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Session 22

User-Defined Nonlocal Models in LS-DYNA
In this paper, we present an implementation technique that aims to easily incor- porate the benefits of a nonlocal formulation to existing local constitutive models. In order to avoid pathological mesh dependency, an approximation of the nonlocal strategy is adopted. The technique is designed in such manner that the nonlocal extension of previously existing local models is carried out straightforwardly, requiring only minor modifications in the local routines. The implementation in LS-DYNA is depicted in detail for which a FORTRAN code excerpt is provided. In order to validate the proposed nonlocal scheme, we have considered two different constitutive models: one of them intended for the description of ductile materials, the other one suitable for the simulation of fiber-reinforced composites. The numerical analysis of different specimens shows that the proposed nonlocal strategy is able to eliminate spurious mesh dependency under different stress states and using different material models.
About Isogeometric Analysis and the new NURBS-based Finite Elements in LS-DYNA
In the context of isogeometric analysis many research activities have focused on the use of Non- Uniform Rational B-Splines (NURBS). These NURBS-based finite elements have been studied in depth and it has been shown, that they are particularly well suited for computational analysis leading to qualitatively more accurate results in comparison with standard finite elements based on Lagrange polynomials. Due to these motivating results, NURBS-based finite elements are currently implemented into LS-DYNA. This work outlines the basic ideas of isogeometric analysis and gives a short introduction into NURBS basis functions. The new keyword *ELEMENT_NURBS_PATCH_2D available in LS-DYNA is presented together with various possible options, like shell theories with and without rotational degrees of freedom. Preliminary results on the performance of these new elements are studied by means of a sheet metal forming example discussed in the Numisheet conference 2005.
An Overview of User Defined Interfaces in LS-DYNA
The user-defined features in LS-DYNA are powerful tools that allow users in academia or industry to verify research results in the context of general and complicated finite element applications. Implementation work concerns only the special field of interest, and there is no need for the comprehensive task of developing and maintaining the complete finite element software. The most popular user interface is for material modeling. But there also exist user interfaces for structural elements, airbag sensors, solution control, friction, interface control, weld failure, loads, output control, adaptivity, thermal contact, and others. An overview of current user-defined interfaces in LS-DYNA version 971 R5.0 will be presented. The aim of this contribution is to bring together the possibilities to add own numerical models and algorithms to the code. Therefore, each interface is described in its functionality. It will be explained, how to activate the particular interface in the input, where to find the corresponding subroutine, and which input/output arguments can be used.
Ply-based composite modeling with the new *ELEMENT_SHELL_COMPOSITE keyword
Because of their superior mechanical properties in combination with a relative low density Fiber Reinforced Composites (FRC) are of great potential in the area of lightweight structures respectively applications. Nevertheless usage and acceptance of FRC highly depends on the fact, whether or not it is possible to prescribe and predict their structural behavior using the Finite Element Method. LS-DYNA has a great potential to simulate the behavior of composite structures since many years. But the model generation of complex layered structures with many different plies in different directions may become very expensive. Starting with LS971 Revision R5.1 LS-DYNA offers the new keyword *ELEMENT_SHELL_COMPOSITE, which is a generalization of the already known *PART_COMPOSITE. This new keyword allows a very comfortable description of composite layers using a ply-based concept. Within one part elements may have different number of through thickness integration points as well as different material angles in each layer. Because of that the model must not be split up in many parts, which makes pre- and also postprocessing much more practical. The ANSYS Composite PrepPost enables a comfortable way to generate layered composite structures using a ply-cased = manufacturing-based concept and export these data to LS-DYNA in the new format. It offers a common composite definition for both ANSYS and LS-DYNA simulations.