Fatigue in compression elements with residual tensile stresses

[bugbus]

Please excuse my lack of expertise in the area of fatigue, but this is a question that I have been thinking about recently.

I am dealing with a simply-supported hot-rolled I-girder that has welded attachments at the top (compression) flange only. The bottom flange is as-rolled and therefore has a much better detail category.

Is it possible that the overall fatigue performance will be governed by the top flange?

Even though the top flange only sees compression forces due to the applied load cycles, I'm aware that there would be a residual tensile stress in the order of 0.3xfy near the middle of the top flange. Therefore under cyclic loading it's possible that this part of the girder is subject to stress cycles that are always in tension.

If a fatigue crack were to form in the top flange, is this a concern? I imagine that as the crack forms, the residual stresses from the rolling process would be relieved to an extent and may help to arrest the problem? Also, it is hard to imagine how a crack in the top flange could lead to the collapse of the girder.

I would be interested to hear a few opinions from some of the more experienced people here.

Thanks!

 

[BJI]

You are correct regarding the fatigue risk in a compressive stress field based on residual stresses. However, there isn't just the hot rolling residual stresses, from the welded attachments you could have localised residual stresses around yield, assuming the beam hasn't had any subsequent heat treatment. Even still, research has shown that heat treatment won't really produce any improvement in fatigue life even for compressive stress fields.

The compression flange is the critical flange for buckling, so it is hard to say if it matters without considering the full member support arrangement. The residual stress profile and the extent to which the residual stresses will reduce cannot really be relied upon without much more detailed analysis. It would likely only be safe to assume cracking would arrest if you could show that your cyclic primary stresses also dissipated with crack propagation. If there are no redundant load paths then you would need a high level of confidence that cracking could be identified and repaired prior to significant loss of section or member capacity.