Crystal Plasticity Based Micromechanical Investigation Of Ductile Failure In An Aluminum Alloy

Ductile failure is an important inelastic behavior for metals in wide range of applications,e.g.,sheet metal forming and structural integrity assessment.It is well known that ductile failure is associated with micro-void nucleation,growth and final coalescence into cracks.As a rather complex phenomenon,the ductile failure of a material can be affected by many parameters,e.g.,stress state,void volume fraction,void geometry and size,constitutive material response,temperature,etc.Among those parameters,many studies suggest that the stress state is the most important factor to govern ductile failure.However,most studies are based on the phenomenological constitutive representation for the inelastic material response to investigate the micromechanical ductile failure.It is somewhat in lack of considerations of the crystal plasticity based deformation mechanisms for the micro-void development under multiaxial stress state.In order to gain more physical understandings into the micro-void governed ductile failure,the present work proposes a crystal plasticity based micromechanical finite element model to account for inelastic crystallographic slip in an aluminum alloy(Al 5083-H116).The micro-void growth at the single crystal level is simulated through the use of a periodic single crystal unit cell containing a spherical void.Structure mesh are chosen to discretize the unit cell and crystal plasticity model is used to represent the constitutive response of the matrix in the unit cell.The crystal plasticity model is calibrated by comparing Taylor-homogenization based modeling of polycrystalline response with the experimental data of the material.A truss is applied in the model(one end of the truss is connected to the unit cell and external load is applied on the other end)such that the stress triaxiality and Lode parameter can be controlled as constant.User subroutine of finite element analysis is then developed with optimized controlling parameters to achieve an effective control.Based on the unit cell model with void embedded,the present work systematically examines the mechanical response of the unit cell to quantify the effect of stress triaxiality,crystallographic orientation and Lode parameter on the void growth and coalescence.The results indicate that the increase of stress triaxiality reduces the void coalescence strain and the void growth direction also depends on the stress triaxiality.Significant effect of crystallographic orientation on the distribution of local stress and strain is identified.The present work also shows that Lode parameter can significantly influence the void evolution,in particular the void coalescence strain.This paper further examines the void growth behaviour for polycrystalline materials based on the so called Taylor-Reuss mean field homogenization method,which is used to connect the unit cell responses for a number of single crystals to the macroscopic response.The multiscale modeling predictions on the stress strain response for the material under multiaxial stress states are compared with the traditional micromechanical modeling,which uses phenomenological plasticity to represent the matrix of the unit cell,and the widely used macroscopic phenomenological constitutive model.The comparison verifies the proposed method and it further indicates that the crystallographic slip and its interplay with the developed micro-voids can lead to significant influences to the ductile failure.