Research On The Forming Limit Of Aluminium Alloy Sheet Based On The Damage Theory

With the advancement of the process of industrialization, aluminum alloy sheet shows a rapid growth in aerospace, transportation, construction and other related emerging industries. Die forming is a basic and convenient processing method for aluminum alloy sheet. As a result of high strength and low plasticity, the aluminum alloy sheet is prone to have defects in die forming process, such as wrinkle, fracture, localized necking and drifting line. The forming limit diagram(FLD) is generally used in industrial production to estimate the forming effects of aluminum alloy sheet parts. Consequently, to realize the low-cost, fast and accurate acquisition of aluminum alloy sheet, that has important significance in guiding the materials selection, structure design and process optimization of aluminum alloy parts.Common methods to obtain the aluminum alloy sheet FLD contain experiment, theoretical analysis and finite element(FE) simulation. Among which, combining the theoretical analysis and FE simulation, may cut the cost and improve the efficiency of FLD acquisition. Since the deformation instability behavior of aluminum alloy has intuitive distinction with common ductile metallic materials, traditional tensile instability theory or essential defect hypothesis is not suitable to obtain the aluminum alloy sheet FLD. As a result, this paper explored the applicability and accuracy of using damage theories in obtaining the aluminum alloy sheet FLD, and offered new ways to obtain the aluminum alloy sheet FLD. This paper proposed formal experimental method for FLD acquisition, established FE model to predict the FLD, compared the applicability of different damage models in FLD obtaining, designed simple tests to acquire the key parameters of damage model, analyzed the influence law of material parameters and loading conditions on FLD, proposed a real time prediction method for aluminum alloy sheet FLD, and conducted related experimental verification.The main contents and conclusions are summarized as follows:1. Experimental method and optimization for obtaining the aluminum alloy sheet FLDThe defects of traditional experimental method and measuring method in aluminum alloy sheet FLD acquisition were discussed. The aluminum alloy 5083-H32 was chosen as an object, the dome bulging process was selected as the experimental method based on the FLD characters. The material failure in various loading paths was realized by changing the effective forming area of the specimen. The specimen center was designed curved to avoid the edge fracture in dome bulging process. The oil and film compound lubrication method was chosen to reduce the impact of friction on the material failure. The motion delay effect of the punch was utilized to obtain the material fracture. The non-contact optical measuring equipment was used to get the global strain. The experimental forming limit diagram(EFLD) of aluminum alloy 5083-H32 sheet was established.2. Establishment of the FE model for aluminum alloy sheet FLD acquisitionThe key issues in establishing the FE model of dome bulging process for aluminum alloy sheet were discussed, including, obtaining the material parameters, establishing the constitutive model, simplifying the geometric model, measuring and optimizing the friction conditions, setting the boundary and loading conditions, In simulation results, the maximum punch force criterion was selected to judge the material failure. The nominal necking forming limit diagram(NFLD) obtaining from simulation results was compared with EFLD. The results show that, EFLD locates upper to NFLD since the nominal necking occurs before fracture, Obvious difference exists between the shape and span of NFLD and EFLD curves, which further validates that the elastic-plastic mechanics is not suitable to express the deformation instability of aluminum alloy sheet.3. The applicability of continuum damage mechanics in aluminum alloy sheet FLD acquisitionThe macroscopic and microscopic tests of aluminum alloy 5083-H32 were conducted to obtain the fracture form and fracture morphology, the results show that, the fracture is more suitable to indicate the material failure, and the material fracture is leaded by ductile damage and shear damage. The crack state variables were independently defined to predict the aluminum alloy fracture. Designed notched specimens were utilized to obtain the key parameters in aluminum alloy damage process, which considered the comprehensive influence of loading paths and loading history on the material failure. The ductile damage and shear damage theories were applied in the simulation of dome bulging process, in order to obtain the fracture location, punch strokes, limit thickness and damage forming limit diagram(DFLD). That were compared with the experimental results and simulation results without damage theory. Error comparison shows that, under the condition of uniaxial tension and plane deformation, FE results with ductile damage and shear damage shows well agreement with experimental results; however, under the condition of biaxial tension, FE results with ductile damage and shear damage are different with the experimental results.4. The applicability of modified Lemaitre damage model in aluminum alloy sheet FLD acquisitionA Lemaitre damage model that based on thermodynamic irreversibility was selected to guide the FLD acquisition. A easily quantized damage variable was set, and a non-linear and path-dependent evolution rule of damage variable was modified. The increment of the damage variable in small deformation range was obtained by integral calculation. The cumulative effect of damage variable on material fracture was considered. The damage threshold and the limit principal strain at fracture were derived. The nonlinear function between the damage variable and the strain was decided by the attenuation rule of the elastic modulus. The VUMAT was used to embed the modified Lemaitre damage model and acquired damage parameters into FE simulation, the punch stroke, limit thickness and fracture forming limit diagram(FFLD) were obtained. That were compared with the experimental results, simulation results without damage model and simulation results with classic Lemaitre damage model. Error comparison shows that, the aluminum alloy FLD can be exactly obtained using the modified Lemaitre damage model and FE simulation.5. Aluminum alloy sheet FLD real-time predictionThe influence law of aluminum alloy material parameters, forming performance parameters and loading conditions on FLD was explored using FE simulation. The limit strain points in generalized deformation conditions were gathered to realize the forming limit zone. The implementation method of simplified FLD was normalized using statistics rule based on the distribution of limit strain points. For aluminum alloy sheets with different material parameters and thickness, the nonlinear function was constructed to predict the simplified FLD. In order to realize the real-time prediction of aluminum alloy FLD, this paper selected a radial basis function network algorithm which was suitable for solving the high dimensional nonlinear problem. The FLD from the simulation results was set as sample data, the prediction model for aluminum alloy FLD was constructed, and the accuracy and applicability of this model was verified. The results show that, the prediction model could avoid heavy experiments and simulations, and simplify the aluminum alloy sheet FLD acquisition process.6. Application of the FLD in predicting the forming effect of aluminum alloy sheetThe FLD of aluminum alloy 5083-P-O sheet with thickness 4mm under the condition of low strain rate was predicted using the radial basis function network model, and the FLD was used with FE simulation to analyze the draw forming effects of the high-speed train curved covering panel, the crack location and crack length were compared with the results from thinning rate method and stress distribution method. The experimental validation was conducted on a combined type stamping die. The results show that, the crack location and crack length obtained from FLD method match the experimental results.