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Publication Number: FHWAHRT04096
Date: August 2005 
Evaluation of LSDYNA Wood Material Model 143PDF Version (8.16 MB)
PDF files can be viewed with the Acrobat® Reader® 7  Additional Evaluation CalculationsA number of LSDYNA parameter calculations were performed in order to understand wood model issues related to: (1) plasticity algorithm iterations, (2) use of fully integrated elements, (3) the erosion criteria, and (4) the assumption of perfect plasticity. These issues are discussed here for both the static post and dynamic bogie impact simulations. 7.1  Plasticity Algorithm IterationsStatic post bending simulations indicate that the calculated loaddeflection curves are insensitive to the number of plasticity algorithm iterations. Figure 45 demonstrates that there is little difference in the loaddeflection curves calculated with one or five iterations. However, bogie impact calculations performed with five iterations produced erroneous behavior in which damage, followed by erosion, was overcalculated. All default parameters were selected from static and dynamic calculations performed with one iteration. Therefore, one iteration is selected as the default number of iterations. The user may override this number. Caution is suggested when using more than one iteration because the iterations parameter has not been thoroughly evaluated. Additional evaluations of the iterations parameter are recommended for future efforts. Figure 45. The default number of plasticity algorithm iterations is set to one because 7.2 Fully Integrated ElementsThe static post peakload deflection comparisons previously shown in figure 27 indicate that the fully integrated S/R (type 2 eightpoint integration) elements produce a more brittle behavior than the standard underintegrated elements. This is probably because the fully integrated elements erode when just one of the eight integration points fails. By failure, we mean that the wood material model calculates a 99.9percent reduction in stiffness and strength (in all six stress components both parallel and perpendicular to the grain) at that integration point. Therefore, seven of the eight integration points could still be loaded in tension when the element erodes. As a check, static bending calculations were performed with and without erosion using both the standard and fully integrated elements (as shown in figure 46). There is essentially no difference between the responses calculated with and without erosion when standard elements are used (not shown). However, some difference is calculated when fully integrated elements are used. This indicates that the fully integrated elements are eroding prematurely (while still carrying load). One possible consequence of premature erosion is a fracture energy that is meshsize sensitive and problemdependent. Figure 46. Erosion affects the fully integrated element curves, but not the underintegrated element Roadside safety applications are primarily dynamic, so bogie impact simulations were also performed with fully integrated elements in the breakaway region for comparison with underintegrated elements. Deformed configurations with damage fringes are shown in figure 47. Energydeflection and velocityreduction histories are shown in figure 48. The damage fringes and histories calculated with eightpoint integration are similar to those calculated with singlepoint integration. These bogie impact simulations suggest that eightpoint integration can currently be used in the breakaway region of bogie impact simulations; however, analysts are urged to use it with caution. Preliminary calculations performed with eightpoint integration in the impact region simulate excessive erosion. Therefore, use of eightpoint integration in the impact region is not currently recommended. 7.3  Erosion CriteriaChecks were also performed on the erosion criteria. As the default setup, elements erode when failure occurs parallel to the grain because all six components of stress are degraded to near zero. Elements do not erode when failure occurs perpendicular to the grain because only three components of stress (perpendicular to the grain) are degraded to near zero. The paralleltothegrain stress components are not degraded with perpendicular damage. Thus, the element is still able to carry load parallel to the grain after perpendicular failure occurs. All default parameters were selected from static and dynamic calculations performed with the default erosion criteria. As an option, a flag is included to request erosion once perpendiculartothegrain failure occurs. For simplicity, this flag is called the perpendicular erosion flag and this erosion option is referred to as perpendicular erosion. Both static and dynamic simulations were performed with and without perpendicular erosion. The static calculations are discussed first, followed by the dynamic calculations. Static deformed configurations and loaddeflection curves are shown in figures 49 and 50, respectively, for calculations performed with and without perpendicular erosion. The deformed configuration calculated with perpendicular erosion is more realistic than the deformed configuration calculated without perpendicular erosion. To see the perpendicular damage in the calculation without perpendicular erosion, one can look at damage fringes (as shown in figure 49(c)). Red denotes elements with damage levels of d > 0.80, blue denotes elements with a damage range of 0.60 < d < 0.80, and cyan denotes elements with a damage range of 0.40 < d < 0.60. Perpendicular erosion also has a minor effect on the static loaddeflection curves. The postpeak softening behavior calculated with perpendicular erosion is slightly more brittle than that calculated without perpendicular erosion (figure 50). Dynamic deformed configurations and loaddeflection curves are shown in figures 51 and 52, respectively, for calculations performed with and without perpendicular erosion. As similarly noted for the static simulations, the breakaway region calculated dynamically with perpendicular erosion looks more realistic than that calculated without perpendicular erosion. However, slight erosion is also calculated in the impact region even though no visible damage was reported in the tests. Perpendicular erosion has little demonstrated effect on the loaddeflection curves. Figure 50. Loaddeflection curves calculated with perpendicular erosion are In fact, in some cases, perpendicular erosion can have an unrealistic effect on the calculated response. Some preliminary calculations performed with the simplest elastic bogie (without neoprene on the cylinder) simulated excessive erosion in the impact region. One such calculation is demonstrated in figure 53. One possible approach is to request perpendicular erosion in the breakaway region, but not request it in the impact region. Therefore, perpendicular erosion is not the default option (it must be specifically requested), nor is it recommended for general use. Figure 53. Use of perpendicular erosion causes excessive erosion to be 7.4 PostPeak Hardening ParameterThe default behavior of the wood model is perfectly plastic in both parallel and perpendiculartothegrain compression. This means that there is no increase or decrease in strength with increasing strain. Perfectly plastic behavior was previously demonstrated in figure 1(c). The FPL clear wood and timber compression data previously analyzed in figures 3 and 5 exhibit perfect plasticity, at least for perpendicular strains as great as 4 percent. However, parallel strains of 20 to 30 percent are typically calculated at ground level in the compressive region of the post in the bogie impact calculations. Recent uninstrumented, unconfined compression tests of pine samples conducted by the user indicate softening at large strain parallel to the grain and hardening perpendicular to the grain. Postpeak softening in compression (parallel or perpendicular) is not currently available in the wood material model. Although the damage model (which is responsible for softening) is applied to the stresses in compression, the stresses do not soften, because a compressive fracture energy of infinity is assumed. Fracture energy in compression is not currently an input value (as it is for tension and shear). Infinite fracture energy is hardwired into the model. To elicit softening, finite fracture energy needs to be included as input. Postpeak hardening in compression is currently available as an option in the wood model. Singleelement simulations with and without postpeak hardening are demonstrated in figure 54 parallel to the grain. Postpeak hardening requires the input of a single hardening parameter. A value of zero models perfect plasticity. Values greater than zero model hardening. At this time, the same parameter is used for both parallel and perpendicular modes (because of the limited input parameter slots available during development as a usersupplied material model). Separate parameters are recommended as a future modification to the model. Figure 54. These singleelement simulations demonstrate postpeak hardening in compression with positive values of G_{hard}. All default parameters were selected from calculations run with perfect plasticity in compression. Small amounts of postpeak hardening have little effect on the static or bogie impact simulations. However, calculations involving high levels of compaction may benefit from postpeak hardening. This is demonstrated in figure 55 for a calculation performed with postpeak hardening (parallel and perpendicular). The post exhibits substantial compression in the elements in the vicinity of the rigid support. However, this same calculation aborted, prior to achieving the deformed configuration shown, when perfect plasticity was modeled. Figure 55. Inclusion of postpeak hardening, both parallel and perpendicular to the grain, prevented this calculation from aborting at a large deflection. Recommendations for future efforts include laboratory compression measurements and wood model enhancements. The laboratory compression measurements should include stress displacement and fracture energy for clear wood and graded wood samples, both parallel and perpendicular to the grain. These measurements should be made as a function of moisture content. If these measurements show softening, as recently measured parallel to the grain, then model enhancements should proceed. The wood model enhancements should include an input slot for compressive fracture energy; smooth variation of the fracture energy between compression, shear, and tension; and identification of default compressive fracture energy values as a function of moisture content. If these measurements show hardening, as recently measured perpendicular to the grain, then the existing postpeak hardening model should be evaluated for accuracy and for selection of default postpeak hardening parameters. Some porous materials exhibit substantial stiffening at high strain levels (70 to 80 percent) after all pores are compacted, which is called lockup. The current postpeak hardening formulation does not model lockup. The need for a lockup model should be assessed with regards to roadside safety applications. 