Study Of Numerical Control Peening On Complex Wing Panel Elongation And Material Fatigue Property

As an important feature of modern advanced aircraft,the integral wing panel has the characteristics of complex geometry and highly integrated inner structure compared with the traditional riveted panel.For example,the thickness direction varies widely,and the inner structure contains such elements as relieving holes,convex platform,mouth frame,subsidence and so on.The forming target is usually double-convex,saddle-shaped and other double-curvature shapes.At present,mechanical shot peening technology is the preferred method for this kind of components.After shot peening and strengthening,the aircraft integral wing panels will inevitably introduce elongation when they meet the requirements of forming curvature and fatigue life.Due to the comprehensive influence of complex size structure and many process parameters,the processing accuracy and the fluctuation of elongation of this kind of components will be affected.These problems are particularly prominent,which will affect the overall assembly accuracy of the wing.Throughout the study of shot peening forming and strengthening,the main focus is on the impact velocity,angle,size and number of projectiles on the forming curvature and coverage.There are few reports on the extension,especially on the process parameters such as projectile flow rate and sample feed speed.It is difficult to use finite element simulation method to model,so it brings some difficulties for the prediction of the extension.In addition,the effect of shot peening on fatigue properties of materials is mainly focused on the study of strengthening process on fatigue properties of materials.The comprehensive mechanism of forming + strengthening process on mechanical behavior of materials is not clear.In order to explore the extension mechanism of complex wing panels,find a fast and accurate simulation method for shot peening forming,establish the relationship between the extension of complex wing panels and the actual shot peening processing parameters,a systematic and in-depth study was carried out:Firstly,based on the theoretical model of shot peening proposed by Homer,the initial stress of the target is introduced to analytically calculate the shot peening extension,and a formula for calculating the extension with regard to section size of reinforcing bars and induced stress is established.The influence of different thickness,induced stress and initial stress field after rolling on the elongation of shot peening is analyzed.The result shows that under uniform shot peening conditions,the elongation is inversely proportional to the elastic modulus and thickness of the target,furthermore,it is proportional to the induced stress.While the initial tension stress and compression stress increase and restrain the development of sheet metal elongation,respectively.In order to establish the relationship between the projectile flow rate,sample feed rate,air pressure and induced stress,the distribution of induced stress with respect to thickness direction is approximately characterized by sinusoidal function..Based on Power function,the relationship between coverage,sheet metal thickness and elongation is fitted,and the theoretical model of elongation about induced stress and sheet metal thickness is further modified.Secondly,parametric study is carried out on the impact process of single projectile with different impact angles and velocities.When the incidence angle is less than 90°,it is found that the indentations are asymmetric.The diameter of the dent increases with the increase of the incidence angle,while the diameter of the pits at 90°and 85°is basically the same and symmetrical.By using Python language to develop the User-written subroutine for ABAQUS,the simulation for the impact forming concerning single peening projectile is realized.The ratio of contour with Uz=0 to node ratio is used to calculate the coverage rate after multiple projectile impact.Through comparison,it is found that the calculated results of stress field of regular array projectile impact model and random projectile impact model are different under high coverage rate.With the increase of coverage,the maximum residual stress produced by random projectile impact model is larger and deeper than that of array projectile impact model.When the coverage is less than a specific value,we find that the two models maintain good consistency,which is mainly due to the interference of stress field caused by the overlap of indentations with the increase of coverage rate.Through residual distribution correlation,it is found that the thickness of sample is the most important factor,followed by the impact velocity,and finally the coverage.When the impact speed exceeds a critical value,the buckling of sheet metal will occur.A method based on finite element-discrete element method(FEM-DEM)is proposed to simulate and predict the coverage and extension.The influence of different process parameters on the saturated coverage(98.2%)is analyzed.The Avrami formula is deduced and the calculation model of the time required for the saturated coverage in a specific target area and the number of projectiles is established to predict the saturated coverage under different process parameters.The influence of initial stress anisotropy on the elongation is studied by using the three-step method of discrete element method-explicit finite element analysis-implicit analysis,and the correctness of the numerical model is verified by experiments.The influence of impact velocity and sample feed velocity amplitude on the elongation is explained by the method of "induced stress integral area";Pressure load is applied to predict the elongation of C919 local complex skin panels.The simulation results are in good agreement with the experiments.Based on Taguchi test,S/N signal-to-noise ratio and principal component analysis,a multi-objective optimization scheme of shot peening forming curvature and elongation was proposed.The process parameters were optimized with the objective of desired forming curvature,expected small elongation and expected fluctuation.The optimal process parameters were obtained,i.e.thickness = 11 mm,air pressure = 0.4MPa,feed speed = 6000mm/min,projectile.The pill flow rate is 10 kg/min,and the normalized results of S/N signal-to-noise ratio and variance are processed by range analysis method.The normalized percentage method is used to deal with the normalization results such as S/N signal-to-noise ratio(SNR)and variance difference.The results of the two optimization methods are consistent,and the reliability of the optimization method proposed in this paper is verified.In order to study the mechanism of the interaction between shot peening and strengthening on the fatigue properties of the samples,the fatigue tests of the samples after shot peening,shot peening and strengthening were carried out on 2024-T351 aluminium alloy;The different shot peening was analyzed by means of metallographic,XRD and SEM observations.The influence of surface roughness,residual stress field along thickness direction and microstructure evolution on fatigue properties of samples under processing method is studied,A method for analyzing the macro and micro mechanical properties of shot peening was proposed;From the viewpoint of local yield strength,the samples after shot peening are divided into different thickness layers along thickness direction.According to the distribution law of residual stress along thickness direction and the Bialey-Hirsch and Kocks-Mecking models,the position is analyzed comprehensively from micro and macro perspectives.Based on the relationship between microhardness and tensile strength,an empirical formula of local fatigue strength along the thickness direction is established.