Fatigue evaluation with equivalent STRAIN method 2

[M.hmk]

Do you think fatigue evaluation with use of equivalent STRAIN method ( such as those mentioned in Para. 5.5.4 of ASME Sec.8-Div2-part 5 or para. T-1420 of ASME III-NH appendix T) could be a solution when there are many problems to obtain the exact value of STRESS in singular points?

there are no exact stresses at a singularity. But how about STRAINS near and at singularities in an elastic plastic analysis?

Thanks in advance.

 

[mskds545]

@TomLee777 The page you linked refers to the local notch stress method in DNV-RP-C203 Recommended Practice for Fatigue Design of Offshore Steel Structures. That code doesn't address things other than fatigue, like crack propagation or local failure, so I assume you are suggesting the 1 mm radius only for fatigue analysis. DNV-RP-C203 uses a correlation between the local notch stress with a 1 mm radius and a specific SN curve that is provided in that code. It is not the same as the ASME SN curve. It seems difficult to justify using the local notch stress analysis approach in DNV-RP-C203 with the ASME elastic-plastic fatigue method without looking into their test data and correlation. Did their test database include thicknesses, geometries, stress states, weld procedures and QC common to pressure vessel construction? Which SN curve would you use with this hybrid approach - ASME or DNV?

A structural stress method from a pressure vessel code seems preferable to me (and also allows a much coarser mesh than local notch stress, or the use of shell elements). I still recommend this approach unless there is some reason it can't be used.

 

[TomLee777]

@mskds545 Yes, you are right, DNV-RP-C203 1 mm radius is only verified in local notch stress method and the associated specific fatigue curve.

I agree that a structural stress method, or hot spot method, maybe preferable. ASME code currently does not provide this method. It is popular in European countries.

 

[TGS4]

TomLee777 - you mean like the Structural Stress Method provided in ASME Section VIII, Division 2, Part 5, Paragraph 5.5.5?

 

[M.hmk]

@TGS4
Would you please tell me your opinion about my post @ 31 Dec 19 12:28 ?
I read the answer of mskds545 @ post 4 Jan 20 22:55,
However, your answer is also very important.

Thank you.

 

[TGS4]

The Code doesn't specifically address this issue - it is one of engineering judgement. It is my experience that the yield "blunting" tends to keep the strains from becoming "infinite".