How to read stress adjacent to rigid elements?

[mmatrix]

Hi,
In nastran, rbe2 is often used to simulate weld and bolt. How to read stress on elements(shell or solid) connected to RBE? The value ot these elements is not right?
Stress is complicated to me. Diffrent average methods result in deiffrent stress value. The max Von mises & max strain locates on same elements(one kind material model),right?
Thanks for your answer.

 

[GregLocock]

Well, go back to the real world. Suppose you had an infinitely rigid part feeding loads into your structure. How realistic would the loads be local to the termination of that idealised part?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TVP]

mmatrix,

As Greg Locock pointed out, stresses near rigid elements are not realistic. One method to improve the results is to determine the reaction forces at the fixed elements, and then use these to change the constraints on the model, i.e. remove the rigid constrain and use reaction forces to maintain equilibrium. I'm sure there are other methods that can be found by googling "finite element bolt" or "finite element weld" possibly with some additional terms such as fixity, constraint, rigid, etc.

 

[GBor]

You can also try to distribute the load by adding additional "ties" to adjacent nodes like the spokes on a wheel.

 

[GregLocock]

I'd put rigid elements in at #3 on my list of ways that people build bad models. #1 is boundary conditions, #2 is loads. #4 is connectivity.

GBor's approach is the usual one, but it locks up the elements in the spider.









Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GBor]

Greg is correct...you need to have a feel for how it impacts the results. I often point people to JohnHors paper on balanced loading and minimal constraints. A quick search of the last 10 threads or so will produce a link to various threads containing the paper. It may help you get rid of the rigid elements.