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Displacement becomes small after activating LGDISP

Hi everyone,

Could you help me understand the phenomema that I describe below ?

I conducted a non-linear analysis with activating LGDISP. After comparing the results with and without LGDISP, I found that there is a big difference in displacement.

If I don't activate LGDISP, the displacement of model is very large. Otherwises, the displacement becomes small with LGDISP. I don't understand this phenomena.

Note : In my model, I have plastic material so with small young's modulus , but just linear comportement is taken into account (I don't have informations on parameters for determining its non-linear comportement). I implemented non-linear analysis because of contact phenomena.

Thanks for your helps.

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Altair Forum User

Hi,

Are you running a large strain problem with NLSTAT without LGDISP?

Altair Forum User

Hi Prakash,

Yes, I ran the analysis with NLSTAT without LGDISP. In this case, the displacement is large. Otherwise, if I activate LGDISP with NLGEOM, the displacement becomes small. I am not sure the problem is qualitified as a large strain problem.

Regards,

Altair Forum User

Hi,

I think you are running large strain problem. So without LGDISP some effects like contact status are taken into consideration.

with LGDISP this is taken care.

Altair Forum User

Hi Prakash,

Thanks for your helps.

I have one more question: What is the differences between NLSTAT and NLGEOM ? I have read Hyperwork Helps. But it is still not clear for me. As NLSTAT works only for small displacement nonlinear analysis, an analysis with NLSTAT and LGDISP should not be allowed by Solver, not ? however, I can do it with my current model.

That means there are 4 combinations :

NLSTAT with LGDISP
NLSTAT without LGDISP
NLGEOM with LGDISP
NLGEOM without LGDISP

Furthemore, I didn't found an explanation for NLGEOM anlaysis type under the descriptions of ANALYSIS in Hyperworks helps.

Best regards,

Altair Forum User

Hi,

LGDISP is used in combination with NLSTAT to analyze large strain problems.

I think with NLGEOM LGDISP will be ignored.

With NLSTAT it is assumed that the simulated physical phenomenon is not influenced by temporal aspects, in other words that the expected results do not depend on the speed of application of efforts. This makes it possible to satisfy the equation of equilibrium with each step, the effects of inertia being ignored. Therefore, the weakly dynamic problem can be solved as a static (quasi-static) problem.

Karthic_22061

Hi @Prakash Pagadala,

Will NLSTAT with LGDISP consider the Geometric non-linearity ?

Altair Forum User

Hi @Karthic

Yes, solver should consider Geometric non-linearity with LGDISP.

See this from OptiStruct help:

Large Displacement Analysis

Large Displacement Nonlinear Static Analysis (LGDISP) is used for the solution of problems wherein the load response relationship is nonlinear and large structural displacements are involved. The source of this nonlinearity can be attributed to multiple system properties, for example, materials, geometry, nonlinear loading, and constraints. Currently, in OptiStruct the large displacement capabilities include large strain elasto-plasticity (MATS1), hyperelasticity of polynomial form (MATHE), contact with small tangential motion, deformation dependent loads (follower loads), and rigid body constraints.

Karthic_22061

Thank you @Prakash Pagadala,

Will all the loads in NLSTAT with LGDISP are follower loads. Or FLLWER card is required along with NLSTAT (LGDISP )

Altair Forum User

@Karthic

FLLWER card is required for follower load analysis.

If FLLWER = option is present above the first subcase, it is applied to all large displacement nonlinear static subcases.

Karthic_22061

Thank you @Prakash Pagadala,

Do I need FLLWER card to specify deformation dependent loads (follower loads) in NLGEOM subcase ?

Altair Forum User

Hi Karthic,

Follower force is currently only supported for large-displacement nonlinear static analysis subcases.