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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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13 - Static Wood Post Test Simulations

13.1 - Static Post Model

Static post testing consisted of both using neoprene and not using neoprene wedged between the wood post and the braces holding the post. Similar results were obtained with or without the neoprene, so eventually the static tests switched entirely to no neoprene being used for convenience.

Because of anticipated contact difficulties with the simulations, two models were made to simulate the physical testing: one model with neoprene and one model without (see figure 74). The area of concern was at the contact interface between the wood post and the edge of the brace. It was thought, and later proven to be true, that the neoprene acted like a softening mechanism between the relatively soft wood post and the rigid clamping frame. However, this effect also proved to be insignificant with regards to the overall post behavior.

In the model, the wood post is divided into three parts: (1) part 8001, the loading area on the post; (2) part 8002, the area where the post breaks; and (3) part 8000, the remainder of the post. All three post parts have the same wood material properties. The post is clamped between rigid, fixed braces (part 8004). Loading is supplied to the post through the loading bolt (part 8003) using a prescribed velocity motion of 1 mm/ms.

After many different model variations, it was determined that the model without the neoprene could be used for simulating the physical static tests. Some difficulties were encountered while developing the baseline static model. A final model variation was developed that incorporated a rounded edge on the brace (as shown in figure 75). This rounded edge eliminated several contact issues that made the previous models very sensitive to the new wood material model.

Baseline Model

A baseline model is established to compare the various static post test simulations. This baseline model uses the default wood material properties for southern yellow pine, which has a grade of 1, a moisture content of 30 percent, and a temperature of 20 degrees Celsius (°C) (*MAT_WOOD_PINE).

Figure 74.

Figure 74. Static post models.

Figure 75.

Figure 75. Rounded edge on brace.

13.2 - Baseline Model Versus Test Comparison

This section compares baseline simulation results to physical testing for the static post test. The initial behavior of the model is good; however, during fracture, the material model is too weak (as shown in figures 76 and 77 below).

The initial fracture strength of the baseline simulation occurs between the minimum and maximum test values. Once the post begins to fracture, simulation forces fall below the minimum force levels seen in testing. Figure 76.

Figure 76. Force deflection: Baseline versus test.

The total fracture energy of the baseline simulation falls below the minimum test value. Figure 77.

Figure 77. Energy deflection: Baseline versus test.

13.3 - Baseline Versus Refined-Mesh Comparison

This section shows that the wood material model is mesh-size dependent. After the initial fracture, the reduced-size mesh is shown to be much weaker than the baseline mesh (as shown in figures 78 and 79). The baseline mesh uses an element size of 25.4 mm by 25.4 mm by 25.4 mm, while the refined mesh uses an element size of 12.7 mm by 12.7 mm by 12.7 mm.

Similar initial fracture strengths occur in both the baseline model and the refined-mesh model. The baseline model retains strength over a longer distance than the model with the refined mesh. Figure 78.

Figure 78. Rounded edge on brace.

The baseline model has higher fracture energy than the refined-mesh model. Figure 79.

Figure 79. Energy deflection: Baseline versus refined mesh.

Figure 80a.
(a) 29 mm
Figure 80b.
(b) 58 mm
Figure 80c.
(c) 116 mm
Figure 80d.
(d) 189 mm
Figure 80e.
(e) 29 mm
Figure 80f.
(f) 59 mm
Figure 80g.
(g) 116 mm
Figure 80h.
(h) 189 mm

Figure 80. Variations by mesh size: Deformed geometry.

13.4 - Parameter Study

Force-Deflection Behavior: *MAT_WOOD_PINE, Static Post Test Simulation

With respect to wood grade, increasing strength is observed as the grade is improved. Figure 18a.
With respect to moisture content, fully saturated wood is weak, but ductile. As moisture content decreases, the material becomes stronger and more brittle. When the wood becomes very dry, the strength decreases, yet maintains brittle behavior. Figure 81b.
With respect to temperature, the wood becomes stronger as the temperature decreases. Figure 81c.

Figure 81. Force-deflection behavior as a function of grade, moisture content, and temperature.

Energy-Deflection Behavior: *MAT_WOOD_PINE, Static Post Test Simulation

With respect to wood grade, fracture energy increases with improved grade. Figure 82a.
With respect to moisture content, fracture energy varies slightly with changes in moisture content. When wood becomes very dry, fracture energy is reduced significantly. Figure 82b.
Fracture energy varies slightly with changes in temperature. Figure 82c.

Figure 82. Energy-deflection behavior as a function of grade, moisture content, and temperature.

1 Default
Figure 83a.
(a) 29 mm
Figure 83b.
(b) 58 mm
Figure 83c.
(c) 116 mm
Figure 83d.
(d) 189 mm
Figure 83e.
(e) 29 mm
Figure 83f.
(f) 58 mm
Figure 83g.
(g) 116 mm
Figure 83h.
(h) 188 mm
Figure 83i.
(i) 29 mm
Figure 83j.
(j) 58 mm
Figure 83k.
(k) 115 mm
Figure 83l.
(l) 188 mm

Figure 83. Variation by grade: Deformed geometry.

30% Default
Figure 84a. Figure 84b. Figure 84c. Figure 84d.
20% Default
Figure 84e. Figure 84f. Figure 84g. Figure 84h.
10% Default
Figure 84i. Figure 84j. Figure 84k. Figure 84l.
1% Default
Figure 84m. Figure 84n. Figure 84o. Figure 84p.

Figure 84. Variation by moisture content: Deformed geometry.

30% Default
Figure 85a. Figure 85b. Figure 85c. Figure 85d.
20% Default
Figure 85e. Figure 85f. Figure 85g. Figure 85h.
10% Default
Figure 85i. Figure 85j. Figure 85k. Figure 85l.
1% Default
Figure 85m. Figure 85n. Figure 85o. Figure 85p.

Figure 85. Variation by temperature: Deformed geometry.

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