and simple micro-model that can be used in the displacement-based nonlinear explicit finite element code DYNA3D. The micromechanical model implemented in the explicit finite element code can be used for simulating the behavior of composite structures under various loads such as impact and crash. The stress-strain relation for the micro-model is derived for shell element. A micro-failure criterion (MFC) is presented for each material constituent and failure mode. The implemented model is validated through several test examples. As a demonstration case of the stability of the developed micro-model, a finite element model of a graphite/epoxy tube structure is developed and simulated under axial crash.

A nonlinear higher order shear deformation shell element for dynamic explicit analysis:: Part I. Formulation and finite element equations

This work presents the finite element formulation of a higher order shear deformation shell element for nonlinear dynamic analysis with explicit time integration scheme. A corotational approach is combined with the velocity strain equations of a general third-order theory in the formulation of a four-noded quadrilateral element with selectively reduced integration. A bilinear isoparametric formulation is utilized in the shell plane resulting in nine degrees of freedom per node. The formulation requires C 0 continuity for the nodal variables. The finite element implementation of the new element in a general explicit finite element code is described in details, including boundary conditions and nodal mass calculation. A simple formula for the explicit time integration critical time step of the higher order element is developed. The described element is capable of correctly representing the through thickness distribution of the transverse shear, which makes it suitable for composite and sandwich shells analysis. In addition, the developed shell can be used for better representation of plastic flow through thickness in isotropic materials. It has been added to the element library of the nonlinear explicit finite element code DYNA3D. Its performance has been evaluated through a series of standard shell verification test problems, which show great promise for many applications. The results are presented in Part II of the present work.

A nonlinear higher order shear deformation shell element for dynamic explicit analysis: Part II. Performance validation through standard tests

This work presents the results from a set of verification shell problems used to assess the performance of the higher-order shear deformation shell elements formulated in Part I of the present study. The developed element has been added to the element library of the nonlinear dynamic explicit finite element code DYNA3D. Several standard verification test problems are performed using the code DYNA3D with the developed shell element. Results are presented for different test problems and are compared with experiments and results from other existing shell elements. The good overall performance builds confidence in the formulation and implementation of the proposed higher-order shear deformable element. The superior advantage of the developed element is evident in one of the examples presented for representation of plastic flow through the thickness in isotropic materials. The element can be used in crash and metal forming simulations in local areas of high transverse shear stresses. Local areas of crack in crash applications and splitting in metal forming applications can be modeled more accurately with the developed shell element.

Development of a Detailed Vehicle Finite Element Model Part I: Methodology

on September 29, 1993, President Clinton and the Chief Executive Officers of the major domestic automakers (Chrysler Corporation, Ford Motor Company, and General Motors Corporation) announced the formation of the Partnership for a New Generation of Vehicles (PNGV). The long-term goal of PNGV is to develop vehicles that will deliver up to three times today's fuel efficiency (80 miles per gallon or BTU equivalent) and cost no more to own and operate than today's comparable vehicles. At the same time, this new generation of vehicles should maintain the size, utility and performance standards of today's vehicles (i.e., the 1994 Chrysler Concorde, Ford Taurus, and Chevrolet Lumina) and meet all mandated safety and emission requirements [1,2]. As part of the PNGV program, The National Highway Traffic Safety Administration (NHTSA) intends on developing a vehicle finite element model representing each vehicle class and size. These FE models will be used in compatibility studies to ensure that PNGV vehicles meet safety standards and that crashworthiness and crash avoidance attributes are not compromised by their lightweight and use of advanced materials. A detailed finite element model of a 1996 Plymouth Neon was developed at the FHWA/NHTSA National Crash Analysis Center as part of the PNGV program. The Neon represents the sub-compact vehicle class. The three dimensional geometric data of each component was obtained by using a passive digitising arm. The geometric data was imported into a preprocessor for mesh generation, parts connection, and material properties. Tensile testing was conducted on specimen to obtain the material properties of the various sheet metal components. Sheet metal thickness was obtained by using an ultrasonic thickness measurement gauge. The non-linear explicit finite element code LS-DYNA, was used to perform the various simulations. This paper will conclude on the following issues; describing and efficient methodology for reverse engineering vehicles; importance of maintaining geometric accuracy; issues concerning material characterization; significance and necessity of component wise testing.

Computer modelling of impact on curved fibre composite panels

This paper presents results of a finite element (FE) analysis study into low energy impact on curved composite panels. The aim of this study was to determine the accuracy and efficiency of a non-linear explicit FE code, MSC.Dytran, and compare results to published experimental data. The study also looked at impact response as a function of composite panel curvature, composite mesh density, impactor weight, velocity and size, and various suggestions are made for improving the accuracy and efficiency of FE analysis procedures in composite low energy impact studies. The paper presents 265 explicit computer simulation results, which show that non-linear FE analysis does provide accurate, efficient and conservative solutions provided various guidelines are followed.

A Finite Element Tire Model and Vibration Analysis: A New Approach

Abstract A complete finite element tire model was developed for the purposes of vehicle dynamics analyses and full vehicle finite element model real time proving ground simulations. The tire model was validated through simulations of some of the very important global, static and dynamic mechanical properties such as the tire radial and lateral stiffnesses, free-drop test, and low-speed rolling cornering stiffness. The three-dimensional free vibration and harmonic/randomly forced vibrations with ground contact of the tire model were studied here. One of the main purposes for the present study is to provide a new approach toward tire and vehicle NVH studies. All the analyses were nonconventional in the sense that, instead of NASTRAN-type modal analysis, the explicit nonlinear dynamic finite element code LS/DYNA3D was used to conduct all the analyses in the time domain, and the vibration modes were decomposed via fast fourier transformation.

Nonlinear dynamic finite element analysis on parallel computers using FORTRAN 90 and MPI

A nonlinear explicit dynamic finite element code for use on scalable computers is presented. The code was written entirely in FORTRAN 90, but uses MPI for all interprocessor communication. Although MPI is not formally a standard for FORTRAN 90, the code runs properly in parallel on CRAY T3E, IBM SP, and SGI ORIGIN 2000 computing systems. Issues regarding the installation, portability, and effectiveness of the FORTRAN 90-MPI combination on these machines are discussed. An algorithm that overlaps message passing and computations of the explicit finite element equations is also presented and evaluated. Several large-scale ground-shock analyses demonstrate the varying combined importance of load balance and interprocessor communication among the different computing platforms. The analyses were performed on only a few to hundreds of processors with excellent speedup and scalability.

Simulation of sheet metal forming using explicit finite-element techniques: effect of material and forming characteristics: Part 1. Deep-drawing of cylindrical cups

The use of the explicit non-linear finite element (FE) code DYNA 3D in sheet metal forming simulation is examined. A 3D FE model for the deep-drawing of cylindrical cups was constructed and the simulation results obtained using different simulation parameters, i.e. punch velocity, sheet material density, Coulomb friction coefficient at the tool–blank interfaces and type and dimensions of the FE mesh-elements, compared with experimental results regarding strain distributions (radial, circumferential and thickness through the sheet) and punch force–punch travel curves carried out on five galvanised steels and an aluminium sheet, for three different geometries of axisymmetric cup: Good agreement was obtained. The evaluation of the CPU time cost, the macroscopic deformation modes, the strain distributions of the deformed material and the process characteristics obtained for the constructed FE models, was directed towards the selection of the most efficient material and punch parameters. Finally, an attempt was made to construct rules from the above-mentioned verification, which may be used for reliable sheet metal forming simulations, using explicit finite-element codes.

CRUSH BEHAVIOR OF HONEYCOMB STRUCTURE IMPACTED BY DROP-HAMMER AND ITS NUMERICAL ANALYSIS

Crush behavior of the bare honeycomb structure with hexagonal cell (without the reinforcing plate members bonded on both edges) is studied experimentally and numerically. The dimensions of honeycomb cell used are 3/8in. (9.525mm) and 3/4in. (19.05mm) and the material of foil is an aluminum alloy, A5052. Their foil thicknesses are 0.020mm, 0.033mm and 0.046mm. In experiment, the impact velocity is ranged from 1.98 to 10m/s. As for 3/8in. honeycomb, irregular folding pattern appears just to a little extent in experiment. On the other hand, very irregular buckling pattern is observed in 3/4in. honeycomb. Crush strength becomes slightly larger as the impact velocity increases. The maximum value of acceleration measured by an acceleration sensor attached to the drop-hammer seems to be almost constant when the impact velocity is greater than a certain value, and increases rapidly as the foil thickness increases. Furthermore, numerical simulation is carried out by using the dynamic explicit nonlinear finite element code DYNA3D. In the computation, due to the geometrical symmetry of hexagonal honeycomb core, the 'Y' letter model is proposed. In computation the buckling with folding pattern appears, though, the irregularity in folding pattern is larger in computation than in experiment. Predicted dependence of the crush strength on the impact velocity and the foil thickness is similar to the corresponding experimental result.