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Publication Number: FHWA-HRT-04-096
Date: August 2005

Evaluation of LS-DYNA Wood Material Model 143

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16 - User's Conclusions

Conclusions that were documented by the user and based on the validation effort detailed in chapters 8 through 15 of this report are as follows:

Single-element models pulled in tension parallel to the grain behaved well and a parametric study involving grade, moisture content, and temperature yielded good results. Because of time limitations, various other single-element simulations were not performed, but are recommended (including compression, shear, and torsion loading both parallel and perpendicular to the grain).

Static post test simulation results did not match the physical test results. The initial fracture strength of the simulation was good; however, the strength of the fracturing post was significantly below the minimum strength seen in physical testing.

Wood material behavior is very sensitive to mesh size. When a refined mesh was used in the static post test simulation, the initial fracture strength was similar to the baseline model. However, the strength of the fracturing post dropped by almost an order of magnitude relative to the baseline model.

The dynamic post test simulations had significant problems with time-step calculations, contacts, and bending behavior as opposed to a snap-off behavior. Further analysis resulted in post vaporization even when using an extremely small time step (0.0001 ms). Also, a significant volume expansion of the wood elements was noted (up to 75 percent volume expansion in individual elements). Additionally, highly distorted elements with damage values of 0.99 did not erode as expected. Because of these difficulties with the dynamic post simulations, a comparison to physical testing of dynamic posts was not made.

Finally, hourglassing in the wood material model appears to be a very significant issue. A parameter study was needed to determine a good hourglass control type and hourglass control parameter for the dynamic post model. When the grade of the wood was changed from grade 1 to grade DS-65, hourglassing reappeared in the model using that same hourglass control. Fully integrated elements are not an option at this time because the eroding characteristics of the wood model do not behave well for fully integrated elements.

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