Hello Fellow Stressers!!

How to set in CII to use Von Mises stress in fatigue evaluation.

Thanks & Warm Regards,

Hi Fellow Stressers!!

I already got it. I have a system where even if I toggle from Max 3D Shear and Von Mises the calculated stress is the same.

I think there is only 1 principal stress S1 in my system with S2=S3=0. For this case Equivalent Von Mises and Stress Intensity will just be equal which is S1 itself.

Warm Regards,

Borzki,

Be careful that if fatigue assessment is performed as per ASME VIII-2 or BS PD5500, ASME B31.3 stresses are not suitable to be employed directly in fatigue assessment.

B31.3 SIFs represent actually 50% from overall Pl+Pb+Q+F peak stress concentration factors. See ASME BPVC III-1/NB considerations regarding Stress Concentration Factors and SIFs inter-dependence.

As a very-rough approximation, B31.3 Von Mises or Tresca stress range (as applicable) should be DOUBLED to be used for ASME VIII-2 fatigue calculation purpose.

Best regards,

Thanks Dorin for that information.

If I'm not mistaken this is also related to smooth bar specimen vs. the buttwelded ASME B31.3 basis for Markl Fatigue Tests for piping and piping components.

Also the difference in Code stress vs. Max Stress Intensity same as sample summary below.

"Highest Stresses: (lb./sq.in.) LOADCASE 14 (EXP) L14=L2-L12
Ratio (%): 225.4 @Node 3608
Code Stress: 67312.5 Allowable Stress: 29862.5
Axial Stress: 1313.8 @Node 3608
Bending Stress: 65990.6 @Node 3608
Torsion Stress: 8109.8 @Node 3619
Hoop Stress: 0.0 @Node 2672
Max Stress Intensity: 80147.3 @Node 3608"

Actually, one of the client from my previous years ask why the Code stress is different than the Max Stress Intensity.

The Code stress as I know is derived from the principal stress out of the longitudinal and shear stress (e.g bending and torsion due to thermal expansion) , while the maximum stress intensity is derived from longitudinal, hoop, radial and shear stress (radial and hoop being a stress related to pressure)

This means that Code stress is pure thermal expansion loads while max stress intensity is a combination of pressure and thermal expansion loads.

And it has been a proven experience from the Code that pressure is considered as primary stress (W+P1) unless there is really a significant amount of pressure cycling in the life of piping component which the designer should be aware about if it's a requirement in the operation of a certain piping system.

But this is rare in the industry, and I myself haven't experienced to do a detailed pressure cycling fatigue calculation of a piping component.

When we use this Max Stress Intensity as a fatigue evaluation, shall we double the stress or lower the allowable by a factor of 2?

I believe yes, because of the presence of buttweld in a piping component comparing it to Smooth Bar Fatigue Curve, which approximately equal to 2 (FSRF).

But how about in a bend where the maximum stress intensity lies in extrados and not on the buttweld itself (but it would be a tedious job to model each weld and input a multiplier of 2 on the stress) so I agree with Dorin to stick to a factor of 2 to simplify the process, which is the original intent of Markl Fatigue Tests.

Your expert opinion is highly appreciated.

Warm Regards,

Borzki,

Failure theory used to evaluate the "equivalent" stress (e.g. Von Mises/Distortion Energy or Tresca/Max. Shear Stress)is less relevant we we talk about Stress Concentration Factors (SCFs) vs Stress Intensification Factors (SIFs) employment.

The essence is that:

SIF = (Local_SCF) x (Secondary_SCF) / 2, or

i = K x C / 2,

as per ASME BPVC III-1/NB and NC.

Local SCF (K) is relevant for welding joints, indeed, meaning butt welds or and/or fillet welds (see the Weldolet classical case).

For seamless fittings (welding tees, elbows, reducers), it is a common approach to separate butt welds' stress concentrations from fitting body's stress concentrations.

Therefore, for these seamless fittings, Local_SCF = K is less relevant and Secondary_SCF = C quantifies the most significant concentration effect.

I suggest to you to have a look in ASME BPVC III-1/Subsection NB, Section 3600 (Piping Design), where there are given in detail the relevant C and K values for all typical fittings and welds.

In general, for seamless elbows, K = 1 (for bending and torsion), so that SIF = C/2.
Therefore, B31 SIF for elbows represent 50% from the actual overall SCF = KxC corresponding to Pl+Pb+Q+F total peak stress range. This is obvious when you'll compare B31 SIF with ASME III-1/NB or IGE TD12 SCF.

When we discuss about bending and torsion SIFs and SCFs, "intrados" and "extrados" sections are irrelevant. Commonly, the maximum stresses found from "beam theory" are superposed in absolute values, applying the SCFs to the nominal stresses calculated by straight pipe spool formulas.

Please also note that ASME Nuclear Code (III-1), or ASME BPVC VIII-3 High Pressure Code use Tresca Theory to evaluate the Stress Intensity Range for fatigue assessment, while ASME BPVC VIII-2 post-2007 uses Von Mises Theory to evaluate the Equivalent Stress Range.

PD5500 uses also Tresca theory for Design by Analysis, but PD5500 fatigue curves are based on Maximum Principal Secondary Stress (S1>S2>S3). The local stress concentrations are accounted by the "de-rated" fatigue curves associated to the typical welding joints and loading schemes.

Therefore, B31 Codes-based stresses (e.g.maximum principal, stress intensity, or Von Mises Equivalent Stress) cannot be used directly with ASME VIII-2 or PD 5500 fatigue curves. The B31 Code-based calculated stresses must be corrected/adjusted in order to make the conversion to overall maximum peak stress range (ASME VIII-2) or to maximum secondary stress range (PD 5500), in the later case being necessary to assume/chose the applicable fatigue curve class.

Please note that so far, I did not have time to review the newly-introduced fatigue analysis method provided by ASME B31.3 / Appendix W (2018, 2020 editions). It looks it is somehow similar to DNV Simplified Fatigue Mathod (based on Weibull probabilistic distribution).
It might be a real progress to use this method for B31.3 Code-based piping systems subjected to high-cycle fatigue loadings (e.g. offshore systems), because such endless discussions about SCFs, SIFs, fatigue curves applicability would bot be required any more.

Best regards,

Thanks Dorin for the detailed explanation of fatigue with respect to each code requirements.

It's really not straightforward to mix match each code to evaluate fatigue.

I have read so fast and was thinking that I have to multiply stress by 2 but in this case it's divide by 2 for elbows.

Anyway, I will try to explore ASME BPVC III-1/Subsection NB, Section 3600 (Piping Design), since it's related to piping components. I have look a quick glance on that Section 3600 and it separates the primary + secondary stress evaluation with C factors and peak stress intensity evaluation with CxK factors. Looks interesting to me, maybe try to have some result comparisons for each Code. But no time for now, a bit busy with some other things.

Thanks for your expert advice.

More power!

Hi Dorin,

Say for example below, I use the bend peak sif of 3.806 (in) and 2.411 (out) from FEA output, in the B31 stress calculation.


Peak Primary Secondary
Bend In : 3.806 3.806 7.612
Bend Out: 2.411 3.067 4.822
Bend Ax : 5.440 9.880 10.880
Bend Tor: 2.409 3.065 4.818
Pressure: 0.563 1.104 1.126

B31 CODE
Peak Stress Sif .... 3.766 Inplane
2.633 Inplane Single Flange
1.841 Inplane Double Flange
3.138 Outplane
2.194 Outplane Single Flange
1.534 Outplane Double Flange



Does this mean that I need to multiply the calculated stress by 2 to do ASME VIII fatigue evaluation?

I have noticed above that secondary sif is 2 times the peak sif which is consistent with K x C = SIF x 2 . And this peak SIF I believe is derived to be B31 based SIF to be consistent with the B31 code and not the PL+PB+Q+F in the ASME BPVC VIII and III (which is KxC). The C value for this particular example from ASME III NB for 20" STD wt elbow is 8.159 (which near 7.612 shown above). Now I'm starting to get your point (since ASME VIII and III is using same basis of fatigue curve therefore the KxC in ASME III will also apply in ASME VIII)

At low cycle fatigue, there is ASME III NB simplified elastic-plastic discontinuity analysis which limits the Se <= 3Sm (Equation 12) (this due to moments in thermal expansion). This is in case equation 10 cannot be satisfied. I believe this requirement is to prevent excessive local deformation.

There is also equation 13 which excludes moment due to thermal expansion.

Is equation 12 in ASME III also in ASME VIII.

Appreciate your expert opinion.

Warm Regards,

Hi Dorin,

Now I understand your point. To reinforce my understanding, I have run a Caesar model (a simple pipe-bend-pipe configuration) using B31 peak SIF from FEA and apply a moment load at the end. I got a max. Von mises stress of 36652.6 psi in the bend.

Similarly, I have made a same model in FEA (BC's and loads are totally the same) and got a Von Mises stress of 59280 psi on the bend.

You're right, that we cannot directly use B31 calculated stress to be used in the ASME VIII fatigue evaluation. In this case the ratio is 1.6.

So a factor of 2 is good enough instead of running FEA on each component.

Warm Regards,