Investigations On Plastic Forming And Typical Forming Defects Of Aluminum Foam Sandwich Curved Surfaces

Aluminum foam sandwich plate is a sandwich composite structure composed of thin metal face sheets and ultra-light closed-cell aluminum foam core.Because of its light weight,high specific strength,high specific stiffness,energy absorption and vibration reduction,impact resistance,heat insulation and noise reduction,electromagnetic shielding and other characteristics,the sandwich plate has been widely used in aerospace,rail transit,shipbuilding,automobile industry,construction,national defense,and military fields.With the increasing application of aluminum foam sandwich plates in the engineering field,the demand for three-dimensional curved surfaces of aluminum foam sandwich plates in engineering is also increasing.The traditional three-dimensional surface preparation of sandwich plate is to form the curved surfaces of face sheet and core respectively,and then,the face sheet and the core are bonded by adhesive or welding process to obtain the sandwich plate surface.This method is time-consuming and laborious,and it cannot guarantee the accuracy of the curved surface of sandwich plate,which does not meet the requirements of rapid preparation of high-quality curved surface of sandwich plate.The flat sandwich plate can be directly formed into a three-dimensional surface using integral plastic forming,which not only greatly improves the processing efficiency but also reduces the production cost.However,unlike the single-layer metal sheet forming,different modes of forming defects such as global buckling,face wrinkling,core fracture,and face sheet/core debonding may occur during the plastic forming of sandwich plate.These defects seriously limit the application of the curved sandwich plate.Therefore,it is of great significance to research the forming of three-dimensional surface of aluminum foam sandwich plate using direct plastic forming technology.At present,the research on aluminum foam sandwich plates mainly focuses on the preparation of flat plates as well as the quasi-static and dynamic mechanical properties of the sandwich plates.There are few studies on the plastic forming and forming defects of sandwich plates.In this paper,the plastic formability of aluminum foam sandwich plate,the typical forming defect modes,the defect prediction,and the influence of sandwich parameters are systematically investigated based on numerical simulation,theoretical analysis,forming experiment,and measurement.The main contents and conclusions of this study are as follows:1.Through the multi-point forming experiments of cylindrical,spherical,and saddle-shaped surfaces for aluminum foam sandwich plates,the feasibility of plastic forming of curved sufaces of the sandwich plate is proved.The mesoscopic Voronoi,reconstructed CT,and macroscopic equivalent aluminum foam three-dimensional models are established,and the corresponding sandwich plate models are used to simulate and analyze the cylindrical panels forming.The stress-strain distribution,finite element mesh number,and calculation time of different models are compared,and the forming accuracy of the experimental part and the simulated part is compared to determine the sandwich model for numerical analysis of plastic forming.Based on the model,the deformation characteristics,laws,and defect modes in cylindrical and doubly curved surfaces forming of sandwich plate are analyzed in detail.The results show that the three-dimensional curved surfaces of aluminum foam sandwich plate can be directly formed by integral plastic forming,and curved parts with different shapes and high forming accuracy can be obtained.The numerical simulation analysis of the plastic forming process using the mesoscopic Voronoi aluminum foam sandwich plate model can relatively accurately reflect the macroscopic and mesoscopic deformation characteristics and defect modes of the sandwich plate in plastic forming with low computational cost.2.Based on Micro-CT and high-precision laser scanning technology,the experimental and numerical results of face wrinkling and dimpling defects in the forming process for aluminum foam sandwich plates are studied,and their characteristics and laws were analyzed.The prediction analytical models of the critical compressive stress for the face wrinkling and the critical equivalent stress for the face dimpling were established.The numerical simulation analyses of the spherical and saddle-shaped panels forming process are carried out by using the macroscopic equivalent and mesoscopic Voronoi aluminum foam sandwich models,and compared with the experimental results,and the reliability of using the critical theoretical stress models of face sheet defects to predict the face sheet forming defects is verified.The effects of geometric and material parameters of the sandwich plate as well as the shape of the forming surface on the face wrinkling and dimpling are studied,and the effects of these factors on the occurrence order of two face sheet defects in the forming process are discussed.The results show that the established face wrinkling theoretical model has a good prediction on the face wrinkling defect in the case of low density core,and the face dimpling theoretical model can accurately predict the face dimpling defects.The face wrinkling defect can be suppressed by reducing the thicknesses of face sheet and foam core,increasing the core density,and using a core with the elastic Poisson’s ratio close to 0 and 0.5.The face dimpling defect can be suppressed by using a thicker face sheet,a thinner core,and a smaller average cell size of the core.The application of face sheet materials with large strength coefficient and strain hardening index can reduce the degree of face wrinkling and dimpling.The face wrinkling is more likely to appear first during the plastic forming of aluminum foam sandwich plate with lower core density and larger ratio of face thickness to core cell size.3.The macroscopic cracks and the microscopic fracture morphology of the core cell-wall during the curved sandwich panels forming are observed,and the characteristics and laws of the core fracture are analyzed.Different fracture criteria were introduced,and the material parameters are calibrated by the finite element simulation method.Combining with the curved panels forming simulation results using the mesoscopic Voronoi aluminum foam sandwich model,the initiation of core cracks during the forming process is predicted.The forming experimental results with different target radii and curved surfaces are compared with the simulated predictions,and the fracture criterion that can accurately predict the core fracture defect of the sandwich plate is determined.Based on the determined fracture criterion,the influence of sandwich parameters on core fracture is analyzed numerically.The results show that Ko ductile fracture criterion can accurately predict the initiation of core cracks during the plastic forming of aluminum foam sandwich plate.The risk of core fracture during the plastic forming can be reduced by decreasing the size of core cells,increasing the thickness of core cell-wall,using aluminum alloy with higher yield strength as cell-wall material,and reducing the thicknesses of core and face sheet.4.The macroscopic characteristics of face sheet/core debonding defect and the fracture morphology of the adhesive layer of the shaped aluminum foam sandwich plate are observed,and the characteristics and laws of the face sheet/core debonding are analyzed.The macroscopic equivalent sandwich model considering the adhesive layer is established,and the cohesive zone model is used to simulate the damage and failure of the adhesive layer.Based on the finite element simulation and experiment of the flatwise tensile and lap shear of the sandwich plate,the parameters of the cohesive zone model of the adhesive layer are determined.According to the corresponding cohesive zone model,the forming of double curvature surfaces of the sandwich plate is numerically simulated.The numerical results of the stiffness degradation distribution on the adhesive layer are compared with the experimental parts,and the reliability of the cohesive zone model to predict the face sheet/core debonding forming defect is verified.The influences of the mechanical properties of the adhesive layer and the sandwich geometric parameters on the face sheet/core debonding defect are analyzed by numerical simulation.The results show that the cohesive zone model can accurately predict the face sheet/core debonding defect.The face sheet/core debonding during forming can be avoided by reducing the stiffness of the adhesive layer,increasing the strength of the adhesive layer and the critical fracture energy.The limited forming radius for curved sandwich plates forming without debonding failure can be reduced by using thinner face sheets and core as well as thicker adhesive layers.