Deformation Behavior And Forming Limit Of Anisotropic Metal Shells Under Continuous Nonlinear Loadings

With the development of transportation equipment in aircraft,aerospace and automobile industries,towards high reliability and long lifespan,it is urgent to use integrated thin shell component to replace the traditional multiple segments structure by welding.At present,such kind of integrated components are manufactured by fluid pressure forming technology using thin sheet and tube as raw blanks.During forming,the blank material undergoes a complex continuous nonlinear loading scheme,which affects the deformation behavior and forming limit of thin metal shell significantly.Furthermore,the anisotropy characteristic of thin sheets or tubes makes deformation more complex,and the prediction of defects becomes more difficult.In this thesis,the deformation law,hardening behavior and forming limit of thin metal shells under complex loading conditions were comprehensively studied by experiments and theoretical analysis,which provide theoretical guidance for the fluid pressure forming of integrated thin shell components.In order to quantitatively determine the deformation law and forming limit of metal sheet under continuous nonlinear loading,an analytical model for sheet bulging considering real-time contact states was established.Moreover,an experimental method,sheet bulging with stepped-dies was proposed,enabling the problems of friction effect and nonuniform large plastic deformation of the existing experimental methods to be solved.An experimental device using stepped-dies dedicated for sheet bulging was developed.Using digital image correlation technology(DIC),real-time instant control and recording of the whole process from initial deformation to final fracture of sheet bulging was realized.Theoretically,the device is able to achieve a continuous nonlinear loading path with a stress ratio ranging from 0.5 to 2.0.For tubes,a method for controlling the stress state of the bulging area was proposed,by changing the end conditions(i.e.controllable loading,fixed and free)of the tube.A dedicated experimental device for providing a continuous nonlinear loading of tubes was developed.Using which,three types of bulging experiments were achieved,and an arbitrary continuous nonlinear loading path within the range of bidirectional tensile stress was achieved.Continuous nonlinear loading paths were realized by the bulging experiment with stepped-dies.The deformation law and forming limit of cold-rolled low carbon steel(ST 16)sheet under continuous nonlinear loading conditions were obtained.During the bulging with an elliptical die,the ratio of principal curvature radii gradually approaches the elliptical ratio ? of the die with the increase of bulging height,correspondingly,the stress ratio approaches 2-?.With regard to the bulging with a stepped-die,the bigger the difference in the cross-section shape of cavity between the two stages,the higher nonlinear extent the loading path is.The limit values of both the curvature radius ratio and stress ratio at the final stage of bulging are also determined by the elliptical ratio of the die.Under the condition of continuous nonlinear loading,due to the experienced equal-biaxial tensile deformation at the initial stage,the limit strain of low carbon steel sheet is significantly lower than that under the linear loading condition.Linear and continuous nonlinear loading paths were realized through the experimental device of tube bulging with controllable end loading.The deformation law and forming limit of extruded aluminum alloy(6061-F)tube under continuous nonlinear loading conditions were obtained.Under linear loading conditions,aluminum alloy tubes exhibit obvious anisotropy,with a strain ratio as high as 3.0 when the stress ratio is 1.0.After the plane strain deformation,aluminum alloy tubes can obtain higher limit strains than those under the linear loading condition.Correlations between the continuous nonlinear loading path and geometric parameters of the bulging region under both fixed and free end conditions were analyzed theoretically.The loading paths of tubes under these two end conditions are mainly determined by the ratio of length to diameter of the bulging region.A new forming limit diagram is presented,with the length-diameter ratio as vertical axis,hoop strain as left horizontal axis and axial strain as right horizontal axis.The direct characterization of limit strain under typical nonlinear loading paths is realized.The new diagram can be used to predict the fracture strain of irregular tubes with varied diameters in the axial direction during fluid pressure forming.An anisotropic hardening model suitable for the plane stress state was established.The model can describe typical anisotropic hardening behaviors including the Bauschinger effect,latent hardening and permanent softening.The anisotropic hardening models of low carbon steel sheet and aluminum alloy tube were determined using the two-step tensile test experimental data.The latent hardening and permanent softening anisotropy hardening behaviors of the two materials during the two-step tensile testing were well predicted by the model.In the meantime,the results of sheet bulging with stepped-dies and tube bulging with a controllable loading predicted by the theoretical models are consistent with the experimental results.A M-K model based on the Clift ductile fracture criterion was established.The characteristics of this new model were analyzed.The forming limit curve obtained using the new model is lower at the two ends corresponding to uniaxial tension and equal-biaxial tension.A peak phenomeon was observed near the equal-biaxial region.Under the condition of linear loding,the smaller the thickness inhomogeneity coefficient,the lower the predicted forming limits and the reduction extent at two ends.While the reduction extent at two ends are greater for smaller material constants of characterizing fracture condition of material.The new model solves the problems of overestimated limit strains of equal-biaxail state for low carbon sheet and uniaxial state for aluminium alloy tube using traditional M-K model.In the meantime,the new model accurately predicted the necking strains of low carbon steel sheet and aluminum alloy tube under continuous nonlinear loading.The effect of the loading path predicted using the new model on the forming limit is consistent with the experimental results.