Study Of Influential Factors And Prediction Model About Edge Cracking Of Advanced High Strength Steel

Advanced High Strength Steel(AHSS) has been increasingly used in automotive industry for it completely conforms to the objectives of current automobile design which mainly aims at improving body lightweighting and crashworthiness. Edge cracking is one of the primary forming concerns in AHSS stamping. The limit strains of the edge are hard to predict by traditional methods because they are influenced by many factors such as material property, process parameters, edge quality and so on. As a result, more deep investigations into the edge cracking and more effective prediction models still need further development for parts production.This research is based on National Natural Science Foundation project(No. 51105246). The experiments(i.e. pre-forming test, hole-expansion test and uniaxial tension test under different temperatures) and relevant simulations were conducted to investigate the effects of pre-strain, curvature and temperature on edge stretchability. Three fracture criteria were utilized in Deform to verify the experimental results and also had a comparison about the prediction accuracy. A new simulation model was proposed to improve the predictability on edge cracking with the utilization of Abaqus. The conclusions are as follows:(1) Pre-forming test and hole-expansion test were conducted to investigate the effects of pre-strain on edge stretchability. The experimental result revealed that pre-strain would lead to the degradation of edge stretchability. The larger the pre-strain was, the worse the edge stretchability was.(2) Hole-expansion test was conducted under three hole diameters to investigate the effect of curvature on edge stretchability. The experimental result revealed that the edge showed worse formability at the position with larger curvature.(3) Three fracture criteria, i.e. Rice-Tracey, Brozzo and Ayada were used to predict the fracture in hole-expansion test with different hole diameter. The comparison between simulated and experimental results showed that the Ayada model performed the best predictability for DP780 and Rice-Tracey model was a relative proper fracture criterion for DP980.(4) Uniaxial tension test was conducted to obtain the stress-strain curves of DP780 under different temperature. An improved material model was proposed for blanking simulation. Four different simulations were conducted under different material models and die temperatures. The simulated results showed that the temperature of deformation zone would had an obvious rise during blanking. However, the temperature of die caused little effect on thin sheet blanking.(5) A new model was proposed for the hole edge with pre-damage and improves the predictability of edge cracking in hole-expansion simulation. This new model was based on Marciniak and Kuczynski(M-K) model and Hill’s localized necking theory to predict the edge limit strain. The problem that the traditional ideal model failed to predict the hole expansion ratio of pre-damage edge was also successfully solved by the new model with the use of the Damage Zone and Effective Failure Strain Ratio(EFSR).(6) A large number of experiments had been done and proved that DP980 had better hole-expansion performance than DP780. During holeexpansion test, the cracks were more prone to occur at the sheared edges in the 45° directions with respect to local tangential direction, which was similar to uniaxial tension test. On the other hand, the cracks were inclined to occur in the rolling direction.