Discussion Groups

Robot Structural Analysis

Reply
Distinguished Contributor
Tuctas
Posts: 166
Registered: ‎10-10-2012
Accepted Solution

Stress analysis

485 Views, 6 Replies
02-13-2013 03:12 AM

 A question that concerns the results of stress analysis. The "extreme normal stresses" in section are the priciple stresses (that are maximum indeed) occured in an inclined plane-cut where are no shear stresses or are the normal stresses that act on the plane of cross section (together with shear stresses)?

  In the "stresses in point" the stresses "σi" are also the priciple ones?   

 

  If the answers are positive  are we able to obtain the angle that these max normal (priciple) stresses occur?

 

Please use plain text.
Distinguished Contributor
Tuctas
Posts: 166
Registered: ‎10-10-2012

Re: Stress analysis

02-13-2013 09:52 AM in reply to: Tuctas

  After making a simple investigation, i finally found out that the answer is NO. Unfortunatelly, the extreme normal stresses are not the principle ones.. I think that this information should be provided too (as well as the principal directions) in order the results to be more usefull for the user. 

Please use plain text.
Distinguished Contributor
Tuctas
Posts: 166
Registered: ‎10-10-2012

Re: Stress analysis

02-13-2013 11:34 PM in reply to: Tuctas

  If it's possible, just a comment from the support...

Please use plain text.
Product Support
pp2008
Posts: 475
Registered: ‎06-23-2008

Re: Stress analysis

02-14-2013 05:09 AM in reply to: Tuctas

Somme comment from the support .. :smileyhappy:

 

1/ Extreme normal stresses are not the principal stresses. These are extreme values of normal stresses (along local X) resulting from bending moments and normal force (Fz, My, Mz)

2/ the "σi" stress is von Mises reduced stress calculated for 3D state of stresses

 vonMises.png 

where: vonMises1.png

Displaying principal stresses and directions of them would be difficult because it is considered as 3D state of stresses and principal directions are independent from XY, UZ and ZX planes available in the interface of stress analysis

 

Moreover in general case of 3D state of stresses the arbitrary setting of directions corresponds to point N in the picture below. Calculating principal stresses from it may be difficult.
3Dstress.png
 
---------------------------------------------
If this post answers your question please click "Accept as Solution". It will help everyone to find answer more quickly!
 
Best regards,


Pawel Pulak
Please use plain text.
Distinguished Contributor
Tuctas
Posts: 166
Registered: ‎10-10-2012

Re: Stress analysis

02-14-2013 06:41 AM in reply to: pp2008

  Thank you for your answer.

 

  As concerns point 2) i just forgot to tell in my previous post, after my simple investigation, that was cleared.

 

  It seems difficult indeed but it is also a very usefull information.. About the graphical presenation, it wouldn't matter if there wasn't anyone, as they are more important the values at it self..

 

  Thank you anyway.

Please use plain text.
Product Support
pp2008
Posts: 475
Registered: ‎06-23-2008

Re: Stress analysis

02-14-2013 07:31 AM in reply to: Tuctas

Stress analysis is performed for bar elements so the state of stresses obtained from it in each point is in reality not full 3D but 2D in some virtual plane resulting from values of tau_xy and tau_xz in specific point.

So it is possible to calculate:

1/ total shear stress tau in specific point (equal to sqrt(tau_xy^2 + tau_xz^2))

2/ angle between for instance y axis and the plane of total shear

3/ perform standard analysis of principal stresses for 2D state of stresses in plane of total shear (considering sigma_x and tau) - resulting in sigma_1, sigma_2, tau_12

 

Of course above operations can be made outside of Robot  in for instance MS Excel - but in such case it is necessary to output necessary components of stresses (sigma_x, tau_xy and tau_xz) point by point.

 

Improvement request to enable it inside stress analysis registered.

 

BTW: why do you need these values if you have von Mises Stresses?

 

---------------------------------------------
If this post answers your question please click "Accept as Solution". It will help everyone to find answer more quickly!Hello,

Additional remarks to my previous answer and answer to your last post:

 

Regards,



Pawel Pulak
Please use plain text.
Distinguished Contributor
Tuctas
Posts: 166
Registered: ‎10-10-2012

Re: Stress analysis

02-14-2013 08:43 AM in reply to: Tuctas

  Dear Pawel,

 

  Thank you for your, as always, illustrative answer.

 

  Actually, i was refering to the case of using stress analysis in the module "section definition" where we can examine individually any kind of section with any kind of material (so, in 2D).

  The Von Mises criterion is used only for ductile materials (i.e usually for steel), and not for other materials. Priciple stresses have a more general use.

 

  Thank you.

 

 

Please use plain text.