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This paper presents a numerical study of a road barrier subjected to a bus crash test. The authors focus on the proper description of a barrier material model. Material model parameters are verified according to a simulated forming process. The solution uses modeling and simulation methods, as well as the finite elements method implemented with LS-DYNA software. The obtained results show that defining a proper road barrier model must consider the manufacturing process. Intensive plastic deformation leads to a significant increase in yield stress only in areas where the highest strain is observed during the forming process. The barrier model is divided into three zones, each of which have material properties characterized by different constants. The described construction, which uses homogenous material, is much more deformed and flatter than that of the improved model. Comparison with the experimental results shows that the new model better describes barrier deformation. In addition, deformation of the barrier under impact is evaluated according to European regulations (EN 1317). In comparison to the performed bus crash test, the crash test parameters obtained in the numerical simulation (static working width and normalized working width) gave more accurate results when the improved material model was utilized. [ABSTRACT FROM AUTHOR]

The paper presents easy-to-use and accurate numerical procedures determining the level of damage to structures subjected to fatigue load. The main goal of the research was the implementation in the finite element analysis program of fatigue analysis procedures for selected multiaxial fatigue criteria, taking into account the exact fatigue characteristics of materials and the certain pattern of cumulative summation the fatigue damages. Selections of three types of criteria were selected for the study: invariant criterion, criterion using the mean values in the elementary volume and the criterion defined in the critical plane. Moreover, tests were carried out for Michell's problem of the topology optimization, which showed the impact of the criteria on the shape of optimal structures. [ABSTRACT FROM AUTHOR]

To achieve high accuracy in finite element simulation, it is beneficial to model forming limit curves (FLCs) for nonlinear strain paths, since FLCs based on linear strain paths inaccurately predict the failure of parts that undergo nonlinear strain paths. Standardized Nakajima or Marciniak tests are used to create a FLC for linear strain paths. For common strain paths, the shapes of the specimens are well-known. Only a limited set of nonlinear strain paths can be generated by using these tests. For example, a biaxial strain path can be generated, and then the test specimen can be cut to generate a uniaxial strain path. However, the other way around is not possible. Therefore, it is important to develop a new tool that can systematically investigate all kinds of nonlinear strain paths. This paper describes a newly developed deep drawing tool that exhibits these properties. Large specimens with homogeneous strain in the centre can be generated for any strain state. These specimens are sufficiently large that a second test specimen can be cut out to generate another strain path using Nakajima or Marciniak tests. The shape of blank for uniaxial, plane and biaxial strains are presented and analysed both numerically and experimentally. A nonlinear strain path consisting of a uniaxial strain path followed by a biaxial strain path is shown as an example. [ABSTRACT FROM AUTHOR]

This paper presents a brief review of Gurson’s damage model, employed to describes the strength degradation in ductile metals submitted to large plastic deformations. The damage model is applied using finite elements and an Arbitrary Lagrangian-Eulerian formulation (ALE), to ensure a better quality to the finite elements mesh. The study of the combined application of ALE and Gurson approach to damage modeling and strain localization is the object of this paper. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

A correct prediction of a possible sheet metal failure is essential to sheet metal forming simulations. The use of the conventional forming limit curve (FLC) is the standard approach on industrial level for this problem. The FLC concept is limited to the case of linear strain paths, however. The initial FLC is no longer valid in the case of nonlinear strain paths. The algorithm Crach allows for a transient prediction of the forming limit for localized necking in the case of arbitrary strain paths. For high strength steels and aluminium sheets there is also a risk of fracture without prior localized necking. This paper presents a fracture model that accounts for ductile fracture (caused by void nucleation, void growth and void coalescence) and shear fracture (caused by shear band localization). For both types of fracture, stress state parameters are introduced which can be applied for the plane stress state and for the general 3D stress state. The fracture limits are defined by the equivalent plastic strain at fracture as a function of the stress state parameter based on different experiments with nearly constant stress state parameter. These fracture limit curves are a basis for an integral damage accumulation in the sheet metal forming simulation for arbitrary nonlinear strain paths. The model Crach for the prediction of localized necking and the two fracture models for ductile and shear fracture are combined in the comprehensive failure model CrachFEM. CrachFEM can be linked via a user material model MF GenYld to different explicit FEM codes. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents the effect of tool modelling accuracy on the simulation accuracy of springback in high strength steels. Simulations of a two-dimensional draw-bending process are carried out by using a polyhedral tool model whose surface is approximated by a polyhedron, and a model whose surface is smoothed by quadratic parametric surfaces proposed by Nagata [Nagata, Comput. Aided Geom. D, 22(2005), 55–59] (Nagata patch model). It is found that not only the shape accuracy but also the normal vector accuracy of tool models are of importance for accurate springback predictions. The use of the Nagata patch model is an efficient approach not only to improve the simulation accuracy but also to make the simulation be hardly influenced by the tool mesh, even for simulations of a high strength steel in which large amount of springback is involved. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

There is currently much focus on the application of magnesium alloys to automotive structural components. This has arisen due to the positive environmental aspects associated with use of magnesium alloys such as weight reduction and recycling potential. In recent years many researchers have focused on the application of various forming processes to magnesium alloys. Magnesium alloys would seem highly suitable for sheet forming due to high N and r values, however, in application their formability has been inferior to, for example, aluminium alloys. It has thus been concluded that, when dealing with magnesium alloys, it is difficult to predict formability based on material properties. In order to improve formability and forming accuracy when using Mg alloys it is necessary to build a database and inference system which could decide the optimal forming parameters for complex automotive components. Currently not enough data is available to build such a database due to the limited number of studies available in literature. In this study an experimental analysis of hemispherical bulge forming at elevated temperature was undertaken in order to evaluate formability and hence build a database for forming process design. A finite element model based on the experiment has been built and validated against the experimental results. A ductile failure criterion has been integrated with the FE model and is used to predict the onset of failure. This paper discusses the development and validation of the finite element model with the ductile failure criterion and presents results from the experimental tests and FE simulations. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Finite element modeling of a prototype friction stir welded blank made of aluminum alloy 2024-T351 is considered in this paper. Feasibility of implementation of the experimentally-obtained mechanical properties of the weld nugget and heat-affected zones in FEM models is investigated. Limiting dome height test is considered as case of the study. Three different finite element models implementing different levels of the weld details are built and compared. It is shown that despite increased simulation time, implementation of the weld nugget and heat-affected zones is justified by significantly improved accuracy of the simulation results. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Within the scope of this article a decoupling algorithm to reduce computing time in Finite Element Analyses of incremental forming processes will be investigated. Based on the given position of the small forming zone, the presented algorithm aims at separating a Finite Element Model in an elastic and an elasto-plastic deformation zone. Including the elastic response of the structure by means of model simplifications, the costly iteration in the elasto-plastic zone can be restricted to the small forming zone and to few supporting elements in order to reduce computation time. Since the forming zone moves along the specimen, an update of both, forming zone with elastic boundary and supporting structure, is needed after several increments. The presented paper discusses the algorithmic implementation of the approach and introduces several strategies to implement the denoted elastic boundary condition at the boundary of the plastic forming zone. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper, a new computational technique is presented based on the eXtended Arbitrary Lagrangian-Eulerian Finite Element Model (X-ALE-FEM) in large plasticity deformations. An arbitrary Lagrangian-Eulerian (ALE) technique is employed to capture the advantages of both Lagrangian and Eulerian methods and alleviate the drawbacks of the mesh distortion in Lagrangian formulation. In order to remove the limitation of the mesh conforming to the boundary conditions in the simulation, the extended finite element method (X-FEM) is implemented by incorporating discontinuous fields through a partition of unity method. The implementation of X-FEM technique in the framework of ALE model is finally presented in modeling of free-die pressing with flexible central core. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper we present a hybrid eXtended Finite Element/Level Set Method to simulate a two dimensional moving interfaces. The discrete discontinuity (material interfaces) is treated by the extended finite element method (X-FEM), in the partition of unity concept, whereby interface discontinuities are completely independent of the mesh. In this context, the discontinuity is represented without explicitly meshing its shape and the moving interface simulations are carried out without any re-meshing. Enriched approximations to the velocities and deformation fields are developed in the context of the eXtended Finite-Element Method, and the level-set representation of the interface was used to model the evolution of the discontinuity interface in time. The X-FEM utilizes the partition of unity property, of the finite element shape functions to incorporate arbitrary enrichment functions into the approximation. An explicit Newmark algorithm and one-point integration scheme with hourglass mode control is used to update the discontinuous fields. Material interface simulations as well as some free surface simulations, follow theoretical aspects presented in. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper deals with the identification of material parameters using an inverse strategy. In the classical methods, the inverse technique is generally coupled with a finite element code which leads to a long computing time. In this work an inverse strategy coupled with an ANN procedure is proposed. This method has the advantage of being faster than the classical one. To validate this approach an experimental plane tensile and bulge tests are used in order to identify material behavior. The ANN model is trained from finite element simulations of the two tests. In order to reduce the gap between the experimental responses and the numerical ones, the proposed method is coupled with an optimization procedure to identify material parameters for the AISI304. The identified material parameters are the hardening curve and the anisotropic coefficients. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Numerical modeling of engineering contact problems is one of the most difficult and demanding tasks in computational mechanics. In this paper, the extended finite element method is employed to simulate the presence of discontinuities caused by frictional contact based on the penalty approach. The FEM approximation is enriched by applying additional terms to simulate the frictional behavior of contact between two bodies. The penalty method, which is one of the most commonly used techniques for contact problems, is used to model the penetration between two contacting boundaries and the normal contact force is related to the penetration by a penalty parameter. Finally, numerical examples are presented to demonstrate the applicability of the XFEM in modeling of frictional contact behavior. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The paper refers to the problem of parallel generation of 3D meshes for complex objects. Simulation of processes using finite element method (FEM) consists essentially of two phases: generation of finite element mesh and solving the appropriate set of algebraic equations. The parallel solving of such problems has been often reduced to sequential generation of a mesh and than decomposing of this mesh. The latter operation was usually performed sequentially as well. In such approach the only part being parallelized is the solver. In the recent years much attention has been directed to the task of parallelization of the mesh generation process. The need for considering this problem results mainly from the fact that the simulations are currently being run on meshes with very large number of elements. In such cases the sequential generation of meshes poses significant problems regarding both the amount of the required memory and the discretization time. In this work there is described the decomposition strategy of the initial surface 3D mesh for subsequent generation of the volume mesh. Surface meshes are the input data which are being decomposed into sub-domains by cutting them with separators. Next, the surface meshes are constructed for the separator, which are closing the new sub-domains. There is ensured the compatibility of the surface meshes at the interface. This procedure is being continued until the prescribed number of sub-domains is reached. Generation of volume meshes can then be performed for each closed sub-domain on the parallel computer. As a result, the whole volume mesh needn’t be stored in the memory of the single computational node. During the simulation phase there is interchanged only the information about the interface meshes, which stay compatible. Moreover, for single-processor unit, the method can give additional benefits. It’s possible to partition the domain depending on the amount of the available memory and then generate the volume meshes sequentially for each sub-domain. This approach allows to use the sequential mesh generator without any modifications. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents an example of the application of a genetic algorithm to a 2-D design problem, which consists in the reconstruction of the function that describes the convective heat transfer coefficient, when the position of the moving interface of the phase change is well-known. In numerical calculations the Tikhonov regularization, the genetic algorithm and the alternating phase truncation method were used. The featured examples of calculations show a very good approximation of the exact solution and the stability of the procedure. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]