hints on ansys modeling for rivets

8/7/2019 Hints on ANSYS Modeling for Rivets

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Hints on ANSYS Modeling

ANSYS gets much easier to use if you take advantage of the FILE.LOG capability. Thisfile saves all the entries you make in the Graphical User Interface (GUI). From the

minute you turn on ANSYS, it records everything in this file. Thus, this file will havewhat you just did, along with what you did last time and all the times before that until thelast time you deleted or erased it!!

So, to use it well…use the SEARCH feature of your operating system to find thedirectory (folder) where FILE.LOG is saved by ANSYS. Place a shortcut to this folder on your desktop. Then, with ANSYS not running, delete all the files in this directorythat start with FILE with various extensions such as FILE.DB, FILE.RST, FILE.ERR,etc…delete them all. Then, turn on ANSYS. Note that it creates a new FILE.LOG.Create a shortcut to this FILE.LOG on your desktop. Click on the shortcut at any time tosee what is in it. Note what is in it as soon as you open ANSYS.

Now, Enter some Parameters. Use the shortcut to look at the current version of theFILE.LOG. ANSYS can write into this file, even if you have it open!!. You can save acopy with a new name such as MYPROJECT_PARAMETERS to indicate the placewhere you left off. You could also give it any name to indicate where you are such asMYPROJECT_INPROGRESS.

Now, go to the preproccessor. Add the element type 95 and pick the option of extraoutput as Nodal Stress. Now look at the FILE.LOG using the shortcut. Notice that youcan see that element type1 is defined as the 20NODE SOLID95. Further, just below, yousee that the 4 options are listed as if you entered commands. The action of the mouse

saved the 4 choices….the extra nodal output is option 5 and choice number 2.Notice that there are !* as lines that separate things making it easier to read. ANSYSignores anything following the exclamation (!) on any line. Thus

!********************REPRESENTS A COMMENT*************

you can enter comments to help you remember what you did in your log files. For example you can say

!**********Select Solid element type 95 and enter the extra output at nodes********

or later

!***************enter the Young’s Modulus and Poisson’s ratio as E and NU********

Once you understand the log files, you can use them to save lots of time because you canrestart using the log files.



To do this, either from a fresh start of ANSYS or after issuing the commandFILE/Clear & Start NEW…..choose FILE/Read Input from… and then select the logfile that you saved with the name MYPROJECT_InProgress.txt. When you issue thiscommand, all the things you did get repeated and the FILE.LOG gets an entry such as

/INPUT,'MYPROJECT_InProgress','txt','C:\FEM81\',, 0 Which basically tells the nameof the input file, its type and location. Mine was in C:\FEM81 …the directory on my Cdrive where I store results from ANSYS version 8.1

Once you have meshed your object, you can save the FILE.LOG with a new name suchas MYPROJECT_MESHED. You can then change any parameter and then get to thispoint easily. You can then add the boundary conditions (the displacement constraints andthe loading forces on the nodes) These are attached to specific nodes by their nodenumbers. You can see the numbering method by seeing what is in the log file. You canalso turn on the node numbers to observe them yourself. Now…consider what happens if

you change the number of elements along a specific side. This will change the number of nodes along that side so when ANSYS numbers the nodes, the particular nodes that youlocked down for example with the ALL DOF =0 command will no longer be in the sameplace or even adjacent to one another. Thus, if you change the number of elementparameters (NL or NH or NT in our examples) you will have to reenter the loads anddisplacements.

If you do not change these, then you can continue on to the solution and to the postprocessor and then save the FILE.LOG file with yet another name….MYPROJECT_XSTRESS for example which shows the solution complete withthe plotted shape and stresses. Then, you can edit this file to change the value of theparameter for the height of the beam for example *Set, H,1 is the old value….change itto *Set, H, 1.5 then you can run the problem again to see the results for a beam of 1.5 inches high. This makes it easy to study geometry effects.

Remember that these solutions are always linear….which means that doubling the loaddoubles the stress and doubles the displacement at any point. Remember you can deformthe sample by prescribing a displacement that is not zero is you hold some other placefrom moving….The same as if you had pulled it to make the nodes move to where youtold them to go.

The maximum and minimum are indicated on every contour plot by MX and MN and themaximum displacement is always shown on the side of the output.

You can use symmetry to model only a portion of a beam. If you think about it, all thesebeam problems are symmetric about the center both left to right and front to back.Therefore, if we modeled only half and constrained the nodes at the left edge to only not



move left and right, we could put the load on the left edge pointing down with the fullvalue and then fix Uy and Uz of the right hand support and get the same answer with onlyhalf the model…Further, we could cut this piece that is left the thin way and set Uz=0along the centerline, change the load to half and still get the same answer. Since we onlyhave one element thick in out examples, this is not useful but if we had had multiple

elements in the thickness direction, we could get away with half the number using thissymmetry about the Z=0 plane.

These symmetry considerations are very helpful. You will not have to model both sidesof your bridges…only ¼!

Now for the rivets…

The rivet expands and fills the hole. Thus it is as if the hole is filled with aluminum. Inother words, a tight rivet acts as if the material has no hole. There may still be a stressconcentration if the loads get large enough to cause the rivet to separate but we can think

of the rivet as effectively filling the hole. We model this then by applying loads to thesurface nodes in the physical location where the rivet is to be. We can use a “circle”when picking the nodes to load, then, turn the view 90 degress and use a box to“unselect” the nodes inside the object. This lets us select just the surface nodes. Nowapply a displacement condition so they cannot move in the vertical direction. Applyforce loads on the other end of the riveted piece. Use symmetry to model only one half of the riveted joint and even more symmetry to model only one half of the lower piece,realizing that on the centerline, the lateral displacements will be zero. See the screenshot below which shows how to pick the nodes for the half rivet hole. These nodes willbe constrained in the Uy direction



The nodes on the left hand side will now be set to have Ux =0 while the nodes on thebottom will be given an Fy= - LOAD/Number of Nodes on Bottom.

One of the nodes in the center of the rivet is Locked with Ux=Uy=Uz=0



Now we solve and look at the stresses.

Below I show the Y stress. Note that it is very uniform over most of the part and justbelow the rivet, shows tension as a stress concentration. There are some “ARTIFACT”or FALSE stress concentrations at the edges on the bottom because we put the same loadon every node…the nodes on the edges should have less load to represent the same stressbecause the load on a node logically represents the force caused by the area surroundingthat node and the area “stops” at the edge. Thus, we really need to apply half the load at

edges and even less at corners…There are better ways to apply loads by applyingstresses. We could also apply a displacement load at the bottom which would be thenuniform because the uniform displacement is what results from a uniform stress. This iswhere we have to realize that FEM is a tool that requires careful attention to all details…

Lastly, we can look at the total reaction load by choosing “List Result---reactionsolution” . As seen in the next screen shot



If we now pick structural forces Fy we will get the following:



Which, after scrolling to the bottom shows 53 as the total load on all the fixed points.This corresponds to the 53 nodes on the bottom of the model because I put one pound oneach node.

After thinking about these results and trying this yourself, this should give you an idea of the stresses associated with the rivets…

You can try holding up your bridge (those plates we did earlier) by circles of nodes ontheir surfaces which represent rivets…we can look at the total shear forces in the rivetsand compare that with the forces we will measure to shear rivets.

This will show that the rivets will tend to twist the structure a bit. Note that the rivetshere actually caused bending of the lapped part…even though we only did ¼ of it…



Hey…This ANSYS FEM lets us experiment a bit before we ever build our firstprototype…that is the whole idea.

Have a good weekend,

Professor Quesnel.

hints on ansys modeling for rivets

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