Introduction to ANSYS LS-DYNA - mescalea.grupossc.com
Transcript of Introduction to ANSYS LS-DYNA - mescalea.grupossc.com
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1 © 2017 ANSYS, Inc. October 13, 2017
Release 18.2
Workshop 04.1: Tubes Quasi-static AnalysisIntroduction to ANSYS LS-DYNA
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Motivation
• In Workshop 03.1 we had a drop test. A tool was dropped and hit the tubes. That case was based on the transfer of kinetic energy of the tool to internal energy of the tubes.
• This example represents a load from a hydraulic cylinder. The speed of a cylinder is based on the hydraulic pump and the control system and takes maybe a few seconds.
• It would take a long time to run the simulation in the physical time frame.
• Can we accelerate the simulation?
• How fast we can run the simulation and which criteria must we fulfill?
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Create the static object
• Return to the Project saved from Workshop 03.1, or, restore the archive “WS04.1_Quasi-Static.wbpz”.
• To preserve the analysis settings use the context menu on B1 to duplicate the workbench object, then open the model from Schematic C4
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Time estimation
• Lowest natural frequency of the outer tube is 229Hz when assuming a simple support on both sides (Roark’s Formulas for Stress and Strain).
• To describe a wave of this frequency we make a trial run encompassing 10 cycles for this frequency.
• We take 1/229 * 10 ≈ 0.045 sec.
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Modification from the dynamic to a quasi static analysis
• Change the “End Time” to 0.045 sec.
• Suppress Velocity in the Initial Conditions.
• Add a Velocity support scoped to the impactor body.
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• Use the EXCEL spreadsheet, Trapezoidal-curve_2.xls, to generate the tabular data for a displacement of -300mm, a duration of 0.045sec and ramp duration of 0.009 sec.
• Copy and paste the table into Tabular Data.
Displacement set up for impactor velocity
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Prepare the output
• Add the Result Tracker objects for contact force impactor/support as well the kinetic/internal energy of all deformable parts.
• Run the simulation
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Energy evaluation
• The internal energy of all three deformable parts is much larger than the kinetic energy of the parts.
• This is necessary but not sufficient.
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Contact force evaluation
• Select “Contact Force” and “Contact Force 2”.
• The lowest natural frequency looks smooth but we have higher frequency oscillations.− Ctrl select with left mouse button two sequential peaks on either curve to read time/force data
− Oscillating frequencies in excess of 500 hertz exist
• Newton’s Third Law “Action and Reaction“ is not fulfilled for the contact force of impactor/support: +20013N / -18353N
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Filtering the Contact Force
• We have an oscillation > 500Hz and to hide this frequency we use a Butterworth filter with a “Cut Frequency” of 100Hz. 100Hz coming through.
• A Butterworth filter is a maximally flat magnitude filter and acts as a lowpass.
• The highest usable cut frequency is, based on the Nyquist theorem, less than half of the frequency of your output frequency of the “Time History Output”.
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A larger time duration
• Duration 0.045sec was too short
• Do a convergence study for a better estimation. Don’t forget to change the velocity support curve based upon each new duration.
• By trial and error a value of 0.25sec as end time seems to be sufficient.
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• Use the EXCEL spreadsheet again to generate the tabular data for a displacement of -300mm, a duration of 0.25sec and ramp duration of 0.05 sec.
• Copy and paste the table into the velocity support.
Displacement set up larger duration
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Contact force evaluation
• Run the simulation and select “Contact Force” and “Contact Force 2”, filter with 100Hz and make a chart object.
• The results are much better balanced than before. The condition for quasi-static is now fulfilled.