Investigations On Plastic Forming Feasibility And Multi-point Forming Experiments Of Bi-directional Trapezoidal Sandwich Curved Surfaces

In recent years,sandwich plates have become a research hotspot in the field of advanced engineering materials with their excellent comprehensive performance and flexible structural design.Due to their light weight,high specific strength,high specific stiffness,impact resistance and other excellent structural performance as well as heat insulation,sound insulation,vibration absorption,electromagnetic shielding and other multi-functional characteristics,sandwich plates have been widely used in aerospace,automotive,high-speed train,ship,construction and other fields.There is more and more requirement for curved sandwich plates with the increasing demand of manufacturing industry for diversification of product shape.At present,the sandwich curved surface is mostly prepared by an adhesive curing method.The production cost of the sandwich curved surface obtained by this method is high and the mechanical properties as well as service life are restricted by the adhesive.However,the traditional all-metal sandwich plate has typical difficult-to-form characteristics,and the failure modes such as skin wrinkling,fracture and core shear failure are prone to occur during the forming process,which causes the sandwich plate is mostly limited to flat plate-type structures and limits the application potential severely.Therefore,it is significant to explore the curved surface forming for sandwich plate using traditional plastic forming method.Bi-directional trapezoidal sandwich plate is a welded metal sandwich plate with a new type of core structure.The core structure is a trapezoidal convex-concave corrugation arranged in two vertical directions in the plane,which overcomes the shortcomings of traditional lightweight cores such as lattice core prone to shear failure when bending due to its low shear strength,meanwhile,the flexible geometric structure improves the designability of the sandwich plate for the plastic forming performance.At present,there are few research results on the curved surface forming directly for all-metal sandwich plates and exploratory research is urgently needed.In this paper,the deformation characteristics,the defect modes,the formability,the equivalence of core,the forming accuracy and the effect of geometrical parameters of the of bi-directional trapezoidal sandwich plate during the forming process are studied systematically through the combination of numerical simulation,theoretical analysis and experimental measurement.The main contents and results of this paper are as follows:1.The finite element model for shaping bi-directional trapezoidal sandwich curved surface using multi-point process is built,and the model which not only reflected the actual forming results but also improved the calculation efficiency is obtained through analyzing the mesh sensitivity and mass amplification factor.On this basis,finite element simulation is used to investigate the deformation characteristics,the defect modes and the forming quality of the sandwich curved surface during the plastic forming process.The results show that the deformation mainly concentrates on the skins and the stress of skin in the welded area is obviously smaller than that in the unsupported area.The core cell is primarily deformed by changing the angle between the inclined plane and the bottom plane,and most of the areas are still in the range of elastic deformation after forming.The defect modes in the forming process mainly include skin wrinkling,skin dimpling,global buckling,local straight surface effects and sidewall buckling on the core.With the decreasing of deformation degree,the forming defects of sandwich surface disappear gradually and the forming quality improves.2.Three main defect modes,i.e.,skin wrinkling,skin dimpling and global buckling,which restrict the curved surface forming of bi-directional trapezoidal sandwich panels,are studied from the macroscopic and mesoscopic aspects.The initiation,development and evolution of defects in the whole plastic forming process are analyzed by numerical simulation.The instability patterns of the various defects are ascertained by observing the mesoscopic geometric characteristics of the defects.On this basis,theoretical prediction models of three kind defects related to material parameters and geometric parameters of sandwich surface during the plastic forming process are established,the theoretical analytical equations for the critical curvature radius of the global buckling of the sandwich surface,the critical curvature radius of the skin wrinkling and the critical stress of the skin dimpling are given,and the reliability of the theoretical prediction model is verified by finite element numerical simulation and experiments.3.The influence of geometric parameters of bi-directional trapezoidal sandwich plate on defect mode is studied.The theoretical prediction and numerical simulation results show that the thin sandwich plate with smaller core size is prone to global buckling defects,while the thick sandwich plate with larger core size is prone to skin buckling defects.For skin dimpling and skin wrinkling of the skin buckling defects,when the length and width of brazing platform are close to each other,the skin dimpling is prone to the first forming defect,while the length and width are quite different,the skin wrinkling becomes the first forming defect.The effects of skin thickness,core thickness,characteristic size of cell and forming mode on formability are analyzed.It is found that increasing skin thickness,reducing core thickness and characteristic size can improve the formability of sandwich plate in varying degrees.For a negative Gaussian surface such as a saddle surface,the larger the length-width ratio of the brazing platform,the greater the difference of the formability between the two kind forming modes of the sandwich plate.Based on the theoretical prediction model,it is found that a reasonable ratio of length to width of brazing platform can improve the formability of sandwich plates,The smaller the critical stress difference between the skin wrinkling and dimpling,the better the formability of its corresponding sandwich plate.4.Based on the assumption of modified Hoff theory,the relationship between stiffness constants and elastic constants of sandwich plates is obtained.The equivalent elastic constants of bi-directional trapezoidal sandwich plate are calculated by semi-analytical method combined with finite element method.This method does not need to account for the complex structure of sandwich plates.The internal stress distribution and the position of stress neutral layer of sandwich surface during cylindrical forming are analyzed.On this basis,the calculation model of bending springback of bi-directional trapezoidal sandwich surface is deduced.Compared with the numerical simulation results,the model can accurately and rapidly calculate the stress and cross-sectional bending moment of sandwich surface.The accuracy of springback calculation is verified by error analysis with the experimental results of multi-point forming.5.According to the above research,the formability range of the sandwich plate is obtained,and three types of curved surface were formed for the sandwich plate within the formable range.Through observation,it is found that the experimental surfaces are smooth and appear no forming defects,which verifies the feasibility of forming sandwich curved surface by multi-point press.The shape point clouds of the experimental parts are obtained by three-dimensional laser scanner,and the thickness and error analysis are carried out.The results show that the thickness of all experimental parts with variable forming radius was reduced,and the thickness reduction of the experimental parts with smaller forming radius is larger than that with larger forming radius.The plastic forming method shows high forming accuracy and stability when forming thin sandwich surface with small cell,and acceptable forming accuracy when forming thick sandwich surface.Plastic forming provides an efficient and economical processing avenue to manufacture sandwich curved surfaces.