双相钢车身板成形的细观断裂判据研究

发布时间：2018-03-10 12:21

本文选题：板料成形　切入点：先进高强度双相钢　出处：《山东理工大学》2016年硕士论文　论文类型：学位论文

【摘要】：汽车的轻量化使得人们越来越多地寻找新材料,以满足轻量化后汽车安全性等方面的要求。由于双相钢良好的力学性能和成形性,被逐渐应用于汽车车体的各种梁结构。但是,双相钢在成形时易发生剪切断裂现象,仅仅从宏观唯象的角度研究双相钢的剪切断裂失效准则具有一定的局限性,细观损伤理论把材料的微观损伤过程和宏观断裂过程联系起来,能更深入理解材料破坏的过程和内在的物理本质,成为近年来研究热点。本文利用GTN(Gurson-Tvergaard-Needlman)及扩展GTN细观损伤模型研究双相钢的不同断裂方式,建立了双相钢断裂失效的细观判据。首先,构建了Hill’48-GTN细观损伤模型,推导相应的本构方程。结合显式有限元法,利用Fortran语言编程实现了Hill’48-GTN模型VUMAT用户材料子程序。其次,针对目前GTN模型较难获得精确损伤参数的问题,提出一种新的基于幂指数材料硬化模型的损伤参数优化方法。利用试验设计的方法选取不同损伤参数组合,建立损伤参数与硬化参数之间的关系,在建立评价函数时采用材料的固有基本参数,不需要寻求新的变量,可以获得较为精确的损伤参数,并采用此方法确定了DP780双相钢的各损伤参数。再次,针对双相钢发生的剪切断裂现象,在原孔洞体积分数增量中引入一项剪切损伤量,将剪切变形对损伤的贡献等效为孔洞体积分数的增长。对拉伸和单拉剪切试样分别采用引入剪切损伤量和无剪切损伤两种不同孔洞体积分数增长方式进行仿真,证明了在纯剪切状态考虑剪切损伤的孔洞体积演化能更好地预测材料的失效,但在较高应力三轴度时预测的损伤值偏高。最后,利用极限剪切塑性断裂应变定义剪切损伤量,并且将此损伤量引入GTN本构方程中。通过设计单拉剪切试验,获得材料失效时剪切损伤与宏观场量应力三轴度的关系,利用最小二乘原理拟合出剪切损伤极限值与应力三轴度的函数关系。根据剪切损伤极限值及韧性孔洞体积分数临界值与应力三轴度的关系建立了双相钢断裂的细观判据。仿真了不同圆角半径下拉弯成形的断裂结果,证明判据的正确性,最后模拟了地板梁冲压成形过程的断裂现象。
[Abstract]:The light weight of automobile makes people look for more and more new materials to meet the requirements of automobile safety after lightweight. Due to the good mechanical properties and formability of dual phase steel, it is gradually used in various beam structures of automobile body. The phenomenon of shear fracture is easy to occur in the forming of duplex steel. It is limited to study the failure criterion of shear fracture of dual phase steel only from the macroscopic phenomenological point of view. The microscopic damage process and the macroscopic fracture process are connected by the meso-damage theory, and the damage process and the intrinsic physical nature of the material can be better understood. It has become a hot research topic in recent years. In this paper, different fracture modes of dual-phase steel are studied by using Gurson-Tvergaard-Needlman (Hill'48-GTN) and extended GTN meso-damage model, and the meso-criterion of fracture failure of dual-phase steel is established. Firstly, the meso-damage model of Hill'48-GTN is constructed. The corresponding constitutive equation is derived, and the VUMAT user material subprogram of Hill'48-GTN model is realized by using Fortran language with explicit finite element method. Secondly, it is difficult to obtain accurate damage parameters for GTN model at present. A new damage parameter optimization method based on exponential material hardening model is proposed. The relationship between damage parameter and hardening parameter is established by selecting different damage parameter combinations by means of experimental design. When the evaluation function is established, the inherent basic parameters of the material can be obtained by using the basic parameters of the material without the need to seek new variables, and the damage parameters of DP780 duplex steel can be determined by using this method. Aiming at the shear fracture of duplex steel, a shear damage quantity is introduced into the increment of volume fraction of the original pore. The contribution of shear deformation to damage is equivalent to the increase of void volume fraction. It is proved that the pore volume evolution considering shear damage in the pure shear state can better predict the failure of the material, but the predicted damage value is higher at higher stress triaxiality. Finally, the shear damage is defined by the ultimate shear plastic fracture strain. The damage amount is introduced into the GTN constitutive equation. The relationship between the shear damage and the triaxiality of the macroscopic field stress is obtained by designing the single tensile shear test. The function relationship between shear damage limit value and stress triaxiality is fitted by using the least square principle. According to the shear damage limit value and the critical value of ductile pore volume fraction and stress triaxiality, the fine fracture of dual phase steel is established. The fracture results of tensile bending forming with different radius are simulated. The validity of the criterion is proved. Finally, the fracture phenomenon in the forming process of the floor beam is simulated.
【学位授予单位】：山东理工大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：U463.82

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