ASME Sec. VIII Div. 2 Fatigue curve

[fabricd]

Hi,

This post concerns Annex 3F design fatigue curves from ASME VIII Division 2.

I am a piping stress engineer and I am trying to find a S-N curve for fatigue calculation.
I am using a Stainless steel TP 316L material, so I guess i should refer either to :

table 3.F.3 (– Coefficients For Fatigue Curve 110.2.2 – Series 3xx High Alloy Steels, Nickel-
Chromium-Iron Alloy, Nickel-Iron-Chromium Alloy, And Nickel-Copper Alloy For Temperatures Not
Exceeding 427 °C (800 °F) Where Sa >195 MPa (28.2ksi))

or to table 3.F.4 (– Coefficients For Fatigue Curve 110.2.2 – Series 3xx High Alloy Steels, Nickel-
Chromium-Iron Alloy, Nickel-Iron-Chromium Alloy, And Nickel-Copper Alloy For Temperatures Not
Exceeding 427 °C (800 °F) Where Sa ≤195 MPa (28.2ksi))

But I don't know what "Sa" stands for for piping stress system issue !

Let's imagine I pick table 3.F.4 Curve A.
What units are used in table 3.F.10 ?
Logically it's supposed to be MPa but I am not sure since these values are very low.
Is table 3.F.10 even a S-N fatigue curve table ?

Thanks.

 

[TGS4]

A couple of comments

Sa if the alternating stress amplitude, as calculated in Part 5.

The tabulated stress amplitude-life dates used units of ksi for stress. This is all explained in the body of Annex 3.F.

What edition are you using? Older editions have the wrong formula for the temperature correction (via the Young's Modulus ratio)...

 

[fabricd]

Thank you for your answer,

I am using 2010 edition.

Sorry but I can't find where you read that table 3.F.10 is in ksi .. But it seems logical seeing the values.

I know that Sa is the alternating stress amplitude and I have already checked Part 5.
But how can I estimate my Sa for a whole piping stress system ?

 

[MrPDes]

Table 3.F.10 specifies in the general note below the table to be used for Customary units.
Table 3.F.10M specifies in the general note below the table to be used for Metric units.

I don't understand your question regarding estimating Sa.

There isn't a single Sa for the entire piping system. You have to scan through the FEA analysis of the pipeline for specific areas of concern and apply the rules of Part 5 including correction factors etc to get a stress range (Sa) for each specific area.

Each suspect area needs to then have its calculated stress range (Sa) applied to the fatigue curve to get a maximum number of cycles.

Are you reading the code thoroughly first and then asking questions in this forum for clarification, or are you using this forum as a "short cut how to guide" and will read the code thoroughly some day when you have nothing else to do?

 

[TGS4]

At least we fixed the temperature correction in the 2010 Edition - it was wrong in 2007, 2008, and 2009. However, changes have been made since the 2010 Edition - I would recommend using the 2015 Edition if you are not required to use an earlier Edition because you are fabricating a Division 2 vessel mandated contractually to be to a specific Edition.

MrPDes - please be very specific with your terminology. Sa is the stress amplitude. It is NOT the stress range, but is calculated from the stress range. However, you are absolutely correct in your reply, otherwise.

Sa is a stress amplitude at a point, and can be modified to be more than the value directly from a pipe stress analysis program or an FEA.

 

[fabricd]

Thank you for your answers.

My questions may look strange i am sorry about it. It is not that simple to understand a technical doc in details in english but I admit I have not read the code thoroughly because i don't have time. But i'll read it someday.