Fatigue Analysis Of Aluminum Alloy Component Of The Bridge Deck Under The Coupling Of Residual Stress And Load

Aluminum alloys are widely used in machinery manufacturing,aerospace,transportation equipment and other fields because of their advantages of light weight,high strength,and corrosion resistance.The phenomenon of applying aluminum alloy profiles to bridge structures abroad has been very extensive,and there are many typical demonstrations in China.In general,China is relatively backward in this field,and there are many related application technology problems that need to be solved urgently.Industry generally improves the mechanical properties of aluminum alloys through heat treatment processes such as solid solution and quenching.However,the inhomogeneous distribution of the temperature field during quenching causes residual stress to form inside,especially for aluminum alloy profiles with larger size and more complicated section shape,the distribution of quenching residual stress is more inhomogeneous.The quenching residual stress will always act on the inside of the material,affecting the subsequent processing process and service condition,and even causing the material to crack and be scrapped.Generally speaking,the large aluminum alloy bridge structure needs to be connected by multiple profiles.The welding technology has the advantages of small deformation and easy processing.It is used in bridge construction,but the welding process will produce welding residual stress near the weld.The combined effect of welding residual stress and quenching residual stress affects the subsequent service life of the bridge.During the service of the bridge aluminum alloy profile,under the joint action of residual stress and alternating working load,initial cracks appear due to material flaws,and then crack propagation occurs.Therefore,it is of great engineering significance to study the crack growth trend and fatigue life of aluminum alloy profiles under the coupling of residual stress and alternating load.Based on the theoretical analysis,this paper uses the finite element method to numerically simulate the spray quenching process and welding process of aluminum alloy profiles,analyze the distribution law and evolution trend of residual stress;and solve the stress intensity factor of the crack under the residual stress.Explore the change law and evolution trend of crack under the action of residual stress;finally,the fatigue life under the coupling effect of residual stress and alternating load is studied.The following conclusions are drawn:(1)During the quenching process of aluminum alloy profiles,the surface thermal stress experienced a transition process from tensile stress to compressive stress,and the core thermal stress experienced a transition process from compressive stress to tensile stress;after quenching,the residual stress of the profile showed "External pressure and internal bracing" distribution.And the maximum value of the surface residual stress is distributed at the corners,and the maximum value of the residual tensile stress at the core is near the core of the profile.The temperature difference in the early stage of quenching is the main cause of residual stress.(2)During welding of aluminum alloy profiles,the closer the measurement point is to the weld,the higher the peak temperature and the shorter the time required to reach the peak.When the center of the heat source moves to the measurement point of the weld center,the temperature rises sharply and reaches a peak.As the heat source moves away,the temperature at this measurement point gradually cools to room temperature,but the rate of change is relatively slow.The peak temperature at the beginning of welding is relatively low,and the peak at the end is the largest.The molten pool area is zero stress.After the weldment cooling,the areas on both sides of the weld bead show compressive stress distribution,while the weld area is tensile stress distribution.(3)During the numerical simulation of the stress intensity factor,the residual compressive stress generated by quenching plays a role in inhibiting the surface crack growth.However,under the influence of welding residual stress,the crack propagation tendency is aggravated.With different residual stresses,the crack propagation tendency is different;the larger the crack radius,the more obvious the propagation tendency.(4)In the process of calculating the fatigue life,the fatigue life of the profile decreases with the increase of the maximum value of the alternating load;the smaller the crack radius,the slower the crack growth rate,which means that the smaller the crack radius,the higher the fatigue life;At the same time,studies have shown that when the crack radius is small,the fatigue life is more sensitive to the change in crack size.