Simulation Research On Ductile Fracture In Multi-point Forming Of Aluminum Alloy Integral Panels

High-stiffened Al integral panel has been widely used in the modern aircraft and high-speed vehicles due to its advantages of light weight,high specific strength and good rigidity.Its forming quality is an important sign to measure the level of aeronautical manufacturing technology.The integral panel is usually formed by incremental bending,creep age forming and shot peen forming with long production cycle and poor forming precision.As a flexible forming method,multi-point forming of integral panel is an innovative application.However,the web and stiffener are prone to fracture when deformation is large,which will lead to the discard of whole slab.The aim of this paper is to predict fracture in multi-point forming of integral panels and propose measures to restrain it.The main contents and conclusions are summarized as follows:1.Fracture prediction of aluminum alloy with different stress statesThe ductile fracture criterion which has taken tensile and shear effects into consideration was selected.Stress triaxiality was chosen as the characterization parameter of stress state and four kinds of tensile specimens with different notch sizes were designed.The distribution of stress triaxiality was analyzed by the numerical simulation and experiments.Fracture location and fracture displacement of different specimens were predicted based on the ductile criterion.Compared with experiment results,the simulation results showed that ductile fracture criterion has high prediction accuracy in the whole range of tensile and shear stress state.2.Fracture prediction in multi-point forming of integral panel specimensThe FEM models of multi-point foming were established and simulations for the I-stiffener,T-stiffener,cross stiffener,parallel stiffener and JING-stiffener specimens were conducted.Fracture position and fracture moment on five kinds of specimens were predicted based on the ductile criterion.And also multi-point press bending experiments were conducted.It is found that the fracture position is consistent with that in experiments.The error of punch displacement at fracture moment is less than 10% compared with experiment results.The fracture reasons of different stiffeners were analyzed based on ductile fracture mechanics and stress triaxiality.The results showed that fracture of I-stiffener and cross stiffener were caused by tensile stress,fracture of T-stiffener,parallel stiffener and JING-stiffener were caused by shear stress.3.Fracture control methods of integral panel specimensThe influences of elastic pad types and deformation on fracture of five specimens were discussed,such as I-stiffener,T-stiffener,cross stiffener,parallel stiffener and JING-stiffener.The stiffener specimens were easy to fracture when deformation is too large and cannot be formed in one step.Therefore,multi-step multi-point foming and continuous load forming methods were adoped to solve above problem.It is found that the more forming steps,the less risk of fracture occurred.But too many forming steps will lead to work hardening.The integral values of specimens formed in the path of equidifferent curvature were less than those in equidifferent radius.Continuous load forming with adjustable upper and lower punch paths can effectively reduce fracture risk and delay fracture time of integral panel specimens.4.Effects of forming parameters on integral panels fractureThe effects of forming parameters,such as forming methods,punch sizes and punch arrangements on fracture were analyzed by FEM simulation.The results showed that multi-point press bending can disperse the concentrated load and reduce fracture risk of integral panels.Moreover,the integral panel has less risk to fracture when punch sizes become smaller.The unsymmetrical multi-point dies can restrain fracture and improve forming quality of integral panels effectively.