difference between 1st and 3rd principal stress

[amirfm]

Hi, I'm now using inventor stress analysis to analyze a gearbox housing made from cast iron which is a brittle material. and i need to know what the difference between 1st dan 3rd principal stress is, since i will use maximum normal stress failure criterion that needs maximum tensile or compressive stress of that material. does anyone know what 1st dan 3rd principal stress is?

regard,

Amir FM

[admaiora · Accepted solution]

Hi Amir,

directly from the software help:

"The 1st principal stress gives you the value of stress that is normal to the plane in which the shear stress is zero. The 1st principal stress helps you understand the maximum tensile stress induced in the part due to the loading conditions."

"The 3rd principal stress acts normal to the plane in which shear stress is zero. It helps you understand the maximum compressive stress induced in the part due to the loading conditions."

In substance for one point  there is a plane where the shear stress is zero.The 3 principal stresses define the stress in this point respect the plane and his 3 direction.

The 1st is the largest

the 3th is the smallest 

(the 2th just in the middle)

[amirfm]

thank you very much

[amirfm]

i'm sorry i wanna ask you again, what does negative value in 1st dan 3rd
principal stress mean?

[admaiora]

Hi Amir,

it's means that they are compressive tensors.

[amirfm]

but you said before that 1st is tensile and 3rd is compressive streght
respectively, why did i get negative value which is a compressive stress in
my 1st principal stress dan positive value in my 3rd principal stress? im
sorry im still confused

[admaiora]

You can follow the second definition i gave to you, the wikihelp definition can be miserunderstood .

In substance for one point  there is a plane where the shear stress is zero.The 3 principal stresses define the stress in this point respect the plane and his 3 direction.

The 1st is the largest

the 3th is the smallest

They can be negative or positive, in substance the opposite limit stress (with shear 0).

The 2nd is not important.

[m.malfatti]

Hello, I find this conversation very helpfoul, can you also explain the notmal stress XX (for example) and the shear stress XY (for example)?

Thanks!

[ManikantaAthukuri]

Thanks for the clarification