Axial Loading Fatigue Performance of a Q690D High-Strength Steel CHS-CHS T-Joint

This study focused on the fatigue performance of a Q690D high-strength steel T-joint with circular hollow section (CHS) brace and chord, under the loading condition of axial tensile cyclic loads at the brace. A total of eight CHS-CHS T-joint fatigue tests were conducted. The fatigue crack initiation and propagation of the CHS T-joint were analyzed, and the fatigue failure modes were clarified. The strain and stiffness development curves were obtained, based on which of the failure warning criteria were determined. The stress concentration factor (SCF) was determined according to the strain measurement results and finite element analysis. The hot-spot stress fatigue design curve of the Q690D CHS-CHS T-joint was fitted and compared with the existing design curves provided in the standard of Committee for International Development and Education on Construction of Tubular structures (CIDECT), Norway’s Det Norske Veritas (DNV), American Petroleum Institute (API), and American Welding Society (AWS). Finally, a fatigue damage evolution model was established, which enables the quick evaluation of T-joints in tubular structures. The results show that the maximum SCF occurred at the saddle point of the T-joint under axial loads. Fatigue cracks were first initiated at the weld toe of the saddle point on chords, then propagated along the weld to the crown point, and finally, fatigue failure occurred. The fatigue crack initiation life accounts for about 65%–90% of the total fatigue life. Compared with normal-strength steel T-joints, the tested high-strength steel T-joints exhibit lower fatigue resistance, as the advantages of higher strength are not reflected in the aspect of fatigue performance. The fatigue tests conducted in this study can enrich the fatigue data pool of CHS-CHS T-joints and further extend the existing database to the range of high-strength steels.