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Fatigue on High Tensile Welded Connections


Optimization of design is very important. With this in mind it can be very tempting to simply choose higher grades of steel in order to slim down members and save on weight. While your boss may be happy you have come up with a design at half the weight, you could well be hearing from your customer in a few years after fatigue has set in!

Background

Surprisingly, for a welded connection the fatigue stress allowables are basically identical whether you have specified standard G250 steel or something as strong as high tensile BIS80 (690MPa yield). So if you’re using high tensile steel to reduce the size of your beams, be aware that any fatigue stresses in the welds are likely rising! Residual stresses, weld undercut and imperfections, heat effected zones etc all contribute to the predicted life of the weld. See below for a classic example:

Example

Weld_Fatigue

Full Penetration T-Joint Connection

Here we are going to have two scenarios for this basic welded connection. The first option is to use 32mm thick G250 mild steel. For scenario two the designer has decided to use BIS80 steel and halves the thickness to 16mm, with the knowledge that the yield strength of BIS80 is much higher than that of mild steel. Scenario two is going to be much lighter but assuming the dynamic force shown above is the same, how will it perform in fatigue?

Weld_Fatigue_2

Table 1, Material Comparison

Outcome

The yield strength of BIS80 is over 2.5 times that of mild steel so you might think it’s safe to assume doubling the stress to half the weight is a great idea. Unfortunately the fatigue stress range has also doubled and now the joint is going to fail within a small fraction of the time.

Conclusion

One situation where high tensile steel would be applicable is if you had very high static loads or the majority of the stress is well away from your welded connections. When designing for fatigue of welded connections, the cyclic stress range is the factor that will determine your fatigue life (assuming adequate weld quality). So if your connection was holding a very large static load, you might need the increased strength of high tensile steel, assuming your welded joint is sufficiently designed for the dynamic loads.

Fatigue design can be complex at the best of times. But without some up front thought, failures of your products in the future can be extremely costly. Here at Motovated we have done some fairly large fatigue projects and we can offer a lot of experienced advice. If this weld fatigue tip has sparked your interest feel free to give us a call to chat about your designs!

Written by Chris Tyree
Motovated Design & Analysis