Weld fatigue criteria wasn’t developed through the same testing as normal material criteria, like various steels. The codes developed the criteria not by treating welds as only weld metal, but rather as a complete welded structure. Such a structure has a number of description properties: it contains random imperfections (flaws), it contains complex residual stresses, and it contains an unknown precise local geometry. Therefore, the weld criteria was developed for distinct overall geometries (butt weld, T weld, etc.), rather than for a simple metal.
The geometry aspect is often misunderstood, and thus deserves further discussion. The code starts with an as-welded configuration, be it butt weld, T weld, or other. Thus, it recognizes that, a priori, in the design stage, one cannot guarantee a local weld geometry, when fabricating a large structure. When welding un-machined parts, which will not be subsequently machined, on ALL surfaces, the code recognizes that even automatic weld processes will not deposit the same exact geometry over large runs. Each weld process, itself, may also provide different geometry to the final cover passes. To limit this variation somewhat, the code expects that good welding practices are included, such as typical weld inspection visual criteria. Implied also (from drawings and test data) is that all fillet welds, or reinforcements, are equal leg (45 degree). A further variation limit is imposed by expecting the welds to have been 100% ultrasonically inspected, and cleared to a reasonable acceptance criteria for flaws. That this is so, is evidenced by the code concentration on weld toe cracks (and expectation that weld throat will be sufficient in fillet welds).
To create a graphical fatigue criteria for such a range of weld structures, that criteria must be based on a uniform testing procedure, and on a defined, measured, stress. Such a stress could be the “far field”, or “nominal” stress. Indeed this P/A + Mc/I stress is the basic stress considered, for codes avoiding the complex geometries of offshore platform weld joints, and the pictorial definitions of joints, in those codes, usually present a loading arrow to show the far field load considered (in the testing). The weld fatigue criteria obtained from that testing, is then graphically presented, per overall weld joint configuration (butt weld, T weld, etc.). This is shown BS 7608 [1], and as the first case in IIW [2].
As weld joints become more complicated/diverse, and the structural analysis by finite element method (FEA) is used, a slightly more complicated stress, called the “hot spot stress”, may come to be used. This is the stress at the weld toe, found by extrapolation from the far field results. In BS 7608, this is the basis for the “T” class, and IIW contains a second set of graphical criteria for use with this stress. Note that the changes between the first and second cases in IIW (nominal stress versus hot spot stress) is nothing more than a recalibration of the stress plots against a differently calculated stress variable. The second case also admits a more complex loading pattern than the original experimental data used to construct the nominal stress plots. Nothing else has changed in the weld joint configuration.
Often mining specifications require fully ground welds, and here is where another confusion arises. The weld codes are predominantly concerned with weld toe discontinuities and cracking. Therefore, the weld grinding considered by the code is TOE grinding. It can be seen that this is true, since weld grinding is given the SAME stress allowable advantage (factor = 1.3), as any other weld toe treatment procedure, such as TIG dressing or hammer peening, for example, which certainly do not produce the same geometry as toe grinding. Therefore, a “ground weld”, according to code, can still have rough, as-welded, surface elsewhere (exception = butt welds), and does not remove all geometric stress concentration effects.
Because of the above, codes do NOT accept the FEA calculated peak stresses, on fully ground welds, to be used with the graphical fatigue curves such as BS 7608, or IIW first two cases. Peak stresses, correctly calculated, can only be used in IIW case three, the effective notch stress. Note that for this case there is only ONE stress allowable plot, which makes sense, since the correct calculation/use of peak stress eliminates the need for all varied overall joint geometries. These geometries are now already fully incorporated within the FEA calculation of the peak stress magnitude.
The above is intended to illustrate the difference between how weld fatigue criteria is obtained, from that of ordinary steels. It shows that one cannot change the weld stress calculations, without correspondingly adjusting the weld criteria graphs, as per IIW cases one, two, and three. The use of the criteria graphs must be consistent with the stress calculation. It should be noted that similar logic is employed in setting the current, relatively low, stress allowables for large grinding mill castings.
[1] BS 7608, Code for Practice for Fatigue Design and Assessment of Steel Structures, including amendment 1, Feb. 1995.
[2] IIW – 1823 – 07, Recommendations for Fatigue Design of Welded Joints and Components, Dec. 2008.