×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

von mises stress or principal stress?
6

von mises stress or principal stress?

von mises stress or principal stress?

(OP)
Can anybody explain the difference between von mises stress and principal stress?

Any help would be great!

RE: von mises stress or principal stress?

3
Von Mises stress is also referred to as the stress intensity, and is equal to:
SI=max(|S1-S2|,|S1-S3|,|S2-S3|)
where Si are the principal stresses.
It also equals two times the maximum shear stress and it is used in the von Mises failure criterion for checking the acceptability of stress states.
You should consult a basic theory book on failure criteria to know more: it's a quite complex subject.

prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design

RE: von mises stress or principal stress?

3
Principal stresses are stresses on planes with zero shear.

The von Mises yield criterion uses stresses on octahedral planes.

This subject is complex, but well described in a variety of textbooks such as Mechanical Engineering Design by Shigley and Mischke or Mechanical Behavior of Materials by Dowling.  Try the following website for an intro:

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/plane_stress_principal.cfm?search_string=principal%20stress

http://www.efunda.com/formulae/solid_mechanics/failure_criteria/failure_criteria_ductile.cfm#VonMises

RE: von mises stress or principal stress?

prex and I posted nearly simultaneously, so I read his response after mine.  prex is in error - the information he provides is for the Tresca yield criterion (also known as the maximum shear stress yield criterion).  The von Mises criterion uses a more complex formula involving the root of the sum of the squares of the differences of the principal stresses.

RE: von mises stress or principal stress?

Oops! Thanks to CoryPad for pointing out my mistake.
The formula for the von Mises stress should be:
SvM=sqrt(S12+S22+S32-S1S2-S2S3-S3S1)

prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design

RE: von mises stress or principal stress?

As CoryPad pointed out, the principle stresses are the three mutually orthogonal stresses acting on planes which bear no shear stresses. For every possible state of stress, the principle stresses and planes are unique. They are directional with magnitude. If a crack is present, the max principle stress will be in the driver's seat.

Von Mises stress is a function of the stress state. Each state has only one value of the Von Mises stress. In continuum mechanics, you learn that any stress state can be broken down into the hydrostatic and deviatoric stress tensors. The hydrostatic stress is the stress which is trying to change the volume of an element of material. The deviatoric stress is trying to change the shape. The Von Mises is related to the deviatoric stress. The Von Mises theory of failure claims that failure of ductile materials is due to the change in shape (twist, pull, bending.)

Von Mises can not be used for predicting fatigue. (See Socie, Multiaxial Fatigue, page 418)

Just my few cents.


Doug

RE: von mises stress or principal stress?

Notice that we're looking at the Stress at a point.

RE: von mises stress or principal stress?

Any discussion of the state of stress is always at a point, so what was your point?

Doug

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources