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Publication Number: FHWA-HRT-04-094
Date: November 2004

Evaluation of LS-DYNA Soil Material Model 147

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CHAPTER 7. LARGE DEFORMATION TECHNIQUES

Soils, in many roadside safety applications, require large deformation capabilities. This requirement is difficult to meet for Lagrangian-based codes. However, it is possible to implement a material that would allow large deformations while maintaining stability. For example, consider material type 126, a modified honeycomb material. This material was specifically developed to handle extremely large deformations for modeling the honeycomb typically used on moving deformable barriers (MDBs). MDBs are used by the automotive industry for side-impact testing. An example of the stable large deformation capability of material type 126 is shown in figure 20.

Another possibility within LS-DYNA is to use the Eulerian capabilities for modeling materials that undergo large deformations. There are several possibilities available, two of which are: (1) using the Arbitrary Lagrangian-Eulerian (ALE) formulation or (2) using the multimaterial Eulerian formulation with Lagrangian coupling.

The ALE formulation basically works by re-meshing the material throughout the simulation so that the mesh stays relatively uniform. A uniform mesh can prevent local instabilities caused by highly distorted elements. A classical problem used to demonstrate the ALE formulation is the Taylor problem, shown in figure 21.

Multimaterial Eulerian formulation is an approach that allows multiple materials to exist within each solid element and lets the material flow from element to element. The solid element mesh is fixed in space in this approach. Lagrangian coupling allows structural elements to be placed within the Eulerian mesh. Interaction between the fluid-like elements (Eulerian) and the structural elements (Lagrangian) is handled by contact algorithms. A contrived example of a post in soil using this method is shown in figure 22.

Unfortunately, time did not permit investigation of these methods with the FHWA soil material model.

(a) Initial condition.

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(b) Moderate deformation.

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(c) Severe deformation.

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Figure 20. Lagrangian approach: Stable large deformations, modified honeycomb material.

(a) Initial Condition:

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(b) Lagrangian Approach:
Highly distorted elements lead to inaccuracies


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(c) ALE Formulation:
Uniform mesh maintained.


Figure 21. Taylor problem.

(a) Side view.

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(b) Close up.

Figure 22. Multimaterial Eulerian formulation with Lagrangian coupling.

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