Min Wan

Effect of flow stress—strain relation on forming limit of 5754O aluminum alloy

A modified Swift type flow stress—strain relation was presented in order to describe the uniaxial tension test curve reasonably. The FLD-strain (forming limit diagram made up of limit strain) of 5754O aluminum alloy sheet was calculated based on the two flow stress—strain relations using Yld2000-2d yield function. By comparing the theoretical and experimental results, it is found that the calculated FLD-strain based on the modified Swift flow stress—strain relation can reasonably describe the experimental results. However, though the common Voce flow stress—strain relation can describe the deformation behavior during homogenous deformation phase accurately, the FLD-strain calculated based on it is obviously lower than the experimental result. It is concluded that the higher the hardening rate of sheet metal is, the higher the forming limit is. A method for determining the reasonable flow stress—strain relation is recommended for describing the material behavior during inhomogenous phase and the forming limit of sheet metal.

FEM equivalent model for press bend forming of aircraft integral panel

Abstract An original plastic equivalent model was proposed to solve the problem of excessive FEM simulation time when designing the press bend forming path and optimizing the process parameters of press bend forming of the integrally stiffened aircraft panels. Based on the in-depth analysis of the mechanics of the bending and springback of the detailed model and the equivalent model of the integral panels, the plastic equivalent model of the virtual material with special initial yield stress and hardening coefficients was constructed. FEM results indicate that the objective of getting the similar contour with the same press bend forming path is achieved with the error less than 6%, and the efficiency of FEM simulation is improved by more than 80%. The plastic equivalent model is valuable and essential for the further research on the press bend forming process of large scale complicated integral panels.

Prediction of stiffener buckling in press bend forming of integral panels

Abstract In order to predict the buckling of stiffeners in the press bend forming of the integral panel, a method for solving the critical buckling load of the stiffeners in press bend forming process was proposed based on energy method, elastic-plastic mechanics and numerical analysis. Bend to buckle experiments were carried out on the designed press bend dies. It is found that the predicted results based on the proposed method agree well with the experimental results. With the proposed method, the buckling of the stiffeners in press bend forming of the aluminum alloy integral panels with high-stiffener can be predicted reasonably.

Wrinkling prediction in rubber forming of Ti-15-3 alloy

Wrinkling is a common failure in the sheet metal forming of titanium owing to the relatively poor ability to shrink. It is important to predict wrinkling accurately in the sheet metal forming without costly trials. The ABAQUS/Explicit code was utilized to predict the wrinkling behavior in the sheet metal forming of Ti-15-3 alloy sheets. In terms of the comparison of wrinkling behavior between the simulation and experiment of the Fukuis conical cup tests at room temperature, the sensitivities of wrinkling simulation to various input parameters were evaluated comprehensively and quantitatively. Prediction of wrinkling and influence of rubber hardness on the winkling behavior in the rubber forming of convex flange were investigated quantitatively and validated by the rubber forming experiments. The excellent agreements between the simulations and the experiments confirmed the accuracy of the prediction.

Friction coefficient in rubber forming process of Ti-15-3 alloy

The forming limit diagram of Ti-15-3 alloy sheet was constituted at room temperature. The effects of different punch and rubber hardness on the limit principal strain distributions were investigated experimentally. Finite element analysis models of the samples with dimensions of 180 mm×180 mm were established to analyze the friction coefficients of different interfaces. Effects of various friction coefficients on the strain distributions were studied in detail. Finally, the friction coefficients in the cold forming were determined by contrasting the strain results between the experimental data and the simulated ones.

Optimization of press bend forming path of aircraft integral panel

Abstract In order to design the press bend forming path of aircraft integral panels, a novel optimization method was proposed, which integrates FEM equivalent model based on previous study, the artificial neural network response surface, and the genetic algorithm. First, a multi-step press bend forming FEM equivalent model was established, with which the FEM experiments designed with Taguchi method were performed. Then, the BP neural network response surface was developed with the sample data from the FEM experiments. Furthermore, genetic algorithm was applied with the neural network response surface as the objective function. Finally, verification was carried out on a simple curvature grid-type stiffened panel. The forming error of the panel formed with the optimal path is only 0.098 39 and the calculating efficiency has been improved by 77%. Therefore, this novel optimization method is quite efficient and indispensable for the press bend forming path designing.

FEM modelling for press bend forming of doubly curved integrally stiffened aircraft panel

In order to establish an FEM model for aircraft integral panel press bend forming process, a special simulation procedure and a calculation method for the punch and die boundary condition based on the bending line coordinates were proposed. The simulation of a seven-step press bend forming process of doubly curved integrally stiffened aircraft panels was realized, and it could well simulate the real fabrication process, so that it could assist in studying this complicated forming process. Stress and strain distributions were analyzed, which reveals the deformation mechanics of this process. With quantitative comparisons, it can be concluded that forming quality of the seven step press bend forming is quite good, considering both the forming precision and the surface quality.

A novel forming method of aluminum sheet based on superposition principle of electromagnetic local forming

A novel forming method aimed at large size aluminum sheet is introduced. In this method, aluminum sheet is clamped between a common flat spiral electromagnetic actuator and a punch matrix. Driven by pulsed electromagnetic force and restrained by punch matrix, the blank sheet will form local shallow dome accordingly. Moving actuator along sheet surface and triggering the pulsed power generator in sequence results in dome matrix that is uniformly distributed. Superposition of those spherical domes leads to macroscopically curved figuration of aluminum sheet. This paper demonstrates the newly proposed method experimentally, which verifies its feasibility. The mechanism analysis of this method is also presented using a simplified analytical model. The results show that this novel method is feasible and can be explained by the proposed mechanism well.

Investigation into Influence of Pre-Forming Depth on Multi-Stage Hydrodynamic Deep Drawing of Thin-Wall Cups with Stepped Geometries

Hydrodynamic deep drawing (HDD) is an effective method for manufacturing complicated and thin-walled parts. Aiming at the forming process of the stainless steel part with 0.4 mm thick and complex stepped geometries, the technology scheme of multi-stage HDD assisted by conventional deep drawing (CDD) is proposed in consideration of wrinkling and destabilization in the unsupported region of the conical wall, and finite element models are built. As a key process parameter, pre-forming depth on the quality of the parts is explored with assistance of numerical simulations and verification experiments. Furthermore, the failure modes, including wrinkling and fracture during forming process are discussed; meanwhile, the optimum pre-forming depth is realized. The results indicate that the technological method is proven to be feasible for integral forming of thin-walled parts with a large drawing ratio and stepped geometries; moreover, the parts with uniform thickness distribution and high quality are successfully formed by adopting optimum pre-forming depth.

Subsequent yield loci of 5754O aluminum alloy sheet

Complex loading paths were realized with cruciform specimens and biaxial loading testing machine. Experimental method for determining the subsequent yield locus of sheet metal was established. With this method, the subsequent yield loci of 5754O aluminum alloy sheet were obtained under complex loading paths. Theoretical subsequent yield loci based on Yld2000-2d yield criterion and three kinds of hardening modes were calculated and compared with the experimental results. The results show that the theoretical subsequent yield loci based on mixed hardening mode describe the experimental subsequent yield loci well, whereas isotropic hardening mode, which is widely used in sheet metal forming fields, predicts values larger than the experimental results. Kinematic hardening mode predicts values smaller than the experimental results and its errors are the largest.