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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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3 - Timber Compression Test Correlations

FPL performed full-scale tests on dry southern yellow pine timbers in compression parallel to the grain.(6) A schematic of the test setup is reproduced in figure 4. The timber cross section is 15.24 centimeters (cm) by 15.24 cm, and the timber length is 304.8 cm. Load-deflection histories were measured for select structural and grade 2 timbers. Moisture content, failure location, and defect-initiated failures were documented. The average moisture content is 12 percent, although measurements as low as 7 percent and as high as 18 percent were recorded. The average strength measurements are:

  • 25.7 MPa for select structural.
  • 22.7 MPa for grade 2.

The default strength of clear wood at 12-percent moisture content is 52.7 MPa.

Figure 4

1 inch = 25.4 millimeters (mm), 1 pound force (lbf) = 0.004448 kilonewton (kN)

Figure 4. Schematic of FPL timber compression test setup.

The developer performed multi-element simulations of these tests to:

  • Evaluate the behavior of the model.
  • Select default quality factors.
  • Select default hardening parameters in parallel-to-the-grain compression.

Comparisons of the simulations with the test data are shown in figure 5 as a function of grade. The black lines are the simulations. The red and colored lines are the test data. The colored lines highlight specific data curves whose strength is approximately average.

Load-deflection curves from two calculations are shown in each plot. One curve is a clear wood simulation that models a higher yield strength than measured. The second curve applies a strength-reduction quality factor (QC) to the compressive strength to correlate the calculated yield strength with the measured yield strength.

A good correlation of the select structural simulation is obtained with a factor of QC = 0.49 applied to the compressive strength. This factor is the ratio of the average select structural timber compression strength (25.7 MPa) divided by the average clear wood compression strength (52.7 MPa) at 12-percent moisture content. A good correlation of the grade 2 simulation is obtained with a factor of QC = 0.43 applied to the compressive strength. This factor is the ratio of the average grade 2 timber compression strength (22.7 MPa) divided by the average clear wood compression strength (52.7 MPa) at 12-percent moisture content. No quality factors are applied to the stiffness, although a factor of 0.8 is probably reasonable for grade 2.

Figure 5a
(a) Select structural
Figure 5b
(b) Grade 2
Figure 5. These comparisons of the model with test data were used to set the
hardening behavior of the southern yellow pine model in parallel-to-the-grain compression.

These comparisons were made with different hardening parameter values for each grade. Good correlations could not be achieved using the same hardening parameter values for each grade. Therefore, the default material property methodology was set up to specify hardening parameters as a function of grade. This is more thoroughly discussed in the wood model user’s manual.(2)

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