Advanced High Strength Steel (AHSS) are increasingly used in the automotive industry due to its higher strength and lower weight. The traditional forming limit criterion cannot accurately predict the unique shear fracture of AHSS, so great efforts have been made to develop failure criteria that can predict shear fracture. In this paper, a series of tensile and shear tests for four steel sheets of AHSS are designed, the stress triaxiality and equivalent strain are measured and solved, and the correlation between them and the performance parameters of steel sheets K and n is studied. In order to study the relationship between stress triaxiality and equivalent strain in the range of low stress triaxiality, the Hill'48 orthotropic model and MMC fracture model were used to establish tensile and shear fracture models of four dual-phase steels. Simulate and study the plastic fracture of AHSS. Solving the relevant parameters enriches the stress triaxiality of the four steel types, and establishes the relationship between the stress triaxiality and the equivalent strain, and verifies its correctness through tensile and bending tests and simulations. The results show that MMC can accurately predict the fracture of these four dual-phase steels, and the quantitative relationship between stress triaxiality and equivalent strain of the four dual-phase steels in the low-stress triaxiality range 0-0.3 is similar, which can be established and expressed by the performance parameters of each steel type.
The Forming Limit Stress Diagram (FLSD) can accurately describe the forming process of high-strength steel. However, obtaining FLSD is relatively difficult. In order to predict fracture in advanced high-strength dual-phase (DP) steels, limit maximum and limit minimum principal strains of sheet were obtained through multiple sets of test and simulation. Two material parameters, strength coefficient K and hardening exponent n are introduced into the FLSD function which is established by the strain-stress transformation function. The function shows that the k-value determines the value of the maximum principal stress, while the n-value affects the curvature of the curve. Verification of correctness by testing and simulation to within 10% accuracy. This paper explores a new approach to FLSD research based on material properties, which can expand the application scope of FLSD.
To predict the shear fracture, tests of advanced high-strength DP steels have been carried out, and fracture models of DP steels have been established using the MMC fracture model. The MMC fracture parameters were obtained through multiple sets of experiments and stress triaxiality solved by simulation. The result was verified by stretch-bending, Nakazima tests and simulations. It shows that the MMC criterion is suitable for predicting ductile fracture of DP980, 1180. The correlation between the parameters of the MMC criterion and DP steel material properties can reduce the amount of tests data required.
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