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Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

(OP)
hi all,
i am reading ASME VIII-2 Annex 3-F,
3-F.1.2 reads:
(a) Carbon, Low Alloy, Series 4XX, High Alloy, and High Tensile Strength Steels for temperatures not exceeding 371°C(700°F). The fatigue curve values may be interpolated for intermediate values of the ultimate tensile strength.

and encounter some issues as follows:
1. it mentions "interpolated for intermediate values of the ultimate tensile strength". what does that mean?
is it referring to a material with UTS larger than 552 MPa (80 ksi), but less than 793 MPa (115 ksi)?
2. what if the steel subject to fatigue analysis have a temperature larger than 371°C(700°F), but still less than the temperature where creep governs?
3. both "3-F.1 SMOOTH BAR DESIGN FATIGUE CURVES" and "3-F.2 WELDED JOINT DESIGN FATIGUE CURVES" refer to fatigue curves in 3-F.5. so i am confused which kind of fatigue curves in 3-F.5 is.
4. the final one, that may i use the curve of 3-F for a steel not covered by ASME VIII-2, for example, AISI 4140?

sorry for so many fundamental questions.

thanks in advance.

RE: Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

1. Yes, that is what it means.
2. Above that temperature, you are into creep-fatigue. Creep-fatigue starts at a temperature lower than when creep governs the allowable stress.
3. 3-F.5 are the figures / graphs for the formulae in the respective paragraphs.
4. That is neither permitted nor prohibited. That would be a judgement call upon you as the engineer.

RE: Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

Jay Dv,
You are perhaps mixing Fatigue and Creep.
Fatigue failures may happen below material yield strength.
Creep effects/failures are independent of fatigue but fatigue and creep can go together.
Materials have their own creep regime and are usually classified in material groups like carbon steel, low alloy steels etc.
The difference between the fatigue curves are that “Smooth curves” can’t be used for welds whereas the “ Welded joint fatigue curves” can be used for both base metal and welds.

GDD
Canada

RE: Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

(OP)
thanks TSG4 and GDD.

GDD, just a minor disagreement,
"“ Welded joint fatigue curves” can be used for both base metal and welds."

i think welded joint fatigue curve can be used only for welds, i have seen such statements by Chris Hinnant.
is there any misunderstanding?

Regards

JIE

RE: Regarding the Fatigue S-N curve in ASME VIII-2 Annex 3-F

Hi,

ASME VIII-2, 3-F.2: "Subject to the limitation of 5.5.5, the welded joint design curves in 3-F.5 can be used to evaluate welded joints".

ASME VIII-2, 5.5.5: "...Weld joints with controlled smooth profiles may evaluating using 5.5.3 or 5.5.4".

For fatigue assessment of weld joints using 5.5.3, a fatigue strength reduction factor ,Kf, shall be included when calculating the effective alternating equivalent stress.

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