FEA; When should we activate large deformation? 5

[Mm.Kaiser]

Dear friends.
Can anyone tell me when should i activate large deformation when using finite element analysis?
I have analyzed a pressure vessel's part (Manhole) with steel structure and trying to find out the real deformation in sealing zone. I don't know which is real deformation: Large def= on or off.
Thanks a lot.

 

[MrMyers]

Run both cases and check the difference in results. If there is a difference, you probably need to leave large displacement turned on.

If your stresses are above the yield strength, then also consider including non linear materials (plasticity).

Where codes/standards support design by analysis, the rules are fairly specific (i.e. turn it on).

I've heard people refer to the following rule of thumb: if deformation is >10% of the nominal thickness, then turn it on.

 

[flash3780]

I've always been told the same thing: turning on large deflections is the safest methodology unless you have the option of comparing both. In general, I don't have that luxury, so it's one of those options that I always turn on.

//signed//
Christopher K. Hubley
Mechanical Engineer
Sunpower Incorporated
Athens, Ohio
--

http://engineeringliberty.wordpress.com

 

[EngineerII]

I know when I analyzed manhole covers for the company I work for. I would run my FEA with large deflection turned off. If my resulting maximum deflection in the center of the cover was greater than thickness of the cover, then I would run a non-linear static FEA.

 

[bxtsafe]

The "Large Deformation" option (or "Large Displacement" option in some FE packages) should be used generally in 2 cases:

- when the displacements of the structure nodes are very large. In these cases, the formulation of the shell elements for bending is no longer valid. I mean... in these element's formulation there is an aproximation: dy/dx=tan(theta)~=theta . This approximation is valid just when there are small displacements. When the large displacement is turned on it's considered that dy/dx=tan(theta). This option should be used when doing FEA of rubber parts, nonlinear elastic analyses, plastic analyses or when the structure displacements are large. It's also recommended to turn it on when the displacement of a plate in out-of-plane bending is bigger than 75% of it's thickness.

- the strain used in the linear FEM formulation is eps=du/dx . However, its an approximation. The real formula, that is used in the nonlinear FEM formulation, have high order terms. These high order terms are considered in the computation of eps when the large displacement is turned on. It's recomended to turn it on when we expect to find large strain, as in rubber parts FEA, in nonlinear elastic analyses or in plastic analyses.