Back to the basics.

Check what is your type of loading 1D/2D/3D. E.g. if you have a round bar with axial tension then your Max principal stress will match the axial stress/Equivalent stress. But this is not the case for 2D and 3D loading conditions. E.g. Take example of 2D problem where Sx = 60 MPa, Sy = 40 MPa, Tauxy = 20 MPa,

Then Max principal stress S1=72.36 MPa, Min principal stress S2=27.63 MPa, third principal stress S3=0 MPa,and equivalent stress will be Seq=63.18 MPa. If only Sx is present then Max principal stress S1=Equivalent stress Seq=60 MPa.

Don't compare apples with oranges. Equivalent stress is theoretical average stress in interested section of component whereas Max principal stress is actual highest stress in the fibers of component that are at orientation to loading plane.

Hi,

the equivalent (von Mises) stress q is a measure for the average shear stress and is calculated from:

q**2 = 0.5*((s1-s2)**2+(s2-s3)**2+(s3-s1)**2),

where s1, s2, s3 are the principal stresses. This should answer your question.

Regards

Dear Prathap,

Both are failure criteria which predict failure based on different mechanisms. von Mises criteria uses Distortion energy theory whereas Maximum principal stress criteria uses (quite unimaginatively) Maximum Principal stress theory.

Try to know their behavior...I hope it is clear now.. 

Back to the basics.

Check what is your type of loading 1D/2D/3D. E.g. if you have a round bar with axial tension then your Max principal stress will match the axial stress/Equivalent stress. But this is not the case for 2D and 3D loading conditions. E.g. Take example of 2D problem where Sx = 60 MPa, Sy = 40 MPa, Tauxy = 20 MPa,

Then Max principal stress S1=72.36 MPa, Min principal stress S2=27.63 MPa, third principal stress S3=0 MPa,and equivalent stress will be Seq=63.18 MPa. If only Sx is present then Max principal stress S1=Equivalent stress Seq=60 MPa.

Don't compare apples with oranges. Equivalent stress is theoretical average stress in interested section of component whereas Max principal stress is actual highest stress in the fibers of component that are at orientation to loading plane.