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

Abstract

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 ( J -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 σ y w = m σ y c , where m denotes the mismatch ratio and σ y c 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 J -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 ( β ) or with a large brace to chord intersection angle ( θ ). For joints with m ≥ 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.