ASME VIII-2 Fatigue Assessment

[cbPVme]

I am trying to understand the fatigue assessment in ASME VIII-2. I am reading the assessment procedure in 5.5.3.2 along with example E5.5.3 of ASME PTB-3-2013.
Step 3(b) says to calculate the stress tensor at the beginning and end of the cycle and then calculate the range of the primary plus secondary plus peak equivalent stress (ΔSP,k) which is the equivalent von Mises stress. This value is then multiplied by Kf in equation 5.36. I take this to mean that the von Mises stress is to be multiplied by the applicable Kf for each point in the model.
In PTB-3 the stress is linearized wherever a Kf not equal to 1 is applied. My questions are as follows:

1. Why does PTB-3 use linearized stresses? It uses ΔSn ,k when applying Kf rather than ΔSP,k. This doesn’t seem to match the method in 5.5.3.2. Am I misreading this?
2. How are the total stresses in Table E5.5.3-3 of PTB-3 computed? In the 2010 version the value of ΔSP,k is always equal to the linearized values in Table E5.5.3-1. In the 2013 version these values are revised for the cases where Kf does not equal 1.0. For instance the value of ΔSP,k at the transition is 31.799 ksi, however the maximum von Mises stress at this location appears to be 33.377 ksi per Figure E5.5.3-2.

 

[TGS4]

Sometimes the rules, as written, don't quite match up with what the rules ought to be. The FSRF was intended to be multiplied by a nominal stress - ostensibly the membrane-plus-bending equivalent stress. This what is written in PTB-1. However, we are still working that change through the Code Committees, and the change hasn't m made it through yet. I expect that change to be finalized for the 2017 Edition.

For the time being, follow the rules as written.

 

[jte]

For the time being, follow the rules as written.

I hate it when people say that! Reminds me of the time my wife got a speeding ticket while crossing a local bridge. The next week they raised the limit from 55 mph to 65 mph and she would have been ok. I had to (gently) remind her that she did violate the law as it existed the day she got written up - and she would likely not prevail before a judge. She paid the fine.