Elastic–plastic crack driving force for tubular K-joints with mismatched welds

This study examines the elastic–plastic driving force in shallow surface cracks located in welds near the crown point of the tension brace toe in a circular hollow section K-joint — with strength mismatch between the chord material and welds. The remote loading at the brace end imposes displacements acting along the brace axis. The 3-D finite element models couple a global, topologically continuous mesh and a separate, local crack-front model through mesh-tieing. The numerical solver computes the elastic–plastic crack driving force (JJ-value) locally along the crack front through a domain-integral approach. The numerical analyses employ stress–strain curves for representative high-strength steels now used in offshore construction. The yield strength of the welds varies as σyw=mσycσyw=mσyc, where mm denotes the mismatch ratio and σycσyc is the chord yield stress. The strain hardening property of the welds remains the same as that of the chord material. Unlike historical research on weld mismatch effects for simple, through-crack fracture specimens, the surface crack considered here in the tubular K-joint resides in the base metal (chord) adjacent to the weld toe of the hot-spot location rather than in the welds. The computed JJ-values demonstrate that the crack driving force increases with increased weld strength — thus a higher potential for initiation of ductile tearing. The numerical results show that a relatively larger elastic–plastic crack driving force exists for joints with a high brace to chord outer diameter ratio (ββ) or with a large brace to chord intersection angle (θθ). For joints with m≥0.8m≥0.8, the welds are sufficiently strong to mobilize significant plastic deformation in the adjacent chord material near the crack surface, and thus prevent large-scale yielding in the welds.