von mises stress vs maximum shear stress
von mises stress vs maximum shear stress
(OP)
hello group,
I have applied pure torque to the circular rod constrained at opposite end and examined the stresses, but i found that von mises stress so different from maximum shear stress,i got von mises as 75ksi whereas maximum shear as 55ksi. I am just wondering how the von mises stress is so different from maximum shear stress.There are no other forces acting on the circular rod. I believe the von mises stresses accounts for all the stresses.can any one describe this behavior?
I have applied pure torque to the circular rod constrained at opposite end and examined the stresses, but i found that von mises stress so different from maximum shear stress,i got von mises as 75ksi whereas maximum shear as 55ksi. I am just wondering how the von mises stress is so different from maximum shear stress.There are no other forces acting on the circular rod. I believe the von mises stresses accounts for all the stresses.can any one describe this behavior?





RE: von mises stress vs maximum shear stress
Maximum shear = (SIGMA1-SIGMA3)/2
Von Mises in 3D = root((1/2(SIGMA1-SIGMA2)^2+(SIGMA2-SIGMA3)^2+(SIGMA3-SIGMA1)^2))
-- drej --
RE: von mises stress vs maximum shear stress
corus
RE: von mises stress vs maximum shear stress
vm stresses account for all the stresses:compressive,bending and shear.but in case of pure torsion to circular rod there should be only pure shear stresses, so what accounts for the rest that vm stresses are higher than shear stresses.
thanks
RE: von mises stress vs maximum shear stress
Tresca, is sort of a simplified Von Mises criterion - sometimes referred to as "Tresca's Hexagon" due to it's shape. Where the Von Mises criterion plots as a 45 degree ellipse, the Tresca criterion would be a hexagon inscribed in that ellipse.
Other criterion like Tsai-Hill and Tsai-Wu are similar to Von Mises except that they manage materials with direction dependant properties. Because of that, it's easier to work the material properties into the equation so that the result is relatable to unity. Thus, we have Tsai-Hill and Tsai-Wu indexes.
RE: von mises stress vs maximum shear stress
RE: von mises stress vs maximum shear stress
Cheers,
-- drej --
RE: von mises stress vs maximum shear stress
How do you then calculate Von Mises' stress?
What I did was use Mohr's circle, with (tau_xy)=55, and calculated the equivalent principle stresses sigma1=55, and sigma2=-55 (sigma3=0). Using these values, Sigma_VonMises would then equal 95.3 ksi. In pure shear, supposedly the VonMises (aka max. octahedral shear stress) and Tresca (aka max. shear stress) theories are supposed to agree; they are also supposed to agree at conditions of pure tensile/compressive loading. If this material had a tensile yield strength of 100 ksi, the max. shear stress theory would predict yielding, but the VonMises would not. Where is my error?
By the way, I know where the error is, but am looking all over the place for a good description of why, and how to properly calculate the VonMises stress for a shaft in torsion.
RE: von mises stress vs maximum shear stress
Where is your error? Aside from your impression that these two theories should agree, you didn't make one. Notice that the results for your example and my results (my first post) agree.
The formula for von Mises stress under a biaxial stress state is sigma_vm = sqrt(sigma1^2 - sigma1*sigma2 + sigma2^2).
RE: von mises stress vs maximum shear stress
Somewhere I thought I'd been taught that the Tresca and Von Mises criteria agreed at conditions of uniaxial tensile load and at pure shear stress. Oops. They agree at uniaxial tensile and conditions of _zero_ shear.
Okay. Somewhere else I've seen people say that there are axial shear stresses in pure torsion (even with circular sections); I thought this was only true for non-circular cross sections?
RE: von mises stress vs maximum shear stress
RE: von mises stress vs maximum shear stress
There is no such thing as Principle Stress!
But you do have Principal Stresses.
RE: von mises stress vs maximum shear stress
RE: von mises stress vs maximum shear stress
RE: von mises stress vs maximum shear stress
when we look at the stresses in a component, generally we ignore the stresses at the bolt hole locations. The reason for this is the way we model, the bolt hole(with RBE2's). My question is how do we judge the stresses around the bolt holes? In some cases we may not ignore them!!!
RE: von mises stress vs maximum shear stress
Simple don't use RBE2's and any other artifical, mathematical conveniences that don't relate to the real physical world. Unfortunately (for you!) this requires a FEA package that can handle contact analyses (eg. Abaqus, Lusas, Marc etcetera).
RE: von mises stress vs maximum shear stress
It also means that you have to explicitly model the hole and the bolt with enough detail to accurately determine the stresses around the hole, and have to get rid of the RBE2 elements and replace them with a reasonable appproximation of the joint flexibility. This will typically increase the size adn complexity of the model be an order of magnitude or more. The other alternative is to make a separate model of the joint region and apply loads/displacements from the global part model. Or just take the fastener loads from your current model and do a hand analysis of the local stresses, though I would be very cautious of loads that result from rigid elements such as RBE2s - they almost always result in models that are too stiff and artificially attract too much load. Joint load distributions from models using rigid elements to connect the joint memeber are almost always wrong - its actually much harder to correctly model a joint with rigid elements than it is with springs, beams, etc.
RE: von mises stress vs maximum shear stress