Research Of Fatigue Life Estimation Approach Of Welded Structures Under Different Loading Modes

Welding processing widely exists in engineering practice,due to the weld toe geometry diversity of weld structure,weld toe is usually stress concentration location.Because of the welding defect caused by the welding prodedure,weld seam and heat affected zone(HAZ)could be the region where fatigue failures are most likely to happen.How to construct adaptive equivalent damage parameters under different loading modes is a precondition to make an effective fatigue life assessment of welded structures.Currently,fatigue life estimation of welded structures is mainly based on the criterion provided by international institute of welding(?W)standard etc.,e.g.nominal stress method,hot spot stress method,local stress method and linear fracture mechanics method etc.The methods mentioned aboved has been successfully applied for the fatigue life estimation of welded structures under uniaxial and torsional loading modes.However,welding joints are generally serving under multiaxial loading modes in engineering practice,and the aforementioned approaches appear a certain limitation.Critical plane method is one of the effective approaches for dealing with fatigue life estimation of material under multiaxial loading modes,while the critical plane methods have not yet been well applied for fatigue life estimation of welded structures.How to effectively combine factors such as stress concentration effect at weld toe,welding residual stress with critical plane method together,and construct appropariate fatigue life estimation for welded structures is a problem demanding prompt solution.So,a series of research work about fatigue life estimation problem for welded structures are carried out under different loading modes.The critical plane method is modified,and the stress concentration effect at weld toe should be taken into consideration of the solution of the equivalent damage parameter.Furthermore,this part of work is also the research foundation of the article.For welded structure with residual stress,a further consideration should be taken into the solution of the critical plane.Geometry discontinuity of welded structures could lead to singularity of the stress calculation at weld toe.In low cycle fatigue(LCF)regime,the stress singularity problem could be solved to some extent using the average strain energy density(ASED)method.Fatigue behavior and mechanism of welded structures in very high cycle fatigue(VHCF)regime could be effectively analyzed based on experimental approach.So the research works are done from experiment and fatigue life estimation model two aspects in this article.For the above-mentioned problems,following works are developed and completed in this dissertation:1)Combining structural stress method and critical plane method together,a fatigue life estimation model considering stress concentration effect is constructed.Firstly,structural normal stress and shear stress concentration factor of welded structure are solved through structural stress method,the structural concentration factors are introduced into the modified Carpinteri-Spagnoli critical plane method and new equivalent damage parameter is constructed.A novel fatigue life estimation model is proposed based on the equivalent damage parameter(Structural Stress based Critical Plane Model).The model is verified through a flange-to-tube welded joint is StE460 material,and compared with results obtained by hot spot stress method(HSS).The comparison results indicate fatigue life of welded joints could be effectively estimated by the SSCP method under uniaxial,torsional,in-phase and out-of-phase etc.loading modes.2)Fatigue life estimation model considering influence of the welding residual stress is constructed for the welded structures without post heat treatment.Firstly,welding residual stress field is numerically simulated through a sequential thermal coupled model.The residual stress is then introduced into the SSCP model,and a fatigue life estimation model(MSSCP model)of welded structure considering residual stress is proposed.The MSSCP model is verified through a flange-to-tube welded joint of AlSi1MgMn T6(6082-T6)material,and the estimated results are compared with results obtained by SSCP model and experimental results.For welded joints with residual stress,the comparison results show that the MSSCP model possesses preferable life estimation effect under multi-axial loading mode.3)A fatigue life estimation model considering average strain energy density(ASED)is proposed based on the SSCP critical plane method.Weld toe is treated as the notch of the welded joints,and the average strain energy density along the thickness direction through the Ramberg-Osgood cyclic constitutive relationship.Then the Manson-Coffin relation equation is integrated for the construction of fatigue life estimation model between average strain energy density and the fatigue life.Then the ASED model is applied for fatigue life estimation of a StE460 steel flange-to-tube welded joint,and the estimated results are contrasted with the strain energy density(SED)model and experimental data respectively.It shows that the estimated life obtained by the SED model are with an over conservative characteristic.4)Fatigue property and failure mechanism of 7020-T651 aluminum alloy and welded joint are studied.A metal innert gas(MIG)protecting welding teconology is applied for the 7020-T651 aluminum alloy plate with 7mm thickness,and different inclination angle of the weld seams are designed,in order to assure the weld seam of the welded joints subjected to uniaxial,multi-axial loading modes.etc.Fractography analysis of the material and welded samples are implemented through the SEM(Scanning Electron Microscope),and the fatigue crack initiation and propagation of the material and welded samples are checked.Comparing with the material,the welding flaw,such as non-metal inclusion,welding porosity.etc,especially the welding porosity should be the crack initiation region.Moreover,the fatigue crack propagation cycles should be also reduced due to the welding porosities.Meanwhile,SSCP model is utilized for very high cycke fatigue life estimation of the welded joint,and the estimated results are compared with the experimental data.The comparison results show that the estimated lives in HCF regime are relatively ideal,while the estimated lives in VHCF regime(Nf>107Cycles)are over conservative.