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gfraulini17-Peridot
17-Peridot
April 4, 2019
Question

Components for Stress or Strain

  • April 4, 2019
  • 2 replies
  • 14290 views

Hi everybody,

I want ask you where and why one need to use Max, Mid, Min or Max Shear principal components of Stress/Strain.
How can I utilize them to understand if a component, in a particular point, will resist or not?

 

I attach two pics where I want to analyse a specific point. One is the vectorized display of the three components. One is the fringed display of the Max,Mid,Min and Von Mises Stress. Then I post also a sheet where I compare Von Mises tension from principals tensions manipulation and directly from Simulate.

 

I've a little bit confusion in my head redarding this argument. Could you let me understand better the point?

 

In my opinion principal tensions are not useful with fringe plot, but only with vector plot, but which one should be considered?

Thanks
Bye

2 replies

skunks
skunks19-Tanzanite
19-Tanzanite
April 4, 2019

example att. (in german)

 

sigmas.JPG

S
SweetPeasHub17-Peridot
17-Peridot
April 4, 2019

You are touching on the area of what criteria to use for a failure.  

Different materials fail in different ways according to the full stress state.  The Von Mises criteria is just one such theory for failure, but it is just a theory and works great for some materials and load cases, yet does not work very well at all for others. 

I like signed Von-Mises which differentiates between compressive and tensile failure.  This can be accomplished in CREO using failure criterion of Modified Mohr with different tensile/compressive failure stresses.  The fringe plot is then "failure index"  The failure index can be confirmed by looking at the Max/Min Principal stresses and doing a ratio to the tensile/compressive material property failures respectively. 

Also, nearly the same information regarding stress is shown between fringe and vector plots.   Min principal shows the compressive, and max principal shows the tensile, The fringe plots give the vector arrow magnitudes and signs (two plots to show).  The only thing not shown by a fringe plot is the orientation of the vector which is best shown by a vector plot.  I would think crack propagation cares about the orientation, but I would not see how the origination point of failure would depend on orientation much if any.

 

This related answer includes references that show more on how and when these stress components might be used. See this answer

 

Signed VM failure CriterionSigned VM failure Criterion

G
gfraulini17-PeridotAuthor
17-Peridot
April 4, 2019

So:

  • Max Principal shows the tensiled points
  • Min Principal shows the compressed points
  • Modified Mohr criterion is to use where you have material, like concrete, that can collapse also in compression

 

@skunks  If I've translated well from german, in the example you are gone to find the normal stress (along Z) and the shear (along Y) to use them subsequently in the welded section calculus.

 

So XX,YY,ZZ stress are useful to see what happen in a particular section. But how can I use usefully Max and Min stresses?
I attach two new pics where I show the assembly (the zones fixed and the ones loaded) and the fringe plots of max and min principal stresses.
I can't understand the plots. For example, in the Min principal plot: why the blue zone (that It should be compressed being where the mass acts through the weighted link) is blue, so negative? If Min principal mean "compressed", a negative compression is a positive tensile.

???

S
SweetPeasHub17-Peridot
17-Peridot
April 4, 2019

Negative stress is always in compression and positive stress is in tension regardless of MAX/MIN principal being shown.  So your MIN principal could be tensile.  For principal stress space by definition MAX >> MID>> MIN.  In principal stress space the stress state matrix is rotated to show where the max possible stress is and then the other 2 orthogonal axis stress states are ordered according to MAX>>MID>>MIN.   It is essential like XX YY ZZ only rotated to the max direction (maybe the max tensile is at 45 degrees to XX) and then ordered from max to min.  (sort of like XX ZZ YY = MAX, MID, MIN because XX was highest then ZZ then YY)

 

For concrete for example, the more compressive MID and MIN are, the higher the magnitude MAX principal tensile stress can be before failure.  Vise- versa.  The more MID and MIN are also tensile, rather than compressive the lower magnitude MAX principal can be before failure occurs.  Basically a compressive "hydrostatic stress" increases the strength of concrete and a tensile "hydrostatic stress" decreases the strength.

 

 

Here is an explanation with Mohrs circle.  This is for planar (2D) stress to simplify.

Ignore the "rotated" line.  The 2D stress state given in some world coordinate system - (blue) is the same but the reference coordinate system is rotated to give the max/min  principal, max/min shear stress.  Thus there is a new local principal stress coordinate frame. The vector plots show the orientation based on how much the angle is between the world coordinate system and the principal stress coordinate frame.

Mohrs Circle - Plane StressMohrs Circle - Plane Stress

 

Maybe another way to think of this is this example.

Two observers, one looking north, one looking east, observe a car appearing to travel west at 60mph and south at 60mph respectively.  The car is actually travelling at 84mph to the south west.  In a similar way stress in X direction and Y direction add to a larger stress in the principal direction.  It only might appear smaller because of the reference frame, not being in the principal direction.

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