This paper presents numerical studies on the static strength of the HSS (high strength steel) thick-walled CHS X-joints with near-toe surface cracks under the brace axial tensile load. The analyses cover a comprehensive array of joint geometric parameters and crack configurations. The 3-D FE models couple a global, topologically continuous mesh and a separate, local crack-front model through mesh-tieing. The plastic collapse loads as well as the crack driving forces (measured by the J-values) have been computed to assess the two competing failure mechanisms: the plastic collapse failure and the fracture failure. The design equation proves to be conservative especially for the high β joints. The effects of fracture failure on the cracked joint strength are also investigated.
Thick-walled circular hollow section (CHS) tubular joints have been widely used in both onshore (Schumacher et al., 2001) and offshore structures (e.g., jackets and jack-ups) due to their unique advantages. For instance, the thick-walled joint has an improved performance in fatigue due to the decrease of the stress concentration factor. Besides, its reduced area can help to decrease the wave load in offshore platforms. In such applications, the joint's outer diameter of the main member (chord) over wall thickness ratio, 2γ, is normally less than 20. However, joints with such low γ values often invalidate the applicability of current strength equations in most recommended codes (e.g., ISO 19902, 2007) and have not been well studied. In reality, the presence of fatigue cracks or otherwise induced defects in such joints causes potential threats to their safety under an overloading event. The ultimate strengths for the cracked tubular joints due to the plastic collapse failure have been extensively investigated in the past decades (Cheaitani and Burdekin, 1994; Stacey et al., 1996; Burdekin and Yang, 1997; Hadley et al., 1998; Burdekin, 2001).
