Large difference between Von mises & Tresca Stresses

[dasumit]

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

 

[prex]

Can you give some more facts and figures?
That gap is not credible.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[dasumit]

Thanks , prex for replying.
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

 

[prex]

So you have at least two loading conditions: pressure and pressure+thermal.
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 [σ][sub]L[/sub]
VM=[√]3 x [σ][sub]L[/sub]

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[dasumit]

Yes, the design temperature is higher than the installation temperature. So, longitudinal stresses due to thermal expansion are also there. And Tresca and Von Mises stresses are calculated as stated in the ASME code i.e,
Tresca = |SH − SL|
Von Mises = (SH^2+SL^2-SH.SL)^0.5

 

[parthanml]

is this section restrained or unrestrained ?

 

[dasumit]

The section is restrained.

 

[parthanml]

Tresca stress is SH -SL
SH = 347.5
SL = 104.3-72.4 = 31.9
SH-SL = 347.5 - 31.9 =315.6

 

[dasumit]

@parthanml

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

 

[parthanml]

But in restrained section , bending stress will be insignificant..no ?
as it will be restrained in all directions

 

[BigInch]

parthanml,

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.

=========================================================
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?