# Vonmises Stress Vs Maximum Principle Stress

Hello All,

Can any one let me know the difference between Vonmises Stress criterion and Maximum Principle Stress criterion ?? And for what kind of materials " Brittle / Ductile " can they be used and why ??

stewartcs
Maximum Principle Stress Theory - According to this theory failure will occur when the maximum principal stress in a system reaches the value of the maximum stress at elastic limit in simple tension. This theory is approximately correct for cast iron and brittle materials generally.

Source: http://www.roymech.co.uk/Useful_Tables/Mechanics/stress.html

Von Mises Stress (Distortion Energy Theory) - This theory proposes that the total strain energy can be separated into two components: the volumetric (hydrostatic) strain energy and the shape (distortion or shear) strain energy. It is proposed that yield occurs when the distortion component exceeds that at the yield point for a simple tensile test.

Source: http://en.wikipedia.org/wiki/Yield_(engineering [Broken])

General information on solid mechanics can be found http://en.wikibooks.org/wiki/Solid_Mechanics" [Broken].

CS

Last edited by a moderator:
HI,

Thank you very much for very useful information and your precious time.

Chandra.

Maximum Principle Stress Theory - According to this theory failure will occur when the maximum principal stress in a system reaches the value of the maximum stress at elastic limit in simple tension. This theory is approximately correct for cast iron and brittle materials generally.

Source: http://www.roymech.co.uk/Useful_Tables/Mechanics/stress.html

Von Mises Stress (Distortion Energy Theory) - This theory proposes that the total strain energy can be separated into two components: the volumetric (hydrostatic) strain energy and the shape (distortion or shear) strain energy. It is proposed that yield occurs when the distortion component exceeds that at the yield point for a simple tensile test.

Source: http://en.wikipedia.org/wiki/Yield_(engineering [Broken])

General information on solid mechanics can be found http://en.wikibooks.org/wiki/Solid_Mechanics" [Broken].

CS
Hi all,

with regards to the max. principal stress vs von mises stress, does a material yield or fail if the von mises is greater than the max. principal stress or visa versa?

secondly, how is the von mises related to compressive strength (or minimun principal stress)?

your help will be greatly appreciated

regards

Tach

Last edited by a moderator:
Hi all,

with regards to the max. principal stress vs von mises stress, does a material yield or fail if the von mises is greater than the max. principal stress or visa versa?

secondly, how is the von mises related to compressive strength (or minimun principal stress)?

your help will be greatly appreciated

regards

Tach

The beauty of Von Mises stress is that in the real world "everything" fails by shear. That's why it has emerged as the favorite failure theory. Having said that, the world of material failure is highly stochastic - subject to statistical variation. So as good as the theory is, you still need significant factors of safety if you don't want your project to come crashing down.

You find Von Mises stress from the principle stresses by using a big ol gnarly equation or three. It is always a smaller value than maximum principle stress (by definition) BUT it is aligned in the direction that has to support the maximum shear load. This can be very helpful in design.

hth (and i hope that I'm not too rusty on this subject)

gedw