Experimental investigation on fatigue performance of three typical welded joints of Q420qFNH weathering steel

Abstract

The novel weathering steel Q420qFNH has significant application potential in modern bridge engineering. This material could meet the urgent demand for high-performance structural materials in contemporary engineering and advance the field of steel material science. In this study, fatigue tests were conducted on three types of Q420qFNH welded joints to obtain their S-N curves and evaluate their fatigue strengths. A comparative analysis was then carried out with existing fatigue codes such as Eurocode 3, IIW, BS 7608, and ANSI/AISC 360 was conducted. A total of 124 pairs of fatigue data were collected from previous studies to analyze the difference in fatigue performance between Q420qFNH welded joints and other traditional structural steels of the same grade by examining the fatigue strength and fatigue resistance index. The results showed that, for all Q420qFNH welded joints, fatigue failure was characterized by multiple crack initiations at the weld toe and their subsequent coalescence and growth along the cross-section near the welding toe. Under the current welding process, the butt-welded (BW) and non-load-carrying cruciform-welded (NCW) joints show that their fatigue strength meet various standards, ensuring safety and reliability. However, load-carrying cruciform-welded (LCW) joints still have room for improving the safety margin. The fatigue strength (P_s=95 %) of BW joints was measured at 88.8 MPa, aligning with the range of 122.7–86.7 MPa seen in other steels of the same grade under as-weld conditions. The NCW joints exhibited a fatigue strength of 103.7 MPa, slightly below the typical range of 112.1–163.9 MPa for ordinary steels under similar conditions. Notably, the LCW joints, featuring near full-penetration welding, demonstrated superior fatigue strength of 76.8 MPa, surpassing most ordinary steels.