Experimental and finite element analysis research on the fatigue performance of CHS K-joints

Fatigue tests on full-scale CHS K-joint models have been conducted, exploring the distribution of hot spot stress, fatigue crack development, and fatigue failure modes. A three-dimensional solid finite element model has been established to carry out a parametric analysis of the main influences on the stress concentration factors for CHS K-joints using the general software MSC.MARC. A novel stress concentration factor method based on the CHS K-joint stiffness has been proposed by combining a theoretical derivation and a regression analysis. The results reveal that the hot spot stress of a CHS K-joint can be obtained with a linear extrapolation. The location of the maximum hot spot stress appears on the crown toe point of the chord and tensile brace intersection welding seam near the chord intersecting line. A fatigue crack in a CHS K-joint belongs to the opening Class I crack category. When the fatigue crack penetrates the full thickness of the chord, the fatigue crack develops rapidly; the fatigue failure of the CHS K-joint belongs to the brittle failure category in the elastic range. A coupling action caused by the geometric parameters is demonstrated to exist in the stress concentration factor of CHS K-joints. Given the current stress concentration factor method for CHS K-joints is based on a regression analysis using a single geometric parameter, this limitation impacts its application in accurately in developing bridge structures. The novel stress concentration factor method encompassing the CHS K-joint stiffness proposed in this paper considers the coupling action on the stress concentration factor caused by the joint geometric parameters, in addition to improving the conservativeness and accuracy in evaluating and calculating the stress concentration factors of CHS K-joints used in bridge structures.