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,