Research On Fatigue Property And Life Prediction Of Welding Joints Used In High-rise Steel Structures Based On Strain Energy Density

Welding is one of the main connections of modern steel structures,which is widely used in the joints of high-rise steel buildings.However,there are usually significant stress concentration effects in the welding joints,and initial defects such as pores,impurity,undercuts,incomplete gaps,residual stresses and deformations are often introduced during the construction process,which might largely weaken the fatigue performance of steel structure welding joints.Under the alternating loading,one even much lower than the static load,the welded joints of the steel structure may undergo brittle fracture,which poses a serious threat to the structure.However,for the lack of reliable methods of calculating accumulative damage and predicting fatigue life,welding jionts of high-rise steel buldings cannot assuringly resist fatigue damage under the influence of earthquake and wind.Therefore,it is necessary to pay high attention to the research on fatigue performance of high-rise steel structure welded joints.This thesis focuses on the fatigue life prediction of high-rise steel structure welded joints,where Strain Energy Density（SED）index is used to evaluate the fatigue crack initiation and propagation of welds.Theoretical calculations,numerical simulations and field experiments are adopted to conduct the research on fatigue crack initiation,propagation analysis and life prediction of high-rise steel structure welded joints under high-period and low-stress conditions.The main work and achievements of this thesis are as follows:1.Based on the local average strain energy density,crack initiation analysis and life assessment of welded joints are carried out.Based on the concept of Notch Stress Intensity Factor（NSIF）,the local strain energy density is introduced as the fatigue parameter,and the relationship is established（W-N curve）between the average strain energy density and the high cycle fatigue life within the region,where has a radius R₀=0.28 mm at the weld toe or weld root.Taking the cruciform welded joint and the beam-column welded joint as an example,the average strain energy density method is used for life evaluation and the results are coMPared with the stress-based analysis method.The results show that the deeper the incomplete penetration depth of the weld root is,the shorter the fatigue life of the welded joint has;the stress distribution along the flange width are W-shaped and V-shaped at the weld toe and the weld root of the beam-column joints;the dangerous position is at the edge of the flange;the fatigue life of the weld root is shorter than that of the weld toe;the average strain energy density method is applicable to both the weld toe and the weld root,and can comprehensively consider the fatigue state around the stress concentration point,while it has a low sensitivity to the mesh and takes a unified W-N curve,which shows a good prospect in the crack initiation evaluation.2.Based on the strain energy density factor,crack propagation analysis and life assessment of welded joints are carried out.The average strain energy density method,the interaction integral method and the displacement extrapolation method are used to calculate the stress intensity factor.A strain energy density factor method based on the element strain energy density is proposed,which simplifies the calculation process and is suitable for3-dimensional problems.A crack propagation model(da/dN-ΔS_minin formula)based on the minimum strain energy density factor was established.The method of sub-expansion step summation was used to evaluate the fatigue crack propagation of CT standard specimens and beam-column joints considering the initial crack of weld root,where the results are coMPared with the Paris model.It shows that the initial crack length has a great influence on the fatigue life and needs to be strictly controlled;the distributions of the fracture parameters of the beam-column joints at the upper and lower flange cracks turn out to be V-shaped and W-shaped,respectively;the dangerous positions are at the edge of the flange.The strain energy density factor propagation model can reflect the direction and rate of crack propagation,and comprehensively consider the effects of stress intensity factor components at the crack tip,which has a promising future in crack propagation evaluation.3.The fatigue crack initiation tests and fatigue crack propagation tests of steel welded specimens are carried out.The high period fatigue parameters of the weld were measured,and the life assessment method was validated.Through the high-period fatigue crack initiation test of the axial tensile and compression cycle of the two typical specimens,the butt weld test specimen and the cruciform K-butt weld test specimen,the characteristic parameters of the W-N curve satisfying the 97.7%guarantee rate were determined.The axial tensile fatigue crack propagation test was carried out on two kinds of CT standard specimens with precast notch at Q235 base metal and welding bead,and the crack propagation rate parameters based on the strain energy density factor model and the Paris model are measured.The results show that there are differences between the crack propagation characteristics of the base metal and weld,and the weld zone exhibits a lower average propagation rate but larger fluctuation than the parent metal.The strain energy density method has a high accuracy for the evaluation of the fatigue life of the test specimen.4.A multi-scale finite element model is established,of high-rise steel structure considering local beam-column welding joints.The average strain energy density method and strain energy density factor method are used to evaluate the fatigue life of the joint welds to form a set of high-cycle fatigue evaluation method for high-rise steel structure welded joints based on strain energy density.Taking a certain high-rise steel frame project as an example,the analysis results show that the beam-column welding joint is a typical high-period fatigue problem under the joint load of gravity and wind.The fatigue damage is the most severe at the root of the upper flange of the beam-column welded joint.Assuming that there is a3-dimensional initial through crack（a₀=0.5mm）at the root of the weld,the crack may propagate to half the thickness of the flange,which may lead to failure within the normal working life of the structure.Therefore,during the design and construction phase,it is necessary to pay attention to the weld fatigue check and welding quality of important welded joints,and conduct monitoring,such as flaw detection,to ensure the safety of welded joints and structures.This research can provide reference and suggestion for fatigue crack initiation,fatigue crack propagation,fatigue life assessment and structural anti-fatigue design of high-rise steel structure welded joints.