Improved Bending Correction Factor for Fatigue Assessment of Welded Joints
By Xing Sun
It has long been recognised that the fatigue performance of welded joints subjected to bending stress is significantly better than those tested under a pure membrane stress. Since published design curves are based almost exclusively on test data obtained under membrane stress, they are conservative with respect to joints in bending. In preparation for the revision of BS 7608:1993, a correction procedure to allow for this was developed using fracture mechanics methods following BS 7910 (2015). Subsequent consideration of published fatigue test data, however, indicated that the correction procedure over-estimated the benefit. Consequently, a new correction, based directly on the available fatigue test data, was produced and included in the revision of BS 7608 (2015).
One possible conclusion to draw from the above is that the current fracture mechanics solutions for bending underestimate the stress intensity factor and are therefore non-conservative. This has wide implications for both industry and BS 7910. An alternative conclusion is that the experimental data are the problem, since the specimens tested axially were not identical to those tested in bending. In view of the importance of this issue in relation to both fatigue design using BS 7608 and fatigue crack growth assessment using BS 7910, it was proposed to investigate it further using both fatigue testing and fracture mechanics analysis.
- The original bending correction factor kb-revision derived in the revision work was non‑conservative and was not validated by the experimental results.
- A new bending correction factor kb-new including the plate geometry effect (W/t) was derived based on fracture mechanics analysis using the 3D Mk solution in BS 7910 (2015).
- The new bending correction factor kb-new was validated by the fatigue experimental results in this work.
- The thickness and bending correction factor ktb in BS 7608 (2015) was verified by the fatigue testing results.
- The experimental results confirmed that the stress intensity factor solutions in BS 7910 (2015) are conservative.
Typical fracture surface with multiple fatigue crack initiation sites
Comparison of kb-revision and kb-new with the bending correction factor derived from the experimental results