Within this study, HFMI treatment and, with a limited scope, TIG dressing were considered as PWTs for fillet-welded joints, showing the efficiency of the 4R method for the fatigue strength assessment of welded joints made of UHSS. To further validate the 4R method for TIG-dressed joints, additional fatigue tests and detailed research on the residual stress state after treatment for different steel grades should be carried out. In addition, the incorporation of other post-weld improvement techniques, such as burr grinding [33] and laser dressing [44], under the 4R methodology needs further verification. In the 4R method, the residual stress relaxation can be considered; see Fig. 4. Nevertheless, in the studied joints, stabilized residual stresses were not measured, and the effects of it on the use of the 4R method should be paid attention in future works.
25
This study showed a decrease in the improvement level of the HFMI treatment when the R ratio increases. Experimental tests were conducted at the intermediate regime, i.e. near 100 000 cycles. It should be noted that the HFMI treatment shows a particularly beneficial effect at the high cycle regime, hence additional experimental tests should be carried out to verify the 4R method at this regime. The fatigue testing of HFMI-treated welded joints at the high cycle regime, however, may result in failures outside the joint area, particularly at low stress ratios, as also observed in this study. To diminish the affecting parameters causing scatter in the fatigue test results, small-scale specimens are typically preferred to study a certain phenomenon. As with many other fatigue design concepts, the 4R method also requires further validation by testing with large-scale specimens and structures.
5 CONCLUSIONS
Within this study, the experimental fatigue test data for fillet-welded joints with transverse attachments made of UHSS was extracted from literature. The tests were complemented with the fatigue tests conducted for the NLCX-joints made of the S1100 UHSS. All tests were carried out using CAL with the applied stress ratio of R = 0.1–0.5. The fatigue strengths were obtained using conventional stress-based approaches; the nominal stress, structural HS stress and ENS method. The 4R method was employed to investigate the mean stress correction by means of the local stress ratio acting at the notch root. On the basis of the experimental work and subsequent fatigue analysis, the following conclusions can be drawn:
• The fatigue test results with low stress ratios, i.e. R = 0.1, exceeded the characteristic curves (Ps
= 97.7%) of the design codes and guidelines [9–12].
• The design curves [9–12] were unconservative for the joints in the AW condition and tested at high stress ratio, i.e. R = 0.5. For the HFMI-treated joints tested at high stress ratio, higher fatigue strength was found than suggested by the IIW Recommendations [15].
26
• The conventional fatigue strength assessment methods, such as the nominal stress, HS and ENS concepts, recall specific S-N curves for different PWTs, and careful consideration of applied stress ratio is needed to reach a reasonable accuracy in the fatigue strength assessment.
• The 4R method enables fatigue strength assessment of transverse fillet-welded UHSS joints with good accuracy regardless of joint condition or applied stress ratio using a single S-N curve with the SWT mean stress correction.
ACKNOWLEDGEMENTS
The authors wish to thank Business Finland and SSAB Europe for the funding through the Digi-TuoTe project that enabled the experimental program and this study to be completed.
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33 APPENDICES
Appendix A. Fatigue test data of S1100 NLCX specimens tested in this study.
Specimen ID
* Corresponding to the failure locations. In some specimens, the failure occurred at the opposite side of specimen to the side of maximum HS stress. The HS stresses corrected to correspond the stress value at the failure location, as shown in [53]. Consequently, Δσhs < Δσnom.
34
Appendix B. Fatigue test data of S960 and S1100 specimens tested in [17,30].
Specimen ID