Study On The Mesoscopic Mechanism Of Fracture Failure Of Advanced High Strength Dual-Phase Steel Sheet During Warm Forming

Advanced high strength dual-phase steel has many advantages such as good energy absorption and high yield strength,which can balance crash safety and light weight,and it is one of the main materials to realize the light weight of automobiles.Compared with commonly used body panels,advanced high-strength dual-phase steel has poor plasticity and low ductility at room temperature.It is prone to process defects such as rupture and wrinkling during the stamping process.In order to improve the plastic properties of the body sheet,the warm stamping technology is applied to the stamping and forming process of advanced high strength dual-phase steel sheet.The warm forming technology is to control the temperature below the recrystallization temperature of the sheet and above the room temperature to improve the plasticity of the material and make it have good elongation and formability.The damage of advanced high strength dual-phase steel sheet under warm forming conditions is not sufficiently studied,because the stress state of the material in the plastic deformation zone is extremely complex during the forming process,especially in the case of warm forming.The theory of detailed observation damage uses average method to connect the micro-damage process of the material with the macro breakthrough process,which helps to more deeply understand and judge the process of material destruction and the internal physical nature.Therefore,it is necessary to carry out a systematic study on the failure mechanism of fine damage of materials in the warm forming process.In this thesis,the fine damage mechanism of warm forming of advanced high strength dual-phase steel sheet is studied by GTN fine damage model and extended GTN fine damage model,the damage parameters of advanced high strength dual-phase steel sheet at different temperatures are calibrated,and the fracture failure behavior of advanced high strength dual-phase steel sheet at different temperatures and different stress states are investigated.The main tasks of this article are:(1)On the basis of generalized plastic forming theory and fine view damage theory,Hill’48-GTN fine view intrinsic structure model is constructed and implemented by ABAQUS VUMAT subroutine by combining Hill’48 criterion;the unidirectional tensile tests of four kinds of dual-phase steel sheet materials at different temperatures are designed,and the material property parameters of the sheet materials are obtained by analyzing the test results.(2)Since the damage parameters of the GTN model may have various combinations of values,the numerical simulation was combined with the central composite test design and the response surface method to obtain the regression equations related to the damage parameters of the four kinds of sheet materials at different temperatures.The fine-scale damage parameters of the four kinds of sheet materials at different temperatures were obtained optimally by using the genetic algorithm,and the variation law of the void damage parameters of the four kinds of sheet materials with temperature was studied to establish the relationship among the damage parameters,temperature and material properties was established to reduce the difficulty of calibrating the damage parameters using experiments.(3)By designing temperature dependent shear tests for four types of dual-phase steel sheets,the shear damage parameters at different temperatures were calibrated by using the Extended GTN damage model.Explored and analyzed the damage evolution law of dual-phase steel sheet during the process from low stress triaxiality to high stress triaxiality under warm forming,particularly focusing on the threshold of shear damage transition from low stress triaxiality to high stress triaxiality pore damage,and constructed an expression for the relationship between the damage threshold and material properties and temperature.A competitive fracture failure model for warm forming of dual phase steel sheets was established,and the effectiveness of the established competitive fracture failure model was verified through warm tensile bending tests of four types of dual phase steel sheets.(4)By designing temperature-dependent expansion tests for four types of dual-phase steel sheets,numerical simulations were conducted using the established fine-scale damage model,and temperature-dependent forming limit curves for dual-phase steel sheets were established based on the obtained rupture failure principal strains.The relationships between the forming limit curve expressions and temperature and material properties were investigated.The validity of the established forming limit model was verified by using stamping tests of box-type parts at different temperatures.