Fatigue Analysis Screening, Method B

[salzrocks]

thread794-423510
Hi All,

I have been stuck at understanding the Fatigue Screening Criteria - method B [5.5.2.4] step # 3. I.e to calculate N(C1 S).[Eq. 5.18]
Where do we get N ? Does this N need to be multiplied by C1 (as obtained in Step #2) and than multiply it by S (Allowable Stress) ?
OR
I just need to multiply C1 with S ?

 

[TGS4]

You are calculating N using a stress of C1*S via the S-N curve.

 

[salzrocks]

Thanks @TGS4 a bunch. Now I am stuck at Step # 4: While calculating, P/C1 x Sa(deltaNp)/S, I have two problems:

1- Units. P/S cancels pressure units but Sa(deltaNp) again gives pressure units hence the final value has some pressure units instead of unit less number.(which is to be compared with deltaPn)
Sa(deltaNp) I suppose is the Stress amplitude calculated via using deltaNp in Annex 3-F from S-N curve using figures or ci coefficients(ASME 2015 or lesser versions).

2- What is the difference in between Full design Pressures cycles including startups and shutdowns and Full operating pressure cycles excluding startup and shutdowns ?. A component goes into full pressure design cycle when it starts from 0 and goes to let's 2 MPa of design pressure. While mostly all equipments go to Full operating pressure cycles and rarely goes into full design pressures.
Plz explain to me with an example as I couldn't understand terminologies used.

 

[TGS4]

1. DeltaPn is in units of pressure. Your supposition, though, is correct.

2. You just need to plot out your cycles. Sometimes an operating pressure cycle may not go to zero, whereas it will when there is a shutdown.