Large difference between Von mises & Tresca Stresses
Large difference between Von mises & Tresca Stresses
(OP)
Hi,
We have a pipeline calculation in which the Von Mises stress comes out to be 341.6 MPa while Tresca stress is only 12.2 MPa. Since both are failure criterion, is it alright to have that much difference between the two ? Von mises is supposed to be higher, but is it acceptable to have it that much higher?
Any help will be much appreciated.
Thanks
Sumit
We have a pipeline calculation in which the Von Mises stress comes out to be 341.6 MPa while Tresca stress is only 12.2 MPa. Since both are failure criterion, is it alright to have that much difference between the two ? Von mises is supposed to be higher, but is it acceptable to have it that much higher?
Any help will be much appreciated.
Thanks
Sumit





RE: Large difference between Von mises & Tresca Stresses
That gap is not credible.
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RE: Large difference between Von mises & Tresca Stresses
The hoop stress we have is 347.5 MPa. And SMYS is 485 MPa
As for the longitudinal stresses,
Stress due to internal pressure is 104.3 MPa,
Stress due to thermal expansion is -72.4 MPa
So considering the allowable limit of 0.9SMYS for longitudinal stress, we are left with 404.6 MPa for bending.
Utilizing 0.75 of this for bending, we get bend radius of 310.4m
And subsequently the Von mises & Tresca stresses as mentioned by me earlier.
So, where in your opinion, the problem is ?
Thanks
Sumit
RE: Large difference between Von mises & Tresca Stresses
I would like to see your calculation of both Tresca and VM stresses for both as a function of stress components.
For a simple condition of pressure only in a cylinder (approximately)
Tresca=2 x σL
VM=√3 x σL
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RE: Large difference between Von mises & Tresca Stresses
Tresca = |SH − SL|
Von Mises = (SH^2+SL^2-SH.SL)^0.5
RE: Large difference between Von mises & Tresca Stresses
RE: Large difference between Von mises & Tresca Stresses
RE: Large difference between Von mises & Tresca Stresses
SH = 347.5
SL = 104.3-72.4 = 31.9
SH-SL = 347.5 - 31.9 =315.6
RE: Large difference between Von mises & Tresca Stresses
That is not the case because of the longitudinal stresses due to bending which will also be present. According to ASME , Maximum longitudinal stresses can be 0.9SMYS which is 436.5 MPa. so that leaves (436.5-31.9) = 404.6 MPa for bending. Using 0.75 of this as some residual stresses might be there , we get the bending radius of 310.4m. So SL is not 31.9 MPa, but 335.4 MPa.
Sumit
RE: Large difference between Von mises & Tresca Stresses
as it will be restrained in all directions
RE: Large difference between Von mises & Tresca Stresses
Let me clarify as to what I think they are doing.
In this scenario they are elstically bending the pipe as they lay it into the trench and before it is restrained by adding the soil backfill, so the lateral movement occurs and bending stresses are developed before restraint is achieved.
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dasumit,
The net longitudinal compressive stress adds directly to the hoop stress to increase the Equivalent Tensile Stress available to cause yielding. The EQUIVALENT tensile stress shall not be allowed to exceed 90% of the specified minimum yield strength of the pipe.
What would you be doing, if you knew that you could not fail?