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Q: When I run vabs.exe, a dos windows appears, and ask me the file name of input data,after I provide the input name,the window disappears.
A: Most possibly, your input file has something wrong. Please prepare your input data according to instructions specified in the ReleastNotes.dat. You are suggested to go to the command window and run VABS under the folder where you have both vabs.exe and input files both available in the same folder. You could use either your own input file or try with the sample input files.
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Q: How to interpret VABS results?
A: VABS results are given with respect to the coordinate system the user chooses to create the mesh. The origin of the coordinate system and the orientation of the axes will affect the VABS calculated results.
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Q: Can VABS provide the values with respect to neutral axes and principal bending axes?
A: The current version of VABS (VABS II, release 6/24/2004) can calculate the location of neutral axes. However, the stiffness matrix in the principal bending axes with origin at the neutral axes is not available in VABS II. To obtain the values in this axes system, one needs to do the following:
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Shift the origin of the input file for VABS to the neutral axes. That is, let the original x2, x3 minus Xe2, Xe3, respectively.
- Run VABS again with the modified input to obtain a new REDUCED CLASSICAL STIFFNESS MATRIX, referring to a new axis system with origin at the neutral axes.
- Calculate the principal axes system using Tan (2*PHI)=-2*Ixy/(Iy-Ix).
- Transfer the new axes system into the principal bending directions using direction cosine matrix to create a new input for VABS, please be advised for composite blade, the layerup orientations must also be modified.
- Run VABS once again to obtain the results in the principal bending axes system with origin at the neutral axes.
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Q: Are there any interfaces between VABS and ANSYS, and some other FEA preprocessors?
A: The interface between VABS and ANSYS has been developed under the sponsorship of Rotor Industry Technology Association at Georgia Tech. The Interface between VABS and PATRAN is currently under development. You need to check Dr. Hodges @ dewey.hodges@ae.gatech.edu for details and availability. If you have other specific FEA package and want to interface with VABS, please let us know.
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Q: What are the use of *.v0, *.V1s, *.V1 files?
A: These three files are intermediate files and only used by VABS. They are in free format storing the warping functions corresponding to each node for difference corrections according to an optimized numbering scheme. A 4X4 Stiffness matrix are also output at the end of *.v0 and *.V1 files. A 6X6 flexibility matrix is output at the end of *.V1s file. They are not supposed to be used by common users. If you want the warping of the section, you need to use the VABS recovery option.
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Q: Regarding the recovery, how do I get displacement and direction cosine matrix terms required for classical beam theory and how do I get other derivative terms required as an input for Timoshenko theory and what are they? I only have forces and moment in three directions.
A: If you are interested to recover the 3D displacements over the cross section, you need to get the 1D displacement and direction cosine matrix term of the reference axis from a beam solver. For their definitions, please refer to our paper inJournal of American Helicopter Society. For Timoshenko beam, f_i and m_i are the distributed 1D applied load and inertial loads as defined in Hodges 1990's paper (Mixed formulation of moving beams). Because they are known as functions of x_1, the beam axis, you should be able to get the derivatives.
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Q: How to obtain the nodal ply angles for each element?
A: If the ply are not very curved, the nodal ply angles can just be the element ply angles. The element ply angles is the same as the ply where the element is. Regarding convention, you can read conventions.ppt. The nodal ply angle is the slope of the ply at the location given by the nodal coordinates. If you know the mathematical representation of the curve, you can calculate it by hand. If you have an arbitrary curve create using a CAD software, you should be able to write some macros to ask the software to calculate the value for you.
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Q: According to VABS manual, shear correction factors can be calculated for isotropic sections. Can it be for composite sections?
A: Shear correction factor has a clear definition for isotropic materials. However, for composite beams, it is recommended to use the shear stiffness from the stiffness matrix directly. If there is indeed such a need, you need to calculate some other value for transverse stiffness and let it divided by the value from VABS.
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Q: Is t possible to extract bending, axial or torsional stiffness values from the stiffness matrix? And the last one, could you verify that in VABS unit system that is used does not affect the results? When we use slug for mass and feet for length and lbf for force, we get the related values in terms of them, do not we?
A: For a closed section beam VABS extracts two appropriate cross-sectional stiffness matrices, one 4 by 4 and the other 6 by 6. There are no restrictions on the material or geometry (other than the section being closed -- for open sections one should use the generalized Vlasov option, a 5 by 5). So, yes, it handles composites just fine. Please refer to papers about VABS to find out the theoretical details. The 4 by 4 handles extension, torsion, and two bending, in that order; the 6 by 6 handles extension, two shears, torsion, and two bending, in that order. All possible couplings are included in both. As long as you're consistent with the units, the stiffness matrix should be consistent in that set of units.
Maximum stiffness optimization of composite beams for wind turbine applications - Blasques and Stolpe - WCSMO8 23042009.pdf
VABS FAQ (Pre 2007)
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