Research On Multiaxial Fatigue Behavior And Life Prediction Method Of Tube-flange Welded Joints

The tube-flange welded joints are broadly used for single-mast arm cantilevered rigid frames that support the traffic signs and signal lights at road intersections.Such welded joints are also used for steel tubular towers which support the wind turbines and transmission lines in power industry.However,the aforementioned steel structures are prone to wind-induced vibration caused by natural wind,and further the welded joints tend to occur multiaxial fatigue damage.The purpose of this research is to study the life prediction method of multiaxial fatigue for tube-flange welded joints.The multiaxial mechanical properties and fatigue performances of Q235 B welded specimens and connected joints are firstly carried out by fatigue experiments.Then a novel critical plane-based fatigue parameter and life prediction formula that incorporates a loading path factor are proposed.Finally,the multiaxial structural strain formula is derived and the multiaxial fatigue evaluation method is established for tube-flange welded joints.(1)The multiaxial cyclic mechanical properties and fatigue performances of welded specimens are studied.Low-cycle torsional and multiaxial fatigue experiments are carried out for Q235 B steel welded specimens under strain control.It is shown that mean stress relaxation occurs for Q235 B steel welded specimen under the partially reversed multiaxial loading with a large plastic strain applied.The non-proportional additional hardening extent in axial direction is: circular path > square path > elliptic path > rhombic path.The hardening extent of axial and torsional components is related to the amplitude ratio of shear to axial strain.The relationship curve between equivalent strain amplitude and experimental fatigue life of Q235 B steel welded specimen under pure torsion is close to that under multiaxial proportional path.(2)The multiaxial fatigue parameter considering non-proportional hardening is studied.The reduction in degree of multiaxial fatigue performance for additional hardening metals closely depends on the relative maximum shear strain and rotational condition of transient maximum shear strain.A loading path factor formular is proposed to gauge the additional hardening effect.A novel critical plane model containing energy-based parameter is established by introducing a weight function of shear strain.The relationship between fatigue parameter and fatigue life is derived based on Manson-Coffin equation.The predicted results for Q235 B steel welded specimens show that the proposed critical plane model effectively reflects the influence of fatigue damage induced by multiple multiaxial loadings.(3)The multiaxial fatigue life evaluation method for tube-flange welded joints is studied.The equilibrium equation between the internal force and the product of the structural stress and cross-sectional area is derived.The procedure of equivalent plastic strain increment is established by introducing a strain hardening constitutive model to form the isotropic hardening yield function.The normal and shear structural strain along the tube’s cross section is calculated by the flow rule.Combined with weighted critical plane model and loading path factor,the life prediction method of tube-flange welded joints is established.(4)The multiaxial fatigue performances and life evaluation for tube to flangeshaped welded joints are studied.Low-cycle multiaxial fatigue experiments are carried out for Q235 B steel welded joints under load control.The cyclic loading process of welded joints includes three stages: stable cycle,stiffness degradation and fracture failure.The joints also exhibit cyclic softening or hardening during stable cycle.The life evaluation method based on structural stress,structural strain terms and loading path factor is used to predict the fatigue lives of tube-flange welded joints.The predicted results show that the proposed method can effectively evaluate the multiaxial fatigue performances of Q235 B tube-flange welded joints under various loading paths.