Weld fracture under non-uniform stress distribution and its mechanism in CHS-CHS X-joints

Abstract

Tubular joints are characterized by the non-uniform stress distribution. In order to study the influence of the non-uniform stress distribution on the bearing capacity of the weld for circular hollow section (CHS) joints and the fracture mechanism, monotonic tensile tests for 8 CHS-CHS X-joints were conducted. The specimens were designed utilizing coarse FE models using a triaxiality-and-Lode-angle-related fracture model to ensure the expected failure mode. Refined FE models were also established which were verified by the test. Failure modes of weld fracture and chord punching shear failure were both achieved by FE analysis, and the corresponding fracture evolution were also obtained. The weld fracture initiated from the welding root at the saddle point while the punching shear fracture initiated from the chord surface at the saddle point. It shows that the non-uniform stress distribution has a great impact on the bearing capacity; the non-uniformity of stress distribution is weakened and the weld bearing capacity becomes higher if the joint stiffness is increased; the formula in Chinese code is not safe if the influence of the non-uniform stress distribution is ignored. A simplified mechanical model is established to study the relative stiffness distribution of the joint and subsequent the stress distribution. It was found that the deformable elements of the chord resist the brace force mainly through extrusion deformation but not the shear deformation, and this is the key reason for the non-uniform stress distribution; the non-uniformity of stress distribution will become higher if the geometric parameter γ and β increase.