What boundary condition techniques do you use to keep FEA models realistic?

[EdwardNigma]

Hello all,

I work in the aerospace industry as a stress engineer. I recently took a very solid aerospace FEA course as part of a master’s program, and one thing I really appreciated was how the instructor emphasized connecting numerical approximations to real-world behavior. Around the same time, a technical fellow at work recommended a practical FEA book that walks through the process of developing and validating models—which reinforced the same idea.

One thing that stood out to me is just how important it is to set up realistic boundary conditions. A model can easily become too stiff or too soft if you’re not careful, especially when you’re trying to represent how a structure interfaces with its surroundings. This seems like one of the most critical aspects of getting meaningful results.

That leads me to my question: what boundary condition modeling techniques or rules of thumb do you use to make your models more realistic?

For example:

• In truss-like structures, using a pin on one end and a roller on the other can allow for lateral movement and prevent over-constraining.
• When modeling plates, allowing for lateral deformation can better capture Poisson’s effect.
• In 3D space, the 3-2-1 rule (restraining three points to prevent rigid body motion) seems like a solid starting approach.

If you have experience creating robust and realistic FEA models, I’d really appreciate hearing about any methods or strategies you’ve developed over time to handle boundary conditions effectively. Thanks in advance.

 

[GregLocock]

I hit the structure that it is fastened to with an impact hammer and an accelerometer. That tells me the impedance in the normal direction. I frequently have had to model the test rig as well as the IUT.

 

[SWComposites]

other than the 3 simple examples you list, there aren't any general simple rules. have to look at the actual structural connections, and ensure that the BC's are realistic and appropriate for expected structural behavior. common mistake is to over constrain the model, by using rigid BC's or constraints. and do not assume test fixtures are "rigid".

oh, and ALWAYS plot the model displacements for each load case BEFORE looking at any other results. if the displacements do not look reasonable, the stresses are likely rubbish.

then examine the loads/stresses at the BC's; if there are wild peaks then the model is likely over constrained.

 

[rb1957]

boundary conditions are one thing, the connection between elements another. It is easy to (unrealistically) weld them together; on the other hand using common nodes can greatly simplify a model. And releasing individual freedoms may be analytically correct, but may create only a small difference in the key results (so is it worth the effort ?).