Well there is an option of inertia relief while defining the solution under the parameters tab in NX Nastran. If you'll work with this parameter the solution will work fine. I hope it helps.
but if you're applying real loads, even if they are a balanced load and reaction set (ie no nett load) you still need to constrain the 6 rigid body motions.
another day in paradise, or is paradise one day closer ?
It would be helpful if you specified what kind of analysis you intend to do without constraints and the purpose of such an analysis. As others have stated, it is common to use unconstrained models in certain cases.
one talked about inertia relief ... ok, that's applying inertia body forces to react the applied load. I guess it barely possible that a FE code could determine the balancing body force to an applied set of loads, but I haven't seen/used it. and if I'm being pedantic I could call that a constraint, in that the analysis would be determining the nett force applied to the body, the body's CG, and the six accelerations required to provide equilibrium, and the internal reactions to these (similar to constraining the CG in 6 dof).
another day in paradise, or is paradise one day closer ?
hi rb1957! The previous posts mentioned inertia relief and modal. Those were the examples. And they are commonly used. I am not sure what we are debating. Either way, my point was to ascertain from the OP what they are trying to accomplish. If they are trying to perform an inertia relief or a free modal, then the answer seems to be a "yes" and they can proceed accordingly.
Though, IMO, the question is vague as it stands and since nobody asked for clarification, I thought I would.
No i am not doing model analysis. I am still at the learning stage. So am not aware of inertia relief. Can u pls explain in detail what is it? How and in which situation i can use it? Constraining for me is big problem, there are manny cases where i dont want to constarin at all but i have to. For eg. Pressure vessels, since the pressere is equal in all direction, wherever i constrain the high stress will be adjecent to the constraining point. I learned that in axisymmetric model i can work with only one degree of freedon fixed but if i use same model in solid i have to fully constrain it. Why is it so?
"inertia relief" is the opposing inertial force that is created when a load is applied, if "relief" is confusing, just call it "inertial force" or "inertial reaction". On a wign the aerodynamic loads are up and the inertial loads are down (hence the inertial loads are relieving the aerodynamic ones).
All stress analysis FEMs need to be restrained in 6 dof (to take out rigid body motion, to "nail" the model in space). these constraints should not generate reactions. if they do it means your loads are not balanced, or your constraints are over constraining the model. in a pressure vessel you should be able to constrain 3 nodes (xyz, xy, x) to take out rigid body motion. if you are new the FEA, read up on this.
If you tell the FEA that this is an axisymmetric model, then it applies the relevant constraints, and so maybe you need fewer rigid body constraints. read up on this. if you have solid elements in your model, you have only three dof at each node ("fully constrain" is usually used to describe constraining all 6 dof).
another day in paradise, or is paradise one day closer ?
Thanks every one,
Inertia relief command has worked for me and i am getting my results. My preesure vessel now dont need any constraining in any direction.
Hello rb1957,
I now little bit understood about inertia relief. Thanks to u But i want to learn it in detail, i searched in help file but there is very little written about it. If u know any reference then pls suggest me.
And yes i understand the requirement of restraining the model in all degrees of freedom as its in space.
I need to read more about it can u pls suggest me something?...
