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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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12 - Single Element: Tension Parallel to the Grain

A single solid element is used to study the developer’s wood material model in uniaxial tension parallel to the grain. Specifically, a parameter study investigating the southern yellow pine option is performed. The parameters investigated are wood grade, moisture content, and temperature. Stress-strain curves, volume-time curves, and changes in the 29 wood parameters (as reported in the d3hsp file) are presented.

The stress-strain behavior of the single element in tension is as desired. In all cases, stress increases linearly with respect to strain until a peak strength is reached, followed by a gradual decrease in stress as the material undergoes damage and eventually fails. With respect to wood grade, increasing strength is observed as the grade is improved. With respect to moisture content, fully saturated wood is relatively weak, but ductile. As the moisture content decreases, the material becomes stronger and more brittle. When the wood becomes very dry, the strength decreases, yet maintains a brittle behavior. With respect to temperature, low-temperature pine is relatively strong, yet brittle, while wood at a high temperature is relatively weak and ductile.

The volume of the single elements in tension parallel to the grain uniformly increases during all simulations by as much as 6 percent. This is an unexpected and undesirable result because it is believed by the user that, at a minimum, volume should be conserved for wood. A significant example of this volume expansion is shown in chapter 14 of this report.

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).

12.1 - Stress-Strain Behavior: *MAT_WOOD_PINE

In all cases, stress increases linearly with respect to strain until a peak strength is reached, followed by a gradual decrease in stress as the material undergoes damage and eventually fails.

With respect to wood grade, increasing strength is observed as the grade is improved.

Figure 72a.
(a) Variation by grade
With respect to moisture content, fully saturated wood is relatively weak, but ductile. As the moisture content decreases, the material becomes stronger and more brittle. When the wood becomes very dry, the strength decreases, yet maintains a brittle behavior. Figure 72b.
(a) Variation by moisture
With respect to temperature, low-temperature pine is relatively strong, yet brittle, while wood at a high temperature is relatively weak and ductile. Figure 72c.
(a) Variation by temperature

Figure 72. Stress-strain behavior of single elements.

12.2 - Volume of Element: *MAT_WOOD_PINE

The volume of the single element in tension parallel to the grain uniformly increases during all simulations by as much as 6 percent. This is an unexpected and undesirable result because it is believed by the user that, at a minimum, volume should be conserved for wood. A significant example of this volume expansion is shown in chapter 14 of this report.

Note: The drop-off of volume on curves is where failure of the element occurs.

Figure 73.

Figure 73. Volumetric behavior of single elements.

12.3 - Material Properties: *MAT_WOOD_PINE

When using the *MAT_WOOD_PINE option, the user specifies parameters such as wood grade, moisture content, and temperature. However, the underlying wood material model is based on a large set of theoretical parameters. By setting the wood grade, for example, the underlying parameters are modified appropriately. This section provides a set of tables (tables 8, 9, and 10) that record the variations in the underlying parameters based on changing a single parameter.

Table 8. Parameters based on wood grade.
Single Solid Element - Uniaxial Tension Parallel to the Grain
Units kg, mm, ms, kN, GPa
Density 6.731E-07 kg/mm3
*MAT_WOOD_PINE
Default Parameters Except for Grade
         
    Grade 1
(default)
Grade DS-65 Grade Clear
Stiffness:        
EL Parallel Normal Modulus 11.350000    
ET Perpendicular Normal Modulus 0.246800    
GLT Parallel Shear Modulus 0.715200    
GLR Perpendicular Shear Modulus 0.087510    
PR Parallel Major Poisson's Ratio 0.156800    
         
Strength:        
Xt Parallel Tensile Strength 0.040030 0.068130 0.085160
Xc Parallel Compressive Strength 0.013320 0.019670 0.021150
Yt Perpendicular Tensile Strength 0.000963 0.001640 0.002050
Yc Perpendicular Compressive Strength 0.002571 0.003796 0.004082
Sxy Parallel Shear Strength 0.004275 0.007277 0.009096
Syz Perpendicular Shear Strength 0.005985 0.010190 0.012730
         
Damage:        
Gf1 Parallel Fracture Energy in Tension 0.020050 0.034130 0.042660
Gf2 Parallel Fracture Energy in Shear 0.041480 0.070610 0.088260
Bfit Parallel Softening Parameter 30.000000    
Dmax Parallel Maximum Damage 0.999900    
Gf1 Perpendicular Fracture Energy in Tension 0.000401    
Gf2 Perpendicular Fracture Energy in Shear 0.000830    
Dfit Perpendicular Softening Parameter 30.000000    
Dmax Perpendicular Maximum Damage 0.990000    
         
Rate Effects:        
Flpar Parallel Fluidity Parameter: Tension/Shear 0.000    
Flparc Parallel Fluidity Parameter: Compression 0.000    
Pow_par Parallel Power 0.000    
Flper Perpendicular Fluidity Parameter: Tension/Shear 0.000    
Flperc Perpendicular Fluidity Parameter: Compression 0.000    
Pow_per Perpendicular Power 0.000    
         
Hardening:        
Npar Parallel Hardening Initiation 0.500    
Cpar Parallel Hardening Rate 1008.000 462.500 400.000
Nper Perpendicular Hardening Initiation 0.400    
Cper Perpendicular Hardening Rate 252.000 115.600 100.000
         
Note: Only those values that changed because of the change in grade are provided in the table.
Table 9. Parameters based on moisture content.
Single Solid Element - Uniaxial Tension Parallel to the Grain
Units kg, mm, ms, kN, GPa
Density 6.731E-07 kg/mm3
*MAT_WOOD_PINE
Default Parameters Except for Moisture Content (MS)
           
    MC = 30%
(default)
MC = 20% MC = 10% MC = 1%
Stiffness:          
EL Parallel Normal Modulus 11.350000 12.560000 15.490000 16.720000
ET Perpendicular Normal Modulus 0.246800 0.461900 0.910100 0.959700
GLT Parallel Shear Modulus 0.715200 0.736900 0.789800 0.811900
GLR Perpendicular Shear Modulus 0.087510 0.165500 0.332300 0.349300
PR Parallel Major Poisson's Ratio 0.156800 0.184200 0.258600 0.303300
           
Strength:          
Xt Parallel Tensile Strength 0.040030 0.052380 0.066190 0.042590
Xc Parallel Compressive Strength 0.013320 0.018580 0.037000 0.054760
Yt Perpendicular Tensile Strength 0.000963 0.001458 0.002139 0.001477
Yc Perpendicular Compressive Strength 0.002571 0.003627 0.007145 0.010310
Sxy Parallel Shear Strength 0.004275 0.005577 0.008526 0.009351
Syz Perpendicular Shear Strength 0.005985 0.007808 0.011940 0.013090
           
Damage:          
Gf1 Parallel Fracture Energy in Tension 0.020050 0.015660 0.013840 0.011670
Gf2 Parallel Fracture Energy in Shear 0.041480 0.046150 0.050160 0.028480
Bfit Parallel Softening Parameter 30.000000      
Dmax Parallel Maximum Damage 0.999900      
Gf1 Perpendicular Fracture Energy in Tension 0.000401 0.00313 0.000277 0.000233
Gf2 Perpendicular Fracture Energy in Shear 0.00830 0.000923 0.001003 0.000570
Dfit Perpendicular Softening Parameter 30.000000      
Dmax Perpendicular Maximum Damage 0.990000      
           
Rate Effects:          
Flpar Parallel Fluidity Parameter: Tension/Shear 0.000      
Flparc Parallel Fluidity Parameter: Compression 0.000      
Pow_par Parallel Power 0.000      
Flper Perpendicular Fluidity Parameter: Tension/Shear 0.000      
Flperc Perpendicular Fluidity Parameter: Compression 0.000      
Pow_per Perpendicular Power 0.000      
           
Hardening:          
Npar Parallel Hardening Initiation 0.500      
Cpar Parallel Hardening Rate 1008.000      
Nper Perpendicular Hardening Initiation 0.400      
Cper Perpendicular Hardening Rate 252.000      
           
Note: Only those values that changed because of the change in moisture content are provided in the table.
Table 10. Parameters based on temperature.
Single Solid Element - Uniaxial Tension Parallel to the Grain
Units kg, mm, ms, kN, GPa
Density 6.731E-07 kg/mm3
*MAT_WOOD_PINE
Default Parameters Except for Temperature
           
    Temperature = 30°C Temperature = 20°C
(default)
Temperature = 10°C Temperature = 1°C
Stiffness:          
EL Parallel Normal Modulus 10.620000 11.350000 11.970000 12.420000
ET Perpendicular Normal Modulus 0.230800 0.246800 0.260200 0.270000
GLT Parallel Shear Modulus 0.668900 0.715200 0.754000 0.782400
GLR Perpendicular Shear Modulus 0.081840 0.087510 0.092250 0.095730
PR Parallel Major Poisson's Ratio   0.156800    
           
Strength:          
Xt Parallel Tensile Strength 0.034840 0.040030 0.044370 0.047550
Xc Parallel Compressive Strength 0.01160 0.013320 0.014770 0.015830
Yt Perpendicular Tensile Strength 0.000839 0.000963 0.001068 0.001144
Yc Perpendicular Compressive Strength 0.002238 0.002571 0.002850 0.003055
Sxy Parallel Shear Strength 0.003721 0.004275 0.004739 0.005079
Syz Perpendicular Shear Strength 0.005210 0.005985 0.006634 0.007110
           
Damage:          
Gf1 Parallel Fracture Energy in Tension 0.021440 0.020050 0.010460 0.002657
Gf2 Parallel Fracture Energy in Shear 0.044360 0.041480 0.021640 0.005498
Bfit Parallel Softening Parameter   30.000000    
Dmax Parallel Maximum Damage   0.999900    
Gf1 Perpendicular Fracture Energy in Tension 0.000429 0.000401 0.000380 0.000367
Gf2 Perpendicular Fracture Energy in Shear 0.000887 0.000830 0.000787 0.000758
Dfit Perpendicular Softening Parameter   30.000000    
Dmax Perpendicular Maximum Damage   0.990000    
           
Rate Effects:          
Flpar Parallel Fluidity Parameter: Tension/Shear   0.0000    
Flparc Parallel Fluidity Parameter: Compression   0.0000    
Pow_par Parallel Power   0.0000    
Flper Perpendicular Fluidity Parameter: Tension/Shear   0.0000    
Flperc Perpendicular Fluidity Parameter: Compression   0.0000    
Pow_per Perpendicular Power   0.0000    
           
Hardening:          
Npar Parallel Hardening Initiation   0.500    
Cpar Parallel Hardening Rate   1008.000    
Nper Perpendicular Hardening Initiation   0.400    
Cper Perpendicular Hardening Rate   252.000    
           
Note: Only those values that changed because of the change in moisture content are provided in the table.

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