Edge Cracking Model Development Considering Forming History during Edge Stretching of AHSS and Experiment Verification

Author:Yu Xiang Yu

Supervisor:chen jun xia zhi yong xia zcedricchen shi


Degree Year:2016





Metal sheet stamping is a modern near net forming mode,and it is widely employed in the automotive industry.More and more advanced high-strength steels(AHSS)are employed because of the increasing demands of energy saving and environment protection.Edge cracking is one of problems in the application of AHSS.And the processing parameters of former procedure,such as cutting,also influence the results of following forming procedures.They make the stamping difficult to control and the quality of productions worse.In this paper,on the basis of theoretical analysis,hole-expansion and stretch flanging simulations and experiments,the failure during edge stretching considering punching(cutting)history is investigated deeply and systemically.The effects of material mechanical properties and geometry of edge on the damaged edge stretchability are studied.The main research contents and findings are briefly presented as follows:The simulation approaches of the hole-expansion based on the shell elements models considering punching history are studied.A modified method to obtain threshold value of Oyane criterion is employed.Based on the axisymmetric model,the simulations of punching-expansion process are conducted using ABAQUS/Explicit.By comparing the fractographs and simulation results,the applicability of Oyane criterion is verified.In order to combine the efficiency and accuracy during the simulations,residual damage values of the fractographs after punching are averaged and embedded in the hole edge using VUMAT subroutine as initial states of the blank in the models based on shell elements.In order to investigate the influence of the forming history on the edge stretching,an analytical method to evaluate effect of edge damage on the stretchability is proposed.This method is based on concept of damage in M-K model and Hill’s localized necking theory.By conducting extensive hole-expansion simulations,an index,Effective Failure Strain Ratio(EFSR),is developed to describe the degeneration of stretchability due to edge damage.In this method,by comparing the maximum circumferential strains under different damage values,the ratio of the effective edge failure strain under certain damage value to the theoretical edge failure strain can be obtained.And the influence of edge damage on the edge stretching can be investigated quantitatively.Based on EFSR,effects of different material properties and processing parameters on edge stretchability can be investigated.With the employment of EFSR,effect of hardening exponent(n-value in Power Hardening Law)of AHSS on the edge stretching performance is studied.Two grades of DP steels with similar microstructure,DP780 and DP980,are used to conduct simulations using ABAQUS/Standard.Results show that the degradation of edge stretchability of materials with higher hardening exponents is more sensitive to the edge damage.The influence of anisotropic coefficient of AHSS on the damaged edge during edge stretching is also investigated based on the analytical method and EFSR.Based on the assumption of transverse anisotropy,Hill48 yield criterion is employed to study the effects of anisotropic and damage zones location on the damaged edge stretching performance.Additionally,the independence of damage evolution and strain growing between different damaged locations on the edge during hole-expansion is firstly investigated.Results show that the degradation of edge stretchability of materials with lower anisotropic coefficient(r-value)is more sensitive to the edge damage,and the direction along which cracks are prone to appearing is the direction along which r-value is the lowest.The comparison between the experimental and numerical results shows that the proposed method is capable to predict a number of phenomena observed in experiments.A method to investigate deformation localization during irregular edge stretching is developed based on maximum circumferential strain.Influences of curvature variation and edge closed or open on edge stretchability in hole-expansion and stretch flanging are firstly investigated with the employment of a new index,Strain Localization Intensity Index(SLII).Results show that with the increasing of hole eccentricity,more severe strain localization causes edge stretchability falling more rapidly.The hole-expansion ratios and flanging limit heights both decrease faster when eccentricity is more than 0.5.It is also found that strain localization during stretch flanging is more severe than that during elliptical hole-expansion under the same eccentricity,and the difference between them is more than 3 orders of magnitude.In current work,the studies on hole-expansion simulation in 3-D shell elements models considering punching history benefit to predict failure combining efficiency and accuracy.On the basis of analytical method with EFSR,the simulation results about influence of hardening exponent and anisotropic coefficient on the damaged edge performance during edge stretching can explain phenomena effectively appearing in experiments.In a unique aspect of considering curvature variation,the effect of the strain localization because of the irregular geometrical parameters on the edge stretching performance is investigated.The achievements of this research provide suggestions on the selection of materials,die quality monitoring,design and CAE analysis of components production.