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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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4 - Timber-Bending Test Correlations

FPL performed full-scale tests of southern yellow pine timbers in parallel-to-the-grain four-point bending.(6) The timber cross section is 15.24 cm by 25.24 cm, and the timber length is 304.8 cm. Load-deflection histories were measured for select structural and grade 2 timbers. Moisture content and failure mode were recorded. The average moisture content is 14 percent, although measurements as low as 9 percent and as high as 23 percent were recorded.

The developer performed multi-element simulations of one or more tests to evaluate the bending response of the wood material model and to select quality factors in tension. Comparisons of the simulations with measured load-deflection curves are shown in figure 6 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 for better viewing.

Load-deflection curves from three calculations are shown in each plot. One curve is a clear wood simulation that models a higher bending strength than that measured. The second curve applies a quality factor to the tensile strength (QT) that is equal to the quality factor applied to the compressive strength (QC). The QC value is that selected from the timber compressive simulations discussed in the previous section. These simulations also model higher bending strengths than those measured. The third curve applies a tensile quality factor that is less than the compressive factor. In addition, a quality factor is also applied to the stiffness (Qstiff). These simulations correlate well with the measured data.

A good correlation of the select structural simulation is obtained with a quality factor of QT = 0.25. This value is much lower than that previously selected in compression (QC = 0.49). A good correlation of the grade 2 simulation is obtained with a quality factor of QT ≤ 0.25. This value is also lower than that previously selected in compression (QC = 0.43). These correlations prompted the developer to model different quality factors in tension than in compression. The tensile quality factors are also applied to the shear strength.

Also note that a quality factor of 0.8 is applied to the stiffness for correlation with the test data (for both grades), although a quality factor of 1.0 is still reasonable. No methodology is currently implemented in the initialization routines of material model 143 to specify quality factors for stiffness. This is because clear wood stiffnesses are adequate for simulating graded wood stiffnesses based on calculations performed to date. However, quality factors for stiffness could readily be added if the need arises.

Figure 6a.

(a) Select structural

Figure 6b.

(b) Grade 2

Figure 6. These comparisons of the model with the parallel-to-the-grain timber-bending

test data demonstrate the need for different quality factors in tension and compression.

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