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Click here to see a video for this article.

2/10/2012

Associative 1D FEA Models 

Click here to see a video for this article.

In this NX A to Z article, I will show you how to use Sketcher to create 1D finite element models that are associative to 3D geometry.

The scenario here is that you are doing an FEA on a beam and plate type structure that isn’t necessarily finalized in terms of the design. You want to minimize the amount of rework that you will have to do should the design change. If the structure changes dramatically and entire components are removed or added, then there really is no way that you can create your finite element model to automatically adjust to this; however you can create a finite element model that will automatically adjust to changes in component lengths and sizes. If the design team is using NX, then you don’t have to worry about imports and all this works great. If the design team is using something else and you are importing the 3D geometry, then when the design changes you will have to decide between importing new 3D geometry and mapping the curves for the linear elements to the new 3D geometry or updating the existing 3D geometry to reflect the design changes and have the curves for the linear elements update automatically.

The model that I will be using is shown below. It is made up of structural tubing and has two mounting plates. This is an assembly model and each structural component is also an assembly component. The assembly is fully constrained.




The Idealized Part and WAVE Linking

First you’ll go into Advanced Simulation and create a new FEM and Simulation. Make sure you create an idealized part.

The next step is to use WAVE linking to create some linked geometry in the idealized part. With a 3D FEA, you always create a linked body and then do all your geometry simplification on the linked body. With a 1D FEA, you don’t necessarily have to link bodies, you can just link points of you want. I am going to link the bodies because using points results in have 20 or so points in the model and that can be difficult to keep track of.

Once you have your idealized part and have created the linked bodies, the next step is to create planes everywhere we will need a sketch. We only need one sketch to create the linear elements for this structure, but if you components of your structure in different planes, you would need a sketch for every plane. You are not required to use Sketcher to create the curves that define the linear elements; however, you have more options for associating the curves with the 3D geometry using Sketcher than you do using regular curves and arcs. The model with the plane is shown below. I changed the color of the linked bodies relative to the original assembly components so that I don’t get them confused. I used the bisector method to create the plane in the center of the tubes.





The next step is to create the sketches for the components in each plane.





The sketch is fully constrained and does not have any dimensions. Basically, the end point of each curve is positioned using a mid-point constraint or a point-on-curve constraint or both. There are also vertical, horizontal and parallel constraints. I also created 2 construction lines (shown in purple) where the cross braces cross each other so I could position the cross brace curves in the middle of each brace. The construction lines are positioned using intersection points where the edges of the cross braces intersect.

So now this sketch is tied to the geometry of the components and as you can see below, a change in the geometry results in a change to the sketch.





You would most likely use a 2D mesh for the base plates and you would also want that mesh to be associative. The way to do that is to extract the top face of the plate and then divide it with the bottom face of the vertical tubes. This is shown below.





Now if the size of the plate, the size of the holes or the size of the tube changes, the plate mesh will update as well.

FEM Geometry

Now we go into the FEM file and the first thing we need to do is edit the geometry options so go into the FEM file and check Edit Bodies to Use and pick Select from the drop down menu and in the CAD Part window, deselect everything except the extracted plate faces.





Then Click Geometry Options and check Sketch Curves and you should see something like what is below.





One last thing we have to do is create mesh points where curves meet and where the curves meet the plates. This is necessary to prevent duplicated nodes in these locations. You can see the mesh points as blue diamonds in the figure below. To create mesh points, you just click the point locations, you don’t have to click Apply every time.





Now you can mesh the curves with 1D elements and the plates with 2D elements and connect the tubes to the tube outlines on the plates with rigid connector elements. That’s it. Hope you enjoyed it.

Dave Holland