Question on Von Mises Stress Envelope

[masshakar]

I have a question about the Von Mises Stress Ellipse:
http://upload.wikimedia.org/wikipedia/commons/4/48/Tresca_stress_2D.png

I understand that this equation if a component is subject to more than just an axial stress, the effect must be account for. I also understand that the ellipse is the result if setting one of the stresses to zero; the resulting equation is the equation for an ellipse.

Let's say that we have three stresses acting on a pipe: radial, axial, and tangential. Let's set tangential stress to zero.
-If you have positive radial (towards the center) and positive axial stress (tensile), why would you need a higher von-Mises stress to yield the pipe (see Quadrant I)?
-If you have negative radial (away from the center) and positive axial stress (compressive), why would you need a higher von-Mises stress to yield the pipe (see Quadrant III)?

 

[nvn]

masshakar: Shear stress is higher in quadrants II and IV. Shear stress is relatively more detrimental than normal stress (quadrants I and III), for ductile materials.