Stress due to bending of fillet weld joint

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Discussion Overview

The discussion revolves around the calculation of stresses in a fillet welded joint, specifically focusing on a circular shaft welded to a base plate under load. Participants are comparing manual calculations with finite element analysis (FEA) results, exploring concepts related to shear stress, bending stress, and von Mises stress in the context of mechanical design.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions the method for calculating von Mises stress, suggesting a formula involving shear stress and bending stress.
  • Another participant clarifies that the shear stress should be squared before multiplying by 3, and provides an alternative approach to calculating stress using the effective throat width and unit section modulus.
  • A participant expresses confusion regarding the reasoning behind multiplying the unit section modulus by the weld throat area.
  • There is a discussion about the correct expression for the unit moment of inertia and its relation to the section modulus, with differing views on whether to use pi*r^2 or pi*r^3.
  • One participant acknowledges a misunderstanding and expresses gratitude for the clarification provided by others.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the method for calculating von Mises stress, with conflicting approaches suggested. There is also disagreement regarding the correct expression for the unit moment of inertia and its application in the calculations.

Contextual Notes

Limitations include potential misunderstandings of the formulas and their applications, as well as the dependency on specific definitions of terms like section modulus and unit moment of inertia.

Mech King
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Evening everyone,

i have been referring to Shigleys "Mechanical engineering design" (pages 349 to 351) regarding a welded joint I am looking at. wanted to compare my manual calcs to sum FEA.

My joint is a circular shaft which is fillet welded to a base plate. A load is applied to the end of the shaft.

Do i compute the shear stress in the joint, mutlipy that value by 3, and the max bending stress in the joint, square them both, add them together and then square root them to get the von mises stress?

Also, Shigley lists the Unit moment of inertia for the joint as: pie*r^3.......... so, if I am treating the weld as a circular line, can i just divide the unit moment of inertia by the radius (the distance to the centroid) in order to get my section modulous Z? I will then divide M/Z to get my max stress?

It's been a very long time since i looked at welded joints, and am a tad confused,

any help much appreciated,

Cheers
 
Last edited:
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Mech King: No, square the shear stress before multiplying it by 3, not vice versa.

Yes, you are computing the unit section modulus, Zbar, correctly, not section modulus, Z. No, do not compute M/Zbar. First, multiply Zbar by the weld effective throat width t, then divide M by this result.
 
Thanks nvn,

i got confused with the text as it didn't explain the reasoning for multiplying by the weld throat by Zbar?

Just to clarify:

So for a circular weld in bending, i mulitly the Unit Moment of Inertia (Z bar) by the throat area (1.414*pie*h*r)?
 
Last edited:
Mech King said:
Do I compute the shear stress in the joint, multiply that value by 3, and the max bending stress in the joint, square them both, add them together, and then square root them to get the von Mises stress?

Mech King: No, don't use that method. Don't compute von Mises stress. Just compute and use the following stress. sigma = M/(Zbar*t), where Zbar = pi*r^2, and t = 0.7071*h.
 
OK nvn,

but shouldn't Zbar be pi*r^3 as in Shigley?
 
Last edited:
Oops sorry,

i understand now, cheers, again
 

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