Study Of Ductile Fracture Based On Mesodamage Mechanisms

The development of modern industry promotes the researches on materials failures.Ductile fracture preceded with large amounts of plastic deformations constantly occurs in the engineering structures of metallic materials.With the advances in engineering design philosophies,there has been an increasing demand for better understand of ductile failure mechanisms and more accurate prediction of the ductile behaviors.Mechanism-based approach,which investigates the interactions between stress/strain filed and mesoscopic structures such as voids and defects,is playing a role to reveal the insights of ductile fracture and establish mechanism-based damage models and has been one of the most promising approaches for perdicting ductile failures.However,the Gurson-Tvergaard-Needleman(GTN)model,one of the most referenced mechanism-based porous plastic damage model,still suffers limitations.As well known,it is incapable of dealing with shear dominated loadings and lack of a failure criterion that is dependent on stress states and mesoscopic structure characteristics.The aim of this thesis is to propose new modifications to the GTN model,develop the relevant numerical algorithms and parameter calibration procedures,so as to make it possible for accurately modeling ductile fracture under a broad range of stress states.Firstly,void-based ductile fracture mechanisms are investigated in detail.Two stress state dependent mechanisms,internal-necking and void shearing,are summarized out of literatures and verified by 2D and 3D cell modeling and analysis.Cell calculation results also show that the two failure mechanisms are related to different mesoscopic structural paramters,void volume fractions and void elongation ratio seperately,and should be characterized by different damage parematers in plastic constitutive relationships.So it is necessary to establish stress state dependent damage evolution laws and failure criteria.Effects of initial porosity,stress traxiality and Lode angle on ductile fracture have also been quantitively studied.The above conclusions are then used in the subsequent work of this thesis.Next,void-based damage models have also been studied and improved.(1)Aware of the drawbacks of the original GTN model and its shear extensions by Nahshon and Hutchinson as well as by Xue,a new modification to the GTN model is proposed for the simulation of ductile fracture behaviors under high,low and negative stress triaxiality loadings.Distinctive damage parameters,respectively related to internal-necking and void shearing,are introduced into yield function as internal variables of degradation process.The void volume fraction,similar to the original GTN model,is still adopted as the volumetric damage arising mainly in tension to characterize void nucleation,growth and subsequent inner-necking.The additional shear damage,due to void rotation,distortion as well as “void sheet” effects,can accumulate in other stress states,even with negative stress triaxiality,with the auxiliary of a new stress–state dependent function.(2)Based on the 3D cell simulation,a macroscopic equivalent strain criterion for 2524-T3 aluminum alloy is established.Thereby new damage models,GTN-E and GTN-L,are proposed by incorporating the GTN model with the criterion and the Thomason's plastic limit-load criterion repectively.In these models,the start of void coalescence is determined by both criteria that take the effects of stress states and mesoscopic structure characteristics into account rather than by a material constant.In the research of elastic-plastic behavior of ductile materials,another important work is the development of appropriate numerical integration algorithm.The two-damage-parameter GTN model is taken as an example to illustrate the procedures of integrating the rate-form pressure sensitive constitutive equations using an implicit Euler method in combination with the return mapping algorithm.An explicit formulation for the consistent tangent matrix is derived based on the frame of the returning mapping algorithm.Since no matrix inversion is involved,the computation efficiency has been improved.The damage constitutive models are implemented into the commercial codes,ABAQUS/Standard and ABAQUS/Explicit,via user defined material subroutines UMAT and VUMAT respectively.Finite models with one single element under tensile,shear and compressive boundary conditions are used to test the developed material constitutive relationships as well as the numerical integration algorithm.In the end,the proposed damage models are applied to study the ductile failures in 2024-T3 and 2524-T3 aluminum alloy.(1)A series of tests under various stress states for 2024-T3 aluminum alloy,including the smooth round bar(SRB),notched round bar(NRB),double grooved plate(DGP)for tension,short column for compression,and thin–wall tube for torsion,are performed to verify the predictions of the overall elastic–plastic response and the crack propagation paths by the two-damage-parameter GTN model.The results indicate that the current model performs pretty well in predicting ductile fractures of various specimens under stress states covering a wide range of triaxiality and Lode parameter combinations.(2)Thinplate specimens of 2524-T3 aluminum alloy with pre-existing multiple cracks are tested under tensile loading.The GTN-E and GTN-L damage models are adopted to simulate the multiple crack growth,coalescence and final fracture.It is shown that the proposed models could be efficient tools for the residual strength assessment of pre-cracked sheets.While predicting ductile fractures in both two materials,special attention is paid to the determination of the model parameters,which considerably affects the simulation accuracy.