Stress-intensity factors for circular hollow section V-joints with a rack-plate chord

Abstract

This study investigates the fatigue crack-driving force, measured by the linear-elastic stress-intensity factors (SIFs), for a surface crack at the root of the welds in a thick-walled, circular hollow section (CHS) V-shape joint, typically installed in modern offshore jack-up platforms. The primary (chord) member of the V-joint consists of two half CHSs welded to both sides of a thick rack plate, while the secondary (brace) member adopts thick-walled CHSs. The surface-breaking crack considered in this study locates at the interface between the rack plate and the weld metal joining the half CHS, and represents an initial flaw introduced by lack of penetration in the welding procedure. The finite-element model incorporates a very detailed, local crack-front mesh in a global continuous mesh through a mesh-tying procedure, which ensures displacement continuity between the independent master surface and the dependent slave surface. A simple plate model verifies the mesh-tying procedure in computing the linear-elastic stress-intensity factors for two remote loading conditions. The computation of the stress-intensity factors employs a linear-elastic interaction integral approach. The comparison of the computed SIF values with a previous experimental measurement for a CHS T-joint verifies the accuracy and feasibility of the interaction integral approach in computing SIF values for surface cracks in welded tubular connections. Subsequent numerical analysis on the gapped V-joints examines the mixed-mode SIF values for different loading conditions and includes an array of practical joint geometric parameters and crack sizes. The nondimensional mode I stress-intensity factors KKI/σbr√πa generally increases with the following variations in the joint geometric parameters: an increase in the chord radius to the wall thickness ratio (γ= d0/2t0), an increase in the brace diameter to the chord diameter ratio (β = d1/d0), a decrease in the crack depth ratio (a/t) or an increase in the crack length c. The current study identifies a practical group of V-joints that requires detailed treatment in the fatigue assessment procedure. These V-joints adopt a large β ratio and demonstrate high mode-mixity angles [ψ = tan-1 (KII/KI)] with correspondingly high mode I and mode II stress-intensity factors. © 2009 Blackwell Publishing.