this exercise shows how to include self weight in a finite

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http://altair-india.com/edu/students/html/publiccontent/tutorialsandexercises.htm

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This exercise shows how to include "self weight" in a Finite Element model. Since the weight of a body acts over the volume, FE models can often do better than simply "lumping" the weight at selected points in the model. In fact, it is often easier to leave it to the FE programme to calculate the relevant loads at the nodes of the model.


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The exercise starts with a model of a plate which already has material data, property data, shell elements, restraints and has an end load applied. We create a new collector for the self-weight, enter the magnitude and direction of the gravity vector, create a collector that combines the end-load and the self-weight, edit the material to ensure that the density has been specified, and solve the problem. Log on to the Technical Discussion Forum for a background on this problem, and to access the related IGES and HM files.

Steps In This Demonstration


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• Press ENTER to move forward from one slide to the next, or click on the blue button at the bottom right of this window

• Use the controls at the bottom to pause, rewind or skip forwards • If you have any questions, email [email protected]


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You will need access to HyperWorks 9.0. Some of the functions described in this tutorial are not available in earlier versions. You should also have access to the Student Guides - you may want to refer to them later both for theory and for advanced use of the software.

Instructions For Use


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An FE analysis often generates several files. The image shows the contents of the working folder used for this demonstration immediately after the successful conclusion of the last of the 3 analyses. It's a good idea to use different folders for each analysis. In any event, always use meaningful names, so you can identify and review the output files.


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Output Files


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The Model Browser shows the contents of the model we start with - it has shell elements, a force, restraints, a loadstep, a material, and a property (the shell thickness).


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The analysis is complete, and the results are in the H3D file as this HyperView session shows.


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Select the Deformed Shape icon


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Pull down the Scale options


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Select Model percent


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Double-click on the Value


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Enter 20 and click on Apply


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Select the Contour icon


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Click on Apply to contour the deflection


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According to the legend, the maximum deflection is 2.69E-1


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Pull down the list of Result Types


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Select Element Stresses


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Click on Apply to contour the von Mises stress


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Note the maximum deflection for this model - this is the deflection with a concentrated load at one corner of the plate. In the next few steps, we will include the affect of gravity. We'll assume that the force due to gravity is in the negative "Z" direction.

Summary of Results


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To create a new Load Collector, right-click in the Model Browser.

Note that if you right-click on Load Collector, you don't need to specify the type of collector you want to create - HyperMesh brings up only the Load Collector menu.


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Select Create


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Enter a name, then pull down the list of card images


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Scroll down the list


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Select GRAV - this is the format for a gravity load


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Click on Create / Edit


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Click on the data entry field labeled G to enter the magnitude of the acceleration due to gravity.


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To calculate the weight of the each element, the solver multiplies the mass of each element with the acceleration due to gravity. Make sure the value you use is consistent with the units you are using. Remember that the mass is calculated from the element's volume and its density. In this exercise, we use g = 9.81, which is appropriate for SI units.


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Also remember that the acceleration is a vector - it has both a magnitude and a direction. The direction of the vector is specified using the direction cosines of the plane that the vector is normal to. In general, if the model orientation is such that the gravity vector is not along one of the axes of the global coordinate system, it's advisable to create and use a local coordinate system to specify the direction. In this example, the weight acts in the negative Z direction. So the three direction cosines are 0, 0 and -1 respectively.

Acceleration due to gravity


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For more details, refer to the online help on the GRAV card image:

Acceleration due to gravity


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To specify the direction of the vector, click on the data entry field for N3


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Enter -1, and click on return


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When the earlier model was built, the loadstep was created.


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Click on loadsteps (in the Analysis page of menus)


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Note that the LOAD referred to by this loadstep has the id number 3


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From the list of load collectors, we can see that id number 3 is the concentrated load.

Since only one load collector can be entered in a loadstep, if we want both the gravity and concentrated load to be used for the analysis, we must create a new load collector. This new load collector will be a

load-combination.


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Click on return to close the loadstep menu


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To create the new Load Collector, right-click in the Model Browser.


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Select Create


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Enter a name, then pull down the list of card images


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Scroll down the list


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Select LOAD - this is the format for a load combination


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Click on Create / Edit


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Click on this data entry field to enter the number of loads we want to combine.


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Enter 2, since we want to combine two loads.

Note that the id of this new load collector is 5 - we will use it when we update the loadstep.


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Also note that since we entered 2, the form has now updated - it allows you to enter the details for two loads that will be combined in load collector number 5


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For more details, refer to the online help on the LOAD card image:

Load Combinations


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The load combination is a scaled value of the weighted sum. Accordingly, the data required is

• the scale factor (multiplier) for the combined load • the weight (multiplier) for each load that is being combined

Refer to the online help on the LOAD card image for more information on the formula.


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Load Combinations


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Select the data entry field for S, the multiplier for the combined load


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Enter 1, then select the data entry field for S1(1), the weight for the first of the loads that will be combined.


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To select the load, double-click on L1(1)


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Select the concentrated load


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Select the data entry field for S1(2), the weight for the second of the loads that will be combined.


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To select the load, double-click on L1(2)


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Select the self-weight


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Click on return


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Click on loadsteps (in the Analysis page of menus)


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Double-click on the data entry field for the Load that is used by this loadstep


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Select the new load collector, the combined load


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Click on update to save the changes made to this load step


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Click on return


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Select the File menu


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Select Save As


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Save the file under a new name


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Click on Radioss


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Click on Radioss again to start the solve. Since the file has been saved under a new name, the output files will be created accordingly.


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When the analysis is complete, click on HyperView


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Pull down the animation options


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Select Linear Static


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Select the Contour icon


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Click on Apply to contour the deflection


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According to the legend, the maximum deflection is 2.69E-1


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The value is no different than that from the concentrated load alone. Note that this could be for a variety of reasons. For instance, there may have been an error in the application of the gravity load. Alternately, it is possible that the self-weight is much less than the concentrated load. In several cases, the self-weight (often called a "dead


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load") is less than the other loads that act on the body (these are usually referred to as "live loads"). An easy way to check is to perform an analysis with the self-weight alone. That is, excluding the affect of the concentrated load.

Deflection under Combined Load


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To try this, close the HyperView session, then close the analysis page in the HyperMesh session


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Right click on the load combination


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Select Card Edit


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Select the multiplier for the load whose ID is 3 (since the Model Browser shows that this is the End Load)


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Set it to 0 and click on return


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Select the File menu


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Select Save As


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Save the file under a new name


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We could also have achieved the same effect (eliminating the affect of the concentrated load) by editing the loadstep and replacing the combined load by the gravity load.

To run the analysis for this new model click on Radioss


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Click on Radioss again to start the solve. Since the file has been saved under a new name, the output files will be created accordingly.


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When the analysis is complete, click on HyperView


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Pull down the animation options


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Select Linear Static


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Select the Contours icon


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Click on Apply to contour the deflections


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According to the legend, the deflection due to self weight is several orders of magnitude less than the deflection due to the concentrated load.


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This does not, of course, mean that we can conclude that the model is correct. Simulation results, like those from any experiment, should be checked and cross-checked. Omitting the density of the materials in the model, or using wrong units, is a very common mistake. Accordingly, the last step in this exercise is a review of how to update material properties.


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Deflection under Self-weight


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Select the Materials menu


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Select Edit


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Click on the mats button to select the material to be edited


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Select the material from the list of materials in the model


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Click on select


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Click on update/edit


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Click on the data entry field for RHO, the density


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Enter an appropriate value - one that has the correct units! - and click on return


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Click on return again to close the edit-materials page of menus


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You can also use this icon on the toolbar


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It opens the same edit-materials page of menus


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Alternately, right click on the material in the Model Browser


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Select Card Edit


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This directly opens the card-image for this material - edit the data and click on return to update the material


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This ends the exercise. Click here to go to a list of the tutorials.

End of Exercise

this exercise shows how to include self weight in a finite

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