NASTRAN- OUTPUT STRESSES

[JayZ]

I have a simple solid plate model with a hole in it, two questions.

1. A tensile load is applied as force on nodes on the 1/2 surface of the hole. The Von Misses stresses are too high (in order of 4-5 times of the calculated Normal Stress)on the plate. What am I doing wrong here?

2. How can I turn on the Normal Stress or the Combined stresses of the solid plate?

Thanks,
Jalal

 

[gavmke]

Your message is a bit short in detail so if some of my comments are off the mark I apologize now. (I wish this thing had a spell checker).

I have a few questions
1) Are you applying the same force to all the nodes or ajusting the distribution in some manor? Generally a pin or bolt will have a graduated force distribution from a maximum at the center point to about 30 or 40 degrees where it approaches zero.
2) What is applying the force? A bolt? A tight pin? This will effect the force area.
3) Is this a symmetric analysis and what are the restrain conditions? I've seen errors were the restraints cause the biggest problems.
4) Are you considering stress concentration factors? You mention 'calculated normal stress'. Do you mean nominal stress? The relative size of the hole determines the stress concentration factor that you would apply to nominal stresses. Holes typically have factors of 3 to 5. FEA by its very nature yields the 'real' stress ie. (nominal)*(stress factor) not the nominal. How accurate it is depends on the mesh density, element type, material model etc.
5) What post processing software are you using? Generally the FEA software outputs ALL the stress values (Von Mises, Normal, Shear etc) into a single file. Even marginal post processors should allow for plotting or reporting any of these.

Is this of any help?

 

[JayZ]

Thanks all for your useful comments, it made me think a little harder.

Gavmke,
The model is symmetric and the boundry conditions are applied such that. I found what I was doing wrong, applying the same load on number of nodes, I learned to check my load! Your point is well taken to factor in the Stress Concentrations & how the load is applied for a pin connection. Now 1 questions for you;

Should the Max Von Misses Solid stresses be used as a fail or pass on a particular material or going through the Mohr's circle and calculating the Max. Combined Stress? Or what your suggestion is?

Thanks again,
Jalal

 

[pjhype]

Jalal,

The question of what stresses should be used has been addressed previously in this forum. Start by looking at the following thread:

thread727-22192