Damage Tolerance And Ultimate Strength Of Stiffened Panel With Residual Stress And Deformation

In the field of aerospace,the pursuit of component lightweight is not only the development trend,but also the main problem for researchers to engage in a large number of experimental research for a long time.Laser welded stiffened panel is the main supporting component of aircraft structure.Its structure processed by integrated manufacturing technology can not only improve the damage tolerance of fuselage structure,but also greatly reduce the weight of fuselage.Laser welding technology is a precision welding method in Colleges and universities,which uses the laser beam with high energy density as the heat source.It will cause serious welding residual stress and deformation of the welded structure and reduce the strength of the structure.In order to predict the welding residual stress and deformation of laser welded stiffened panel and study the ultimate strength and failure mode of stiffened panel with initial welding defects,this paper carries out numerical simulation and experimental test on small and large stiffened panel respectively,puts forward an inverse inherent deformation method to predict the welding residual stress and deformation of large stiffened panel,and analyzes the failure mode of laser welded aluminum lithium alloy stiffened panel from the micro point of view.The main contents and conclusions are as follows:(1)The thermal elastic-plastic finite element method is used to predict the welding residual stress and deformation of laser welded stiffened panel.The results show that the welding residual stress is mainly distributed in the weld,in the form of tensile stress,which is about the yield strength of the material.Based on the thermo-elastic-plastic method and natural deformation method,an inverse natural deformation method for predicting the welding residual stress and deformation of large stiffened panel is proposed.Compared with the results of thermo-elastic-plastic method,it is found that the prediction results meet the expected accuracy,which is verified by double stiffened panel,and the accuracy of inverse natural deformation method is verified.(2)The welding residual stress and deformation of the impact model are predicted by the thermo-elastic-plastic finite element method,and the influence of the initial welding defects on the ultimate compressive strength of the stiffened panel is analyzed.The results show that the combined action of welding residual stress and deformation reduces the ultimate compressive strength of stiffened panels.The compression after impact simulation of stiffened panel is carried out,and the influence of impact dent on the ultimate compressive strength of stiffened panel with residual stress and deformation is studied.It is found that when the stiffener is impacted,the initial impact dent damage has the greatest influence on the ultimate compressive strength of stiffened panel;When impacting the weld,the impact is the second;When the skin is impacted,the impact on the ultimate compressive strength of the stiffened panel is the least.With the increase of impact energy,the residual compressive strength of stiffened panel decreases.When the impact energy reaches the energy of skin failure,the reduction degree of ultimate compressive strength of stiffened panel decreases.(3)The plane shear experiment of laser welded dissimilar Al-Li alloy multi stiffened panel was carried out.The buckling evolution behavior and failure mode are studied by using the projection fringe method and strain gauge.It is found that with the increase of shear load,the stiffened panel skin first has local buckling,the peaks and troughs are spaced on the inter rib skin,and the peaks and troughs are antisymmetric on the adjacent skin on both sides of the rib.At the same time,with the increase of load,the tendon also begins to buckle in the post buckling stage,bearing the main load until the overall instability occurs.(4)The micro fracture mechanism of the weld was discussed by metallographic observation and SEM fracture morphology analysis.The results show that a large number of non-dendritic equiaxed zone(NDE)are formed at the weld fusion line.Due to the segregation of solute elements in the NDE,the continuous existence of liquid film at the grain boundary and the softening of partial melting zone(PMZ),the weld strength is weakened.When the stiffened panel is subjected to shear load,the crack nucleates preferentially in the weld and expands along the weld direction,and the stress at the weld first reaches its strength,the final weld of stiffened panel is desoldered and invalid.Most of the fracture surfaces are cleavage steps and intergranular fracture in NDE,indicating that the failure mode of the weld is brittle fracture with a small amount of shear dimples.(5)The virtual simulation of large stiffened panel with and without welding initial defects is carried out respectively.Compared with the test results,it is found that the virtual finite element method can effectively simulate the modal evolution and failure mode of stiffened panels.By introducing the form of welding deformation obtained by the inverse natural deformation method and considering the simulation of welding initial defects,the prediction accuracy of ultimate shear strength of stiffened panels is effectively improved.