B31 Code Stress

"The peak stress intensification factor comes from Markl fatigue testing and theoretical work. Markl's approach has been extended by many into the realm of finite element analyses for common types of materials and weldments.

The secondary stress intensification factor is intended to trap the highest discontinuity effect without trapping the effect of concentrations, i.e. stresses at the toes of fillet welds. In our classes we tell attendees that secondary stresses exist in an area that is a percentage of the diameter. In general secondary stresses attenuate with the distance root(RT) from the stress discontinuity. Peak stresses attenuate with distance through the wall thickness.

The peak stress intensification is intended to trap the highest peak stress in the body, while the secondary stress intensification is intended to trap the highest discontinuity stress in the body. The peak stress intensification factor is often lower than the secondary stress intensification factor however. This is primarily true for the following reason: The peak stress to be computed in the B31 piping Codes as per Markl uses a girth butt weld as the baseline peak stress, i.e. a girth butt weld has a stress intensification factor of 1.0 in the B31 Codes. In reality a girth butt weld in a carbon steel piping system has a stress intensification factor of between 1.7 and 2.0. Thus Markl type peak stress intensification factors are reduced by about 0.5 from what their actual values would be.

All of the allowables are adjusted for this and so there is no problem, the only result is confusion.

For the majority of cases with FE/Pipe the peak stress intensification factor will be seen to be smaller than the secondary stress intensification factor. It is because it is only accounting for half the stress that this occurs.

The primary stress intensification factor is even smaller than the peak stress intensification factor because its value is based not on the peak stresses, or the highest discontinuity stresses, but rather on the highest membrane stress at the discontinuity. And so the primary stress intensification factor will be smaller than both the peak and secondary stress intensification factors by the ratio of the peak to membrane stresses at the point of interest."

Conclusion:

Secondary Stress Intensification Factor does not have any relation with the Brittle Failure of material.

Secondary Stress Intensification (or Concentration!) Factor is related to ASME VIII-2 Code Secondary Stress PM+Pl+Q identified for a gross structural discontinuity and it represents the ratio between the maximum Secondary (Pm+Pl+Q) Stress Intensity (Coulomb-Tresca Theory) or Von Mises Equivalent Stress Value, on the one hand, and the NOMINAL Stress value in the absence of that discontinuity, on the other hand.

Note: Von Mises Equivalent Stress is currently used by ASME VIII-2 post-2007 Editions. ASME B31.3 Code is still using Stress Intensity concept based upon Tresca Theory.

For pipe stress engineer immediate interest, the Peak Stress Intensification Factor (SIF) represents the ratio between the the actual peak stress in the component (e.g.fitting for instance) and the nominal stress in the same sized straight pipe, both stresses developed by the same external load - e.g. bending moment, torsion moment, axial force.

As emphasized in the quoted text above, ASME B31.3 SIFs are HALF (50%) from the corresponding theoretical ASME VIII-2 Peak Stress (Pm+Pl+Q+F) Concentration Factors, because ASME B31.3 SIFs incorporates the "intrinsic" stress intensification factor of the girth butt weld, which was postulated to be 2.00.

As pointed out by PRG Documentation and PRG Software Help explanations, for the Weld Local Stress Intensification Factor's typical assumed value K = 1.35, the correlation between ASME B31.3 (Peak) SIF (i) and Secondary Stress Intensification/Concentration Factor (C) is: i = (1.35/2) x C.

So, the SIFs values that should be introduced in Caesar II models are the "Peak SIFs" values as given by FEA Reports - one value for each individual load: in-plane bending, out-plane bending, axial load, torsion moment.

The ASME B31.3 Sustained Stress Indexes (SSIs) are NOT equal to Primary Stress Intensification Factors. The ASME B31.3 SSIs are calculated based upon limit load analyses.