ASME Fatigue Screening Method B - Used Allowable Stress 1

[Alizera]

Hello All,

Do you have any idea when using ASME BPVC fatigue screening method "B" for austenitic SS materials with two sets of allowable stresses (as described in Sec II App.1, 1-100(a)), which allowable stress (higher or lower one) should be employed in screening method formulas (5.18 & 5.19)?
Using a lower allowable stress could results in a higher limit for significant pressure fluctuation cycles.

 

[DriveMeNuts]

Use the S value that is used for the Plastic Collapse design.

 

[Alizera]

Hi DriveMeNuts,

Thanks for your reply. This is a valid point when the plastic collapse design has been done using fully elastic methods which are based on an allowable stress value. How about the cases for which the plastic collapse has been verified employing the elastic-plastic approach? The Elastic-Plastic method in ASME for checking the plastic collapse (ASME BPVC Div.2 Part.5) use the factored loads instead of an allowable stress.

 

[TGS4]

There is no guidance provided by the Code. Refer to the Foreword:

This Code contains mandatory requirements, specific prohibitions, and nonmandatory guidance for construction activities and inservice inspection and testing activities. The Code does not address all aspects of these activities and those aspects that are not specifically addressed should not be considered prohibited. The Code is not a handbook and cannot replace education, experience, and the use of engineering judgment. The phrase engineering judgment refers to technical judgments made by knowledgeable engineers experienced in the application of the Code. Engineering judgments must be consistent with Code philosophy, and such judgments must never be used to overrule mandatory requirements or specific prohibitions of the Code.

 

[Alizera]

Thanks TGS4, it makes sense.

 

[DriveMeNuts]

I expect that the screening method is so conservative that either allowable stress will do.
However, justifying that in the report so that it complies with the Forward is perhaps the tricky part.
Perhaps you could back calculate the result of the Elastic Plastic analysis to find an equivalent allowable elastic stress, S. If this stress exceeds the higher Section II option, then use that higher Section II value in your fatigue screening calc.

 

[Alizera]

Thanks DriveMeNuts,

Calculating an equivalent allowable stress seems to be a good approach.