AFS-i6070202 w Dynamics

1/1 Introduction to ABAQUS/Standardand ABAQUS/Explicit

W4.1

ABAQUS

Workshop 4

Dynamics

Goals

• Become familiar with the input and output for frequency extraction, implicit dynamic,and explicit dynamic analyses.

• Become more familiar with the status (.sta) and message (.msg) files.

• Learn how to plot eigenmodes and create history plots using ABAQUS/Viewer.

• Learn how to perform a “restart” analysis.

Introduction

In this workshop the dynamic response of the cantilever beam shown in Figure W4–1 isinvestigated. The beam is modeled using B21 beam elements. A frequency extraction analysisstep is performed to determine the 10 lowest vibration modes of the beam. The problem is alsosolved in both ABAQUS/Standard and ABAQUS/Explicit by performing direct integrationdynamic analyses to simulate the vibration of the beam upon removal of the tip load.

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Figure W4–1. Problem Description

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Workshop 4: Dynamics


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Frequency Extraction Analysis

Change to the abaqus_solvers/workshop4 directory, and copy w4_beam.inp to anew file named freq.inp.

Input Specification

1. Make the following changes to freq.inp. Refer to the online documentation asnecessary.

a. Include a density of 2.3E-6 in the material definition. Add the following optionblock below the ∗MATERIAL option:*DENSITY

2.3E-6,

b. Change the procedure type from ∗STATIC to ∗FREQUENCY, and select theLanczos eigensolver. Request 10 modes. The finished option block should look likethe following:*FREQUENCY, EIGENSOLVER=LANCZOS

10,

c. Remove the loading, but retain the boundary conditions (one end built-in).

d. Limit the printed node output to the data (.dat) file by restricting output to a nodeset:

i. Create a node set called TIP that contains the node at the loaded end of the beam.*NSET, NSET=TIP

11,

ii.Add the following output requests within the first step:*NODE PRINT, NSET=TIP

U,

*OUTPUT, FIELD, FREQUENCY=999

*NODE OUTPUT

U,

2. Submit the frequency extraction analysis as an ABAQUS job.

3. After the analysis has completed, check the printed output file and make any necessarycorrections to the input.

Examining the Eigenmodes and Eigenvalues

1. Open the printed output file in an editor of your choice.



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2. Search for the second occurrence of “S T E P” to find the beginning of the analysisresults. The first table gives the eigenvalue output. Find the frequency (cycles/time) forthe lowest mode.

3. Visualize results:

a. Start ABAQUS/Viewer, and open the output database associated with this analysis.

b. Click the Plot Deformed Shape tool in the toolbox.

c. From the right side of the prompt area, click Deformed Options.

The Deformed Plot Options dialog box opens.

d. From the Labels folder, toggle on the node labels (numbers) and node symbols andclick Apply.

e. Select the first eigenmode by choosing Result→Frame from the main menu andthen selecting Mode 1 in the dialog box.

f. Using the arrow keys in the prompt area, select different mode shapes. Notice thatthe location of node symbols help to visualize the extensional modes.

Question W4–1: Are there modes of the physical system that cannot be captured byyour model because of limitations in element type or mesh?(Remember that the elements are planar and the mesh is somewhatcoarse.)

Question W4–2: Do any of the mode shapes for your model look nonphysical?

Implicit Dynamic Analysis

We now investigate the free vibration of the beam upon removal of the tip load.

Input Specification

1. Copy w4_beam.inp to a new file named dynam.inp.

Use the following steps to modify the file so that the tip of the model is loaded and thenreleased and allowed to vibrate freely:

a. Include a density of 2.3E-6 in the material definition.

b. Add a second step to the analysis history with a ∗DYNAMIC procedure type. Give asuggested time increment of 0.01 and a time period of 1.0.

c. Remove the tip load in the dynamic step by specifying ∗CLOAD, OP=NEW. Thisoption removes all existing concentrated loads.



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d. Request that the tip displacement be written to the output database (.odb) file. Asbefore, use the ∗NSET option to define a node set called TIP containing the node atthe loaded end of the beam. Add the following output requests to the input file:*OUTPUT, HISTORY

*NODE OUTPUT, NSET=TIP

U,

e. Request that the displacements for all the nodes in the model be written as field datato the output database (.odb) file.*OUTPUT, FIELD

*NODE OUTPUT

U,

f. It is useful to be able to monitor the progress of an analysis by noting the value ofone degree of freedom. To do so, add the following option to the first analysis step:*MONITOR, NODE=11, DOF=2

g. A restart file should be saved in order to have the data for the last increment of eachstep available for continuing (“restarting”) the analysis from that point withouthaving to rerun the first part of the analysis. To obtain the data, add the OVERLAYparameter to the ∗RESTART option.

2. Submit the new analysis as an ABAQUS job.

While the job is running, you can check on the progress of the analysis by looking at thestatus file.

Checking the Output Files and Results Visualization

1. Check the bottom of the printed output file to see if there are warning or error messages.These messages may refer you in turn to the message file. In this case the message issimply that the analysis did not go to the end of the step because the default number ofincrements is only 10.

2. Note that the message file also contains information concerning the solution progress.

3. Open dynam.odb in ABAQUS/Viewer.

4. Create a history plot of the displacement of the tip node. Refer to the description of X–Yplotting in Workshop 2, if necessary. You will see that less than one full period ofoscillation is plotted.



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Restart Analysis

In this section you will “restart” the analysis to continue it without starting over from thebeginning.

1. Create a new file called dynamr.inp in the text editor, and add the ∗HEADINGoption to the top of the file.

2. The ∗RESTART option is the only other option needed in the model data of the inputfile. Add the following option after ∗HEADING:

*RESTART, READ, STEP=2, INC=10, END STEP, WRITE, OVERLAY

This option specifies that the analysis is continued from Step 2, increment 10. TheEND STEP parameter terminates the current step at this point. If this parameter isomitted, the 10 increment default limit for the step will again prevent the analysis fromcontinuing. The WRITE parameter writes restart output at the default frequency of 1(every increment). The OVERLAY parameter causes restart data from each incrementto overwrite data from the previous increment so that only the last increment is retained.

3. Specify the history data:

a. Begin the history data with the ∗STEP option. Set the INC parameter to 200 toensure that the analysis will not stop before the end of the step due to the incrementlimit.

b. Specify a ∗DYNAMIC procedure with the time period set to the additional time(from the end of the second step) required for the analysis to complete a total timeof 1.0.

No loading is necessary since this is a continuation of the previous free vibrationanalysis.

4. Use the following command to submit this job:

abaqus job=dynamr oldjob=dynam

Results Visualization with ABAQUS/Viewer

1. Open dynamr.odb in ABAQUS/Viewer.

2. Plot the displacement history of U2 at node 11.

Notice that the plot traces the displacement history of the node in the restart analysis.Since the restart analysis is a continuation of an earlier job, it is often useful to view



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results from the entire (original and restarted) analysis. Thus, to create a plot ofdisplacement history from these two analyses, follow the steps outlined below:

a. Save the current plot by selecting Result→History Output from the main menu.Ensure that the correct quantity has been selected, and click Save As. Name theplot RESTART.

b. Open dynam.odb in ABAQUS/Viewer. Following the procedure outlined above,save the plot of U2 at node 11. Name this plot ORIGINAL.

c. From the main menu bar, select Tools→XY Data→Manager.

Notice that the plots ORIGINAL and RESTART are listed in the XY DataManager dialog box.

d. In the XY Data Manager dialog box, select plots ORIGINAL and RESTART andclick Plot to create the plot of displacement U2 at node 11 for the entire simulation.

3. From the graph, find the period and calculate the frequency of the vibration.

Question W4–3: How does this compare with the frequency calculated in theeigenvalue analysis?

4. Animate the results.

a. Click the Plot Deformed Shape tool in the toolbox.

b. To animate the deformed shape, click the Animate: Time History tool in thetoolbox.

c. Click Animation Options and Deformed Options, located in the prompt area,to modify the speed of the animation and to customize the plot, respectively.

(Continue this workshop with the next section on Explicit Dynamic Analysis after Lecture 8,Using ABAQUS/Explicit.)

Explicit Dynamic Analysis

We now investigate the vibration of the beam using an explicit dynamic procedure(ABAQUS/Explicit) and compare the results to those obtained with an implicit dynamicprocedure (ABAQUS/Standard).

Input Specification

1. Copy w4_beam.inp to a new file named xdynam.inp. Use the following steps tomodify the file so that the tip of the model is loaded and then released and allowed tovibrate freely:

a. Include a density of 2.3E-6 in the material definition.



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b. Define an amplitude function named RAMP by adding the following to the inputfile:*AMPLITUDE, NAME=RAMP

0.0, 0.0, 1.0, 1.0

c. Modify the step definition by setting the parameter NLGEOM=NO on the ∗STEPoption, and change the procedure option for the step to ∗DYNAMIC, EXPLICIT.Specify a total step time of 1.0 s, as shown below:*DYNAMIC, EXPLICIT

,1.0

Question W4–3: Why is the NLGEOM parameter set to NO in this analysis?

d. Add the AMPLITUDE parameter to the ∗CLOAD option, and set it equal to RAMP.

This will have the effect of linearly ramping the load from zero to the specifiedvalue over the course of the step. By loading the structure slowly enough, we cansimulate a quasi-static response. Simulating quasi-static processes with explicitdynamic methods is discussed in more detail in Lecture 10.

e. Request that the tip displacement be written to the output database file. As before,use the ∗NSET option to define a node set called TIP containing the node at theloaded end of the beam. In addition, write the whole model internal and kineticenergies to these files. The following output requests should be added to the inputfile:*OUTPUT,HISTORY, TIME INTERVAL=0.005

*NODE OUTPUT, NSET=TIP

U,

*ENERGY OUTPUT

ALLIE, ALLKE

f. Request that the displacements for all the nodes in the model be written as field datato the output database (.odb) file.*OUTPUT, FIELD

*NODE OUTPUT

U,

g. Add a second analysis step with a ∗DYNAMIC, EXPLICIT procedure type. SetNLGEOM=NO on the ∗STEP option, and specify a total step time of 1.0.

h. Remove the load by specifying a ∗CLOAD option with the OP=NEW parameter:*CLOAD, OP=NEW



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i. A restart file should be saved in order to have the data for the last increment of eachstep available for continuing (“restarting”) the analysis from that point withouthaving to rerun the first part of the analysis. To obtain the data, add the OVERLAYparameter to the ∗RESTART option

2. Submit the new analysis as an ABAQUS job.

You can check on the progress of the analysis by looking at the status file if the job isrunning in background mode. If the job is running interactively, the contents of thestatus file are printed to the screen.

Checking the Output Files and Postprocessing

1. Check the bottom of the printed output file to see if there are warning or error messages.These messages may refer you to the status file.

Note that the status file also contains information concerning the progress of thecalculations, provided the job was run in background mode.

2. Open xdynam.odb in ABAQUS/Viewer.

3. Plot the tip node’s displacement history, and compare the results obtained withABAQUS/Standard against the results obtained with ABAQUS/Explicit by plottingboth results on the same plot. Your results should look similar those shown inFigure W4–2.



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Question W4–4: How do the results compare with one another? What factorscontribute to the discrepancies in the solutions?

4. Compare the internal and kinetic energy content of the model by plotting thesequantities on the same plot. Do this only for the ABAQUS/Explicit results.

Question W4–5: Was the loading rate small enough to ensure a quasi-static responsein the first step?

STD (DT=0.01)

XPL

Figure W4–2. Comparison of the Tip Node Displacement History

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AFS-i6070202 w Dynamics