SPNTID behaviour in pretension

Vidyaprakash G S

Query:

1) Why in the second step of loading , new node 108153 is created which is equivalent to 39 kN pretension force at the end of loading. The node number is not found in the model

2) In SPCF file ,The direction of pretension ( 39 kN ) is given in z direction but the reaction magnitude equivalent to pretension obtained is in X direction . kindly explain

3) At the 1.1 loading , the same node 108153 magnitude is minimum - 3828.7 N and slowly increases to 39kN at the end of 2 . Why the trend is slowly increasing?

at 1.1 only 39 kN must be reached  is the expectation. 

 

Observation:

108153 is SPNTID obtained from pret file

 

Kindly refer the attachment

1) pret file

2) SPCF file

3) Out file

3) presentation

 

Robinson Ferrari_20451

Hi Vidyaprakash,

1) From the .out file you received the message
*** INFORMATION # 3071
SPOINT # 108153 is automatically generated for PRETENS # 1.
This means a scalar point is generated (internally for pretension proposal) not a new node in your model.

 

2) OptiStruct will apply the pretension in DOF1 locally.
For 1D elements, it is along the axis of the respective 1D element.
For 3D pretension, it is defined as the average normal to the respective SURFace (average of respective element normals).

3) You are using statsub(pretens) to your load step 2. Thus, OptiStruct is keeping fixed the nodes after the adjustment done to step 1 (please, notice in the .pret file that ADJUST-TOT remains constant since total time = 1.0).
SPCF shows the external forces applying to your model. For the specific 'node' 108153(spoint) an external force is applied incrementally to keep the external and internal forces in equilibrium with this node.
Please also notice that FORCE-INCR from time 1.1 until 2.0 is changing. If you do FORCE-TOT - FORCE-INCR you will obtain the pretension load applied.
Thanks,