What condition defines a principal stress?

[buxZED]

What condition defines a principal stress?

thx

 

[Loki1342]

Since I'm studying Engineering I should probably be able to explain this to you in my own words, but this site does a good job of it:
http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/plane_stress_principal.cfm

If you consider a given stress state, principal stresses are defined as stresses that are normal stresses only. So take the easy case of a thin plate, and apply a tensile stress to one end (i.e. try to stretch it). In that case, the principal stresses would be normal to the sides of the plate. If you had a beam with a rectangular cross section in pure bending, the principal stresses would just be equal to the bending stresses, and are tensile and compressive stresses on the top and bottom surface of the beam, assuming you've got a point force (for example) P acting normal to the bottom surface (in the positive direction, bending the beam into an upside down U). Things like I beams are a little different, and in that case you use Mohr's Circle to calculat principal stresses and shear stresses; the website covers that in a good amount of detail.

If I've missed anything or explained anything ambiguously, someone let me know and I'll try to do a better job of it. :P

 

[buxZED]

this covers the confution I had, thank you verry much

Since I'm studying Engineering I should probably be able to explain this to you in my own words, but this site does a good job of it:
http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/plane_stress_principal.cfm

If you consider a given stress state, principal stresses are defined as stresses that are normal stresses only. So take the easy case of a thin plate, and apply a tensile stress to one end (i.e. try to stretch it). In that case, the principal stresses would be normal to the sides of the plate. If you had a beam with a rectangular cross section in pure bending, the principal stresses would just be equal to the bending stresses, and are tensile and compressive stresses on the top and bottom surface of the beam, assuming you've got a point force (for example) P acting normal to the bottom surface (in the positive direction, bending the beam into an upside down U). Things like I beams are a little different, and in that case you use Mohr's Circle to calculat principal stresses and shear stresses; the website covers that in a good amount of detail.

If I've missed anything or explained anything ambiguously, someone let me know and I'll try to do a better job of it. :P