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May be I am bring up the old topic again., but I've lost enough sleep over this topic. I understand that we use Von-Mises criteria for ductile material failure by comparing to yield limit and max principal stress is used to check failure for brittle materials. My question is bit more basic in nature.

By principal stress do we mean perpendicular stress acting on X, Y & Z planes ? Or it is combination of all three components ? Do we also consider shear stress ζx, ζy & ζz in calculating principal stress ?

Is Von-Mises stress a real stress value, or its kind of an index which is calculated to check for failure criteria ?

If yes, what is the physical significance of Von-Mises stress ?

Is it only used as a pass fail criteria to understand if the component will pass or fail or we can extract additional information (such as fatigue life) ? How ?

Is Damage factor (calculated from FEA) related to Von-Mises stress ? How ?

I might be asking basic questions (and a lot of them), but need to sort things out once and for all.

Regards

Saumya