Research And Application On A Ductile Fracture Model For Aluminum Alloys Based On Void Evolution

Aluminum alloys,as a lightweight material with high strength-to-weight ratio,impact resistance and recyclability,have been increasingly employed in many fields,such as automobile manufacturing.However,aluminum alloy tends to crack without diffuse instability and localized instability due to its weak formability at room temperature.As a result,ductile fracture for aluminum alloy cannot be predicted by traditional forming limit curve(FLC)based on tensile instability,which cannot also predict shear-induced ductile fracture behavior.It is urgent to establish a ductile fracture model to predict damage failure over a wide range of strain path for aluminum alloys.In addition,high-speed forming technology can improve the formability of aluminum alloys at room temperature,resulting in different forming limits and ductile fracture behavior at various strain rates.Therefore,the effect of strain rate should be considered in the ductile fracture model to enable it to be applied to dynamic impact field.In view of above problems,the ductile fracture prediction model is deeply studied,providing theoretical guidance for accurately describing the fracture behavior under quasi-static and dynamic loading.The main research contents and conclusions are as follows:Aluminum alloy 6016-T6 was selected to conduct ductile fracture experiments on six specimens with different shapes,which can cover a wide range of stress state from compression-shear to balanced biaxial tension.Fracture occurrence,as well as fracture initiation and stress state distribution for each specimen,was acquired through a robust hybrid numerical-experimental approach.Combined microscopic fracture morphology with the stress state of various paths on fracture surfaces of specimens,the behavior of void growth under different stress state was quantitatively and qualitatively analyzed,demonstrating that void volume/dilation and void shape/elongation were influenced by hydrostatic stress and maximum shear stress,respectively.The behavior of void coalescence under various stress state was qualitatively analyzed,illustrating that voids tended to link-up along the direction of maximum shear stress.Based on the micro-mechanism of void nucleation,growth and coalescence,an uncoupled ductile fracture prediction model was proposed considering hydrostatic stress and maximum shear stress.Then,the effect of model parameters on 3D ductile fracture surface and forming limit curve was studied.The proposed model was compared with three other typical uncoupled ductile fracture models for different materials in order to illustrate the applicability.The experimental and theoretical forming limit curves were obtained through Nakajima tests and the proposed model.Fracture initiation and fracture pattern were captured by employing the VUMAT subroutine of the proposed model with a prediction error of 4.67%during stamping process.Finally,the shear-induced ductile fracture behavior was predicted by using forming limit damage criterion based on the proposed model.Uniaxial tension tests for AA 6016-T6 were conducted under different strain rates(0.1-100/s).The effect of strain rate on mechanical properties was investigated,demonstrating that fracture strain of AA 6016-T6 was sensitive to strain rate.A modified Johnson-Cook constitutive model was established by integrating the strain rate hardening coefficient with the strain rate.VUMAT subroutine of this model was developed.The prediction performance of this model was verified by using high-speed tension tests and finite element simulations,founding that it can more accurately predict the deformation behavior of materials under different behavior.The above research provided a foundation for the subsequent dynamic simulation.High-speed tension tests and corresponding finite element simulations were conducted for five specimens with different stress state at strain rates of 0.1,1,10 and 100/s,respectively.Ductile fracture data for specimens was determined at each strain rate.The effect of strain rate on macro-and micro-fracture morphology was analyzed,demonstrating that the increased strain rate was beneficial to improve the plasticity of materials.The relationship between model parameters and strain rate was investigated,and the model parameters were converted from a constant to a function of strain rate.Consequently,the dynamical ductile fracture model was further developed,which comprehensively reflected the relationship among void evolution,stress state,strain rate and fracture strain.Finally,the impact bulging fracture behavior of aluminum alloys was studied based on the proposed model related to strain rates.