Using shell/plate element types

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In this unit, we will be discussing Shell Elements and when they are best used in a Inventor Nastran Analysis.

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For this example, we'll be working with this fender component here.

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This part is made from sheet metal, meaning that the thickness is constant throughout its entire length.

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When you're considering using Shell Elements with Inventor Nastran, there's two things you'll want to look for.

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The first and usually the most obvious is when the parts length is significantly greater than its thickness.

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What that means in this case is if you were to take some measurements on this part,

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and look at, for instance, the length of this section right here, you'll notice that it's around 425 millimeters.

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If you divide that by the thickness, so 425 divided by the thickness of three, you get 141.

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Any time that the ratio of length or width divided by thickness is greater than 100,

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that would make your part a good candidate for Shell Elements.

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The reason that Shell Elements will perform well here is that because of how thin this component is,

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you would need a lot of solid elements through that thickness to capture bending accurately.

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However, a Shell Element simplifies that thickness dimension into a surface and eliminates the need for a three dimensional element.

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The second thing you'll want to look out for when using Shell Elements is that your part has a consistent wall thickness throughout.

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This sheet metal part has a thickness of three millimeters on every face.

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And that also makes it a great candidate for using Shell Elements.

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So I'll go ahead and open up Inventor Nastran here.

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And when I do so, you'll see that a solid idealization is automatically created just like we've seen in prior exercises.

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However, if you are planning on using Shell Elements,

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you will want to remove that solid idealization so that it's not meshed on top of the additional Shell Element that we will be adding.

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So I'll right click on "Solid 1" and remove that Idealization.

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Now, I'm ready to create my Shell Elements.

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A Shell Element can only be applied to a single surface, which means if you are working with a three dimensional model as we are here,

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you'll need to use this Prepare Panel where you see Offset Surfaces to create that mesh.

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If I use the drop down menu, you'll see there's three options.

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There's Offset Surfaces, Fine Thin Bodies and Midsurfaces, Midsurfaces and Fine Thin Bodies.

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Both create a Midsurface. Offset surfaces allow you to choose the distance you'd like to offset each face.

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In general, Midsurfaces are recommended.

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And that's what the Fine Thin Bodies tool is going to do. If I select that tool, it says, one or more thin bodies found.

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Would you like to generate Midsurfaces? I do, I'll select "Ok".

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It says Body 1, which is the only solid body in this part file. So I'll select "Ok".

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And now the Midsurface has been automatically generated.

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In addition,

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under idealizations, you'll see Midsurface 1 is a Shell Element idealization with Mild Steel being applied from the part file.

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If I right click on "Midsurface 1" and go to "Edit",

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what you'll see is it pulled all of those faces and automatically offset to the mid plane or the average of the thickness.

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And it gave it an overall thickness of three millimeters.

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Now because it's a Midsurface, that means that 1.5 millimeters will be applied to each side of the shown surface here.

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Now, as I've mentioned before, I do recommend giving it a name, so I might say three millimeter steel,

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so that I know the thickness without having to open up this idealization again.

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And then I would select "Ok" to confirm my idealization.

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And now when I go to generate a Mesh, you'll notice instead of three dimensional elements,

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two dimensional surface elements are generated across that surface.

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The Midsurface tool is going to do the exact same thing as the Fine Thin Bodies tool.

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It just allows you to choose your bodies manually instead of automatically.

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So for instance, if I did not have this Shell Element Idealization or Midsurface created, so I'll delete those.

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And instead I go to Midsurfaces.

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You'll notice it asks me for a Solid Body. So I can choose this one here and select "Ok".

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And then I'm left with the same exact result as using Fine Thin Bodies.

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It just gives the user a little bit more control.

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Now, the last option we haven't discussed yet is the Offset Surfaces tool.

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This works really well. When you have a specific face, you need to keep intact.

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For example, on this part, you will notice there is a split face that has been created on the original part,

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you'll notice it right here that is not maintained when you go to create a Midsurface automatically.

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However, you can maintain split faces if you use the Offset Surface tool instead.

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So for instance, here I can go to the Offset Surfaces tool.

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And this time I'm going to select "Faces", Automatic Face Chain is automatically chosen,

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which means if I choose one face anything tangent to that will be grouped into the selection.

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So I'll select here and it pulls all the faces on that side of my component.

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And then I need to put in an Offset Value to offset to the mid plane.

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You would type in 50% of your original thickness, in this case, that's 1.5 millimeters.

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So I'll select, "Ok".

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It generates the offset surface, but notice I do have that split face intact if I'm using that for a Constraint or Load.

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So that's where Offset Surfaces is a better option than using a Midsurface.

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And if at any point, you do need to go back and make a modification to the surfaces or the offset from that command,

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you can always go back over to the Analysis Tree where you see Offset Surface.

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it opens that dialog box back up where you can Deselect or Reselect faces as well as change the offset or thickness value if you need to.

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I'll select "Ok", and then we are ready to mesh this and use it for an analysis.

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One more example here,

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if you create a surface body within the part file instead of a three dimensional body,

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that can also be meshed as a Shell Element within an Inventor Nastran,

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you'll notice here this cylindrical surface was generated from a series of sketches and a boundary patch.

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If I open up Inventor Nastran, I can't use the Offset Surfaces or Fine Thin Bodies tool.

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However, I can directly jump into a new idealization where I will change the element type here from Solid to Shell.

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And now I'm able to type in a thickness value, I can assign a Material and I can choose the Associated Geometry.

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So first, I'm going to update my Material.

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This will just be a simple Alloy Steal. So I'll go to the database and pull that value.

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Next, I will check the box for Associated Geometry and I will choose that surface that was already generated.

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And then for thickness, let's say this is going to be a quarter of an inch thick.

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I'll type in 0.25, assuming I'm working in inches, select "Ok".

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And now I can go ahead and generate the mesh,

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and those surface elements are applied directly onto the pre-generated surface

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and it will act by default like a Midsurface like we discussed before.