Fatigue Performance Study on Q690 High Strength Steels and Their Welded Sections

High strength steel with nominal yield strength, f_y, no smaller than 460 MPa has been progressively applied in buildings and bridges due to its good material properties. Compared with normal strength steel, high strength steels have higher yield strength which may reduce the size and dimension of members. In addition, the thinner steel plates may help avoid welding problems associated with thick steel plates as reducing the amount of welding work and improving the welding quality. Fatigue damage is one of the most critical service problems of steel bridge structures. There is no special design standard for fatigue performance of high strength steels in the current design code. This paper presents an experimental investigation on fatigue performance of Q690 high strength steels and their welded sections to determine the fatigue strength, so as to promoto the application of Q690 high strength steel in practical engineering. The monotonic tensile tests were carried out on Q690 high strength steels and their welded sections to examine the material properties. The welded section was etched before test to distinguish the position of base metal, heat affected zone and welded metal during testing, and determined the position of necking was heat affected zone. The various applied loadings of fatigue tests were determined according to the mechanical properties examined in the tensile test, and a series of high-cycle-low-strain cyclic tests on smooth cylindrical coupons had been conducted to determine the S-N curves for Q690 high strength steels and their welded sections. The fatigue limit of Q690 welded sections is 420 MPa, and that of base plate is 668. 8 MPa. It means that the fatigue performance of Q690 welded sections is worse than that of Q690 base plate, while the fatigue resistance of both the Q690 base plate and their welded sections are significantly larger than those given in Eurocode (base plate: 118 MPa, welded section: 82.5 MPa). The hardness test was conducted on the Q690 welded section to determine its hardness distribution along the longitudinal direction. It is found that the weld metal exhibits the larger hardness value as 328 HV, while the softening was identified in the heat affected zone with a hardness of 240 HV, which indicated that the hardness distribution of Q690 welded section is uneven and the difference is significant. Based on the results of etching tests and SEM observations on the fracture surfaces under fatigue loadings, it is found that the crack initiated at the fusion zone rather than the heat affected zone. What’s more, non-metallic inclusions have been identified in the fracture surfaces of Q690 welded sections, it means that the initial imperfections may be a critical parameter affecting the fatigue performance of Q690 welded sections.