Understanding Principal Stresses

[BekirHan]

Hello everyone,

I have an issue with evaluating principal stresses and quite confused and needed your priceless comments.

Normally what we assuming is; max. principal stress (s1) is stress which generally tensile,

and minimum principal (s3) is compressive.

With this assumptions i am trying to comment results of my fea case in ansys workbench. (for a brittle material)

What is confusing me is in the results of loadstep which i am looking for maximum value of maximum principal stress is negative.

Normally i am looking max value of maximum principal stress which is positive and tensile direction, and min value of minimum principal stress which i assuming compressive direction.

So how to evaluate that negative value? What does it mean? I am waiting for your priceless comments. Thanks!!!

 

[rickfischer51]

The first principle stress is not by definition tensile. It is the largest of the three principal stresses. If my principal stresses were -10 MPa, -24 MPa, and -4 MPa, then s1 = -4 Mpa, s3 is -24 Mpa, and s2 is-10 MPa.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[NRP99]

Principal stress at location may have positive or negative sign but is dependent on the applied loading. The negative value of principal stress means the component is in compression and positive vale indicates tension. Is your component in compression?

 

[corus]

Negative means it's in compression. Check for buckling or in a severe case check for residual tensile stresses if you've gone above yield. Also check the stress range over different load cases in case the secondary stress range exceeds twice yield. Fatigue damage would also have to be considered when looking at the stress range.

 

[ThomasH]

Hi

This is probably easier in 2D.

You have three stresses: sigma_x, sigma_y and tau_xy. Now imagine a line in the plane, you can find an orientation for that line so there will be no shear in that coordinate system.

Now you have three new values: sigma_1, sigma_2, angle. Sigma_1 and sigma_2 are the principal stresses and the angle describes their orientation. They can be both tensile (+) and compressive (-) depending on the loading.

For more detailed info (if you need it):

https://en.wikipedia.org/wiki/Cauchy_stress_tensor#Principal_stresses_and_stress_invariants

Best Regards

Thomas

 

[stressebookllc]

rickfischer51 hit the nail on the spot.

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