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Publication Number: FHWA-HRT-04-095
Date: November 2004
Manual for LS-DYNA Soil Material Model 147
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CHAPTER 4. SUMMARY
This report presents the theory manual, user's manual, and typical examples for the FHWA soil material model implemented into LS-DYNA. This model was developed for use in roadside safety applications. The model is a modified Drucker-Prager plasticity model. In addition to the plasticity model, the FHWA soil material model includes pre-peak hardening, post-peak strain softening (damage), strain-rate effects (strength enhancement), pore-water effects (moisture effects), and erosion capability. These enhancements to the standard soil material models were made to increase the accuracy, robustness, and ease of use for roadside safety applications.
The theory manual gives a detailed description of the model, including the justification, equations, and methods of implementation of the equations. Developers should be able to use the theory manual to make modifications to, or maintain, the FHWA soil material model. Appendix A presents the details of the determination of the plasticity gradients.
The user's manual is a contractor's user's manual for the FHWA soil material model. Also included is a table that shows the correspondence between the symbols used in the theory manual and the input variables in the user's manual. A brief discussion of the use of the model in roadside safety applications is also included in this section.
Finally, typical examples are presented. These examples should help the user prepare the input and check out the model for their versions of LS-DYNA. The direct shear test simulation was stopped at approximately 46 ms because of shear locking and premature erosion (deletion) of the selectively reduced integration (8 gauss integration point) elements. The problem of shear locking is well known for this type of element when the element is subjected to large distortions. Premature erosion of this element type is a deficiency of the current version of LS-DYNA (version 970 Beta) and is not ascribable to improper functioning of the FHWA soil material model. The use of ALE with constant stress elements (1 gauss integration point) may produce results with greater simulation times.
Presently, the FHWA soil material model has been shown to be accurate for small soil samples at all levels of deformation and for large simulations that involve small to intermediate deformations. This restriction is caused by limitations in the current version of LS-DYNA.
Additional work is recommended to provide a more robust FHWA soil material model. This work includes investigation of the use of ALE for simulations involving large distortions, investigation/development of nonreflecting boundaries, and further simulations investigating moisture and strain-rate effects. Additional testing would be advantageous for the determination of accurate material properties (soil material model input).