What condition defines a principal stress?

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Principal stresses are defined as normal stresses that occur in a given stress state, specifically acting perpendicular to the material's cross-section. In a thin plate under tensile stress, these principal stresses are oriented normal to the plate's sides. For a beam in pure bending, principal stresses correspond to the tensile and compressive stresses on the top and bottom surfaces. In more complex shapes like I beams, Mohr's Circle is used to calculate both principal and shear stresses. Understanding these concepts is essential for engineering applications involving material mechanics.
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What condition defines a principal stress?

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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
 
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this covers the confution I had, thank you verry much

Loki1342 said:
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
 
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I am following someone on Youtube who has built a rocket shaped (4 proprllers) drone for the world speed record ... He was having problems with internal heat from electical control gear and battery inside the sealed rocket (about 1kw waste heat) Speed is 500km/hr , I suggested a 4mm hole in the tip of the nose and 8mm hole at rear which should alow sufficient air cooling .. I said this will increase thrust because the air exiting is hotter .. a bit similar to a ram jet ... was I correct...
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