Modeling washers in Nastran

In an FEM I'm working on (in NASTRAN), fasteners (beam elements) are
creating very high regions of stress near the node they are connected
to. In reality, I realize that the fasteners will have washers to help
distribute the stress across the surface more effectively.
I've tried running some simulations with interpolation elements, but am
not quite sure what DOF's I need to contraint for the independent and
depenedant elements. The closest thing I've come to is using multiple
springs (with a very high K) attached nearby the region the fastener
inserts into.
Are there any better ways to approximate washers (i.e. realistic
fastener loading) in NASTRAN?
Thanks in advance,
Dave
Reply to
dave.harper
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I'm assuming you are concerned with out of plane loading here, and not in plane loading. Anyway you could post a link to a picture of your model?
Model the fastener hole and use a rigid element (RBE2) to connect the bolt node to all nodes covered by the bolt head or nut whichever is smaller. The bolt node will be the independent node and the other dependent. The dependent DOFs can be 123 (or x, y, & z translations). You could get more complicated with this if you wanted to model bearing better. This same type thing works with stiff bar elements (pin flag rotations at outer ends of bars) too.
Does this make sense?
Reply to
Jeff Finlayson
connected
multiple
fastener
translations).
Thanks for the good advice. I tried that, but due to the geometry there's still really high stresses at the end of some of the rigid elements. It led me in the right direction, and I think I've put together a good equivalent of a washer now. I have an RBE2 constrained on TX, TY, RX, and RY attached to nodes "covered" by the washer, and on the same nodes I have DOF springs operating in the TZ direction. This configuration is mirrored for the mating surface too. The stresses look similar to what I'd expect to see in reality.
Thanks, Dave
Reply to
dave.harper
Generally similar to what I suggested. I think covering the bolt head or nut area would be more conservative.
If you have covered the washer area, I don't think you should connect the rotation on out nodes in the RBE2. Contact does not necessarily mean the surface and washer will rotate together.
Best of luck David..
Reply to
Jeff Finlayson

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