Introduction to ANSYS LS-DYNA - mescalea.grupossc.com

1 © 2017 ANSYS, Inc. October 13, 2017

Release 18.2

Workshop 04.1: Tubes Quasi-static AnalysisIntroduction to ANSYS LS-DYNA


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?


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


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.


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.


• 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


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


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.


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


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”.


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.


• 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


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.

Introduction to ANSYS LS-DYNA - mescalea.grupossc.com

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