Large-scale fatigue tests have shown that the crack propagation through the chord wall in square hollow section T-joints is very different from those observed in circular hollow section T-joints, especially the fatigue crack surface under the weld toe. This paper details an experimental investigation on the crack surface under the weld toe in four (4) large-scale square hollow section T-joint specimens. The test results revealed that the crack has a twisted crack surface. The crack not only curved under the weld toe but also changed significantly with the crack location around the weld toe. Further numerical investigation is conducted to study the significance of the crack surface on the distributions of the stress intensity factors around the crack front. A general formula is proposed to define the crack surface for fracture mechanics analysis to predict the residual life of cracked square hollow section T-joints.
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