Fracture Mechanism And Finite Element Simulation In Roll Forming Process

People pay more and more attention to energy conservation and environmental protection, the lightweight of automobile has become an inevitable trend of automobile industry. Advanced high strength steel(AHSS) has good strength and toughness, it can reduce the weight of car body, and at the same time improve the car crash performance by using this kind of steel. Therefore, the advanced high strength steel is widely used in automobile industry, which is of great significant to improve the safety of automobile and the development of lightweight of automobile.A lot of parts used in automobile can be produced by roll forming process. Roll forming is a kind of forming process of metal sheet, it is very complicated which contents the knowledge of geometry, kinematics, dynamics and material science. Therefore, roll forming discipline is a very strong empirical engineering technology. With the increase of strength, the toughness of AHSS will reduce, which leads to the fact that it’s more likely to fracture for AHSS during roll forming process.Advanced high strength steel DP980 is studied in this paper. In order to predict the fracture of AHSS in roll forming process and draw conclusions about it, many approaches are used in this paper, including the conduct of the basic tensile test of AHSS, the establishment and application of constitutive model involving strain rate and temperature rise effect, the establishment and application of ductile fracture criterion, and the establishment of finite element model.The uniaxial tensile tests, plane strain tests, and shear tests of AHSS are conducted under five different kinds of strain rate based on the metallic materials tensile testing at ambient temperature to obtain the basic mechanical parameters of DP980. During the test, the temperature of deformation zone of specimen is measured in order to get the relation between strain rate and temperature rise. The material parameters are applied to the finite element model of tensile test, then the results of simulations and tests are compared to verify the reliability of the obtained material parameters and the finite element model.Based on the experimental data, a constitutive model involving strain rate and temperature rise effect which is suitable for DP980 is established. Then the constitutive model is applied to the finite element models of basic tensile tests. The results of tests and simulations are compared to verify the reliability of the constitutive model.In this paper, an U-type bending roll forming device with 80°bending corner is designed, and the experiment of roll forming with three bending corners is conducted. Experimental results show that the fracture of DP980 only occurs in the 180°bending corner. Then the finite element models of these two roll forming process are established.The ductile fracture criterion which is suitable for DP980 is established, then it is embedded into finite element software. The results of simulation and experiment show that the established ductile fracture criterion can predict the fracture of DP980 in roll forming process precisely.