Residual Stress or Pre-stressing a beam


Hello Everyone, 


I want to know if there is any way in NX to have a solution on which the load is applied on a model which is pre-stressed already (elements or nodes). 


Here is what I want to do:


1- I have two beams: both of them have a square section of 10x10 mm. The first beam is straight and is 5000 mm long. The second beam is also 5000mm long but is slightly curved.


2- I modelled both beams on NX


3- The beams are to be welded together. 


4- In practice, and with a jig, the curved beam will be clamped to the straight one, so I can weld them together. Meaning that with an external load (the clamps of the jig), the curved been will be elastically straightened for welding


5- I want to know the amount of stress in the weld, due to the curved beam wanting to spring back to its original position.


6- I know the load needed to straighten the curved beam and the amount of stress in it. 


7-I want to estimate the residual stress in the weld as follows: I model 2 straight beams that are perfectly watching each other, with one of them pre-stressed as the curved beam. 


8- I ''glue'' them in NX, with the weld modelled too. The spring back effect of one of the beams will induce a stress in the weld. That's what I want to estimate and quantify. 


Is there any way in NX to do that?


Thank you  



Re: Residual Stress or Pre-stressing a beam


Hi, I think there is no easy way to do so. Keep in mind that are different solvers with different capabilities which has to be taken into account.


But I think, there is no straight way to analyze it. But, my experiences are made in NX10.


Here are two ideas to get the pre-stress situation you want to have.


Curved beam is I, straight beam is II. Normal direction of weld is n. The curved beam I is above the straight beam II and the gap is in the middle.

+Z is normal direction from  straight beam to curved beam,

+X is in parallel direction of beams (and weld seam)

+Y is in lateral direction of beams.


Assuming you have identical meshing size on both beams, there are grid points which can be provided as "PARTNERS in normal direction" of straight beam.


1. If you want to close the gap between curved beam and straight beam you could define a set of multi-point constraints between the partner-grid points. They have to be moved into their direction vice versa in a way, that the difference of displacements in normal direction is exactly the starting gap. The problem is that you can't define a forced user defined constraint, as  the difference hasn't  to be fixed by absolute displacements. The trick is to have a set of THIRD GRID POINTs at a new free surface. Here you must define the user defined constraint dealing with the curved geometry as absolute displacement with positive values. Then you have to take into account the displacement of that grid point in multi-point constraint:


u.I = u.II - u.3


Check for plausibility: If the upper beam remains undeformed (u.I = 0 mm), the lower beam has to deform fully.

u.II = u.3


I guess it is important to connect the both transverse directions, too. But here only manual coupling is necessary, because there is no gap in X or Y.


Unfortunately, I don't know a way to define it by means of geometry objects. One has to do it node by node, I think.


That's why here is my second idea:

2. If you want to get the stress values for closing the gap, try to see the problem in invers manner. That means, deal with a penetration in a contact problem instead of a gap: The contact algorithm will eliminate the penetration and you will get a stress tensor which is negated to that one of the gap problem in elements (I assume they are solid meshes made by hexahedrons (CHEXA)).

You could subtract that invers (penetration) stress components instead of adding the closing-gap stress components to your real working loads.

But here is the problem: The mesh for working load simulation has to be well connected without contact and without starting curved geometry. How on earth one could include that pre-stress values in a negated way into the unstressed model of two straight beams? - I only could imagine to do it in Excel.

And additionally, it's not clear if contact pressure remains at al spots of penetration surface. But this is important for negating the problem.


I hope you could understand what I mean.


If there is a better way I'm really keen to know it!!!!

Best wishes, Michael

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