Nonlinear Spring Elements

Altair Forum User

I am running a non-linear quasi static analysis and would like to insert some 1D spring elements that do not have a constant stiffness.  Is there a way to reference a table of load-displacement data for this?

Altair Forum User

Hi WOB_Watson,

 

Please refer to PBUSHT card in HyperMesh reference guide. 

 

Using PBUSH>>K_LINE>>PBUSHT>> KN_LINE one can introduce force Vs deflection table to the spring.

Altair Forum User

Thank you, Prakash.  I'd like to ask for some further clarification:

 

I have 2 DOFs that I want to reference TABLED1 data for.  The other 4 DOFs have constant stiffness values.  When the TKNID is being populated in the PBUSHT fields for a DOF, do I leave the Ki value as zero in the PBUSH fields?

Altair Forum User

Hi Watson,

 

 

 

 

could you please define the problem?

 

 

 

 

Altair Forum User

Prakash,

 

A very specific definition of the problem would probably have to be done over email.  What is being represented with the 1D spring element does in fact have linear stiffness values in 4 DOFS and nonlinear (cubic) load-displacement relationships in 2 DOFs.  This has been validated with test data.

 

My question in post #3 comes from being unable to 'depopulate' the K_LINE DOFs even if a table ID is referenced in the corresponding KN_LINE.  I am able to change them to zero, but I am not sure if that is going to do what I am expecting it to.

Altair Forum User

Hi,

 

Can you get in touch with US commercial support team?

 

Please get in touch with the local support and they can help you out on mail/phone

 

 If you are a commercial customer, you can contact your local support @ http://www.altairhyp...ortProduct.aspx

sbalazs

Hi Prakash Pagadala,

 

1. I have a same problem, 3 DOFs linear and 3 DOFs neliniar. How should I model my modell in this case?

 

Is this correct:

 

PBUSH          3       K0.0     0.0     0.0     28650.0 28650.0 28650.0
PBUSHT         3      KN      29      30      31

TABLED1       29                
+         -10.12-38799.0   -9.77-34584.0   -9.44-30866.0   -9.13-27587.0
+          -8.84-24695.0   -8.57-22144.0   -7.98-17030.0   -7.46-13186.0
+          -6.98-10296.0   -6.53 -8124.0   -6.11 -6490.0    -5.7 -5263.0
+          -5.32 -4340.0   -4.95 -3646.0   -4.58 -3124.0   -4.22 -2732.0
+          -3.87 -2438.0     0.0     0.0    3.87  2438.0    4.22  2732.0
+           4.58  3124.0    4.95  3646.0    5.32  4340.0     5.7  5263.0
+           6.11  6490.0    6.53  8124.0    6.98 10296.0    7.46 13186.0
+           7.98 17030.0    8.57 22144.0    8.84 24695.0    9.13 27587.0
+           9.44 30866.0    9.77 34584.0   10.12 38799.0ENDT    
$$
TABLED1       30                
+         -20.15-96415.0   -19.7-93975.0  -19.26-91574.0  -18.81-89212.0
+         -18.38-86887.0  -17.94-84601.0  -16.97-79487.0  -16.02-74558.0
+          -15.1-69808.0  -14.19-65230.0  -13.31-60817.0  -12.43-56564.0
+         -11.59-52465.0  -10.75-48515.0   -9.95-44707.0   -9.16-41037.0
+          -8.38-37500.0     0.0     0.0    8.38 37500.0    9.16 41037.0
+           9.95 44707.0   10.75 48515.0   11.59 52465.0   12.43 56564.0
+          13.31 60817.0   14.19 65230.0    15.1 69808.0   16.02 74558.0
+          16.97 79487.0   17.94 84601.0   18.38 86887.0   18.81 89212.0
+          19.26 91574.0    19.7 93975.0   20.15 96415.0ENDT    
$$
TABLED1       31                
+          -4.52-13173.0   -4.28-11825.0   -4.06-10626.0   -3.87 -9560.0
+          -3.68 -8612.0   -3.52 -7769.0   -3.28 -6637.0   -3.08 -5689.0
+          -2.89 -4894.0   -2.72 -4228.0   -2.57 -3671.0   -2.43 -3203.0
+           -2.3 -2812.0   -2.18 -2484.0   -2.06 -2209.0   -1.96 -1979.0
+          -1.86 -1786.0     0.0  -450.0    1.86   886.0    1.96  1079.0
+           2.06  1309.0    2.18  1584.0     2.3  1912.0    2.43  2303.0
+           2.57  2771.0    2.72  3328.0    2.89  3994.0    3.08  4789.0
+           3.28  5737.0    3.52  6869.0    3.68  7712.0    3.87  8660.0
+           4.06  9726.0    4.28 10925.0    4.52 12273.0ENDT 

 

 

2. What does this text mean?:

 

 *** WARNING # 3065
 PBUSH/PBUSHT 3: Ki (i=1) is zero and the corresponding KN-TIDi is given.
 In linear analysis, Ki is overwritten by the slope of KN-TIDi at zero deflection.

 

I get this text when I'm running a linear analysis with PBUSHT.

 

In my first example means this that Optistruct takes for :

 

Tx = 0 N/mm, Ty = 0 N/mm and Tz = -450 N/mm ?

 

Thanks in advance and best regards,

Szilard Balazs

 

Altair Forum User

HI,

 

If you look at the warning message, it says the slope on KN curve will be used for K for linear analysis.

 

If you are running non linear analysis I think KN will be used directly. 

sbalazs

Hi Prakash,

 

thanks for your answer.

regarding my 2. question : if optistruct takes a slope, which slope does it take - each pair of values define a slope, which one is taken, how can I understand the 'at zero deflection' therm?

 

What do you think about my first question, can it be modelled as I did?  Because above at 15.June.2015 you wrote : ' A spring will be either linear or Non linear in all DoFs. So, if X, Y are non-linear then physically other DoFs should be non linear. '

 

Best regards,

Szilard Balazs

Altair Forum User

Hi,

 

If the Ki is left zero and KNi specified, slope of the curve will be used for Ki. I will check regarding the slope as I am not sure about which slope of the curve will be considered.

 

2) I dont remember on what context I made that comment. But when as said from the warning slope will be considered  when solver finds zero for Ki

sbalazs

Hi Prakash,

 

thanks a lot for your answer.

 

You made that comment in this same topic, into I added my question too, a few posts higher. I hope my model works properly in the way I modelled it, with the springs that have 3 DOF's nelinear and 3 DOF's linear.

 

I'm waiting your answer regarding the slope.

 

Best regards,

Szilard Balazs

 

 

Altair Forum User

Hi,

 

I will let you know soon. 

 

Anyways, I removed that comment which is misleading.