摘要
部件的连接对整车的碰撞安全性和车身刚度有着至关重要的影响。胶焊技术作为一种新的连接方式,已经在汽车车身装配中得到越来越多的应用。但迄今为止,尚没有一种胶焊接头的有限元模型能够满足整车耐撞性分析的要求。本论文的目标是建立能够准确预报胶焊接头在碰撞载荷下变形和失效的模拟方法,并利用其分析胶焊接头中胶层与点焊的交互作用以及各设计参数对胶焊结构承载能力的影响,从而为胶焊车身结构的设计提供帮助。
胶焊结构可以认为是胶粘和点焊的累加,论文分别开发了胶粘接头和点焊接头的有限元模型,再将两者结合用以模拟胶焊接头。为了能够在整车耐撞性分析中模拟胶层的变形和失效,论文首先基于不同应变率下的胶粘剂材料拉伸试验数据,提出了可以表征胶粘剂应变率相关性的弹塑性材料模型,并建立了胶粘接头的有限元模型。胶层的失效参数通过准静态和动态下的简单结构件试验来确定。论文分别针对点焊接头的脆性和韧性失效模式,建立了点焊接头有限元模型,采用有限元软件LS-DYNA中的MAT_SPOTWELD_DA材料模型来预报焊核的失效,采用Gurson材料模型来模拟母材的失效。论文将胶粘接头模型以及点焊接头模型相结合来模拟胶焊接头,并通过胶焊剥离试验、搭接试验以及胶焊连接的直管轴向压溃试验对该模拟方法进行了验证。结果表明,所建立的胶焊接头模型能够准确预报胶焊连接结构的变形和失效,其计算效率和模拟精度均满足整车耐撞性分析的要求。
论文通过对胶焊简单结构件的模拟,研究了母材厚度、母材强度以及胶层厚度对胶层失效的影响,考察了点焊对于胶焊接头中胶层失效的止裂作用,并分析了胶层对点焊失效模式的影响。通过对胶焊连接的复杂结构件的模拟,讨论了引入胶层后,焊点数量对于结构承载能力的影响。论文以胶焊结构的轴向压溃、结构的三点弯曲以及T型构件的冲击为例,考察了各设计参数如胶层厚度、胶粘剂材料、母材厚度、母材材料等对结构承载能力的影响。在研究结果的基础上,对点焊连接的汽车前纵梁和胶焊连接的B柱进行了轻量化设计,达到了在保持结构承载能力的前提下减轻车身质量的目的。
Joining is crticial to vehicle crashworthiness and stiffness. As a new joining technology, weld-bonding is becoming increasingly common in the automotive industry. Toughed adhesives have become available and they significantly enhance bonding performance in vehicle bodies. However, due to the complexity of the combined use of toughed adhesive, spot weld and high strength steel, it is challenging to predict deformation and failure behavior of weld-bonded vehicle body structures under impact loading. The objective of this study is to develop an efficient modeling technique for crash analyses of weld-bonded vehicle body structures.
In this thesis, the modeling of weld-bonding is based on the no-coupling effect between spot-weld response and adhesive-bond response, revealed in a previous study. Firstly, a simplified finite element model was established for modeling of the toughened adhesive-bonded joint. To better predict the impact response of the adhesive, a user defined material model was developed for adhesive, in which the strain rate effect of both the pre-failure material properties and the failure criterion is characterized. The input failure parameters were identified with simulations of adhesive-bonded coupon tests. Then simplified models of spot-welded joint were developed for the nugget failure mode and pull-out failure mode, respectively. In the former case, the spot weld was represented by a single solid element, and the material model of MAT_SPOTWELD_DA in LS-DYNA is used to characterize its behavior. In the latter case, the failure of base steel was predicted by the Gurson model. The Gurson parameters were optimized from modeling several different test types of the base steel. The combination of the finite element models of the adhesive-bond and the spot-weld were used to simulate the weld-bonding. The developed model was used in the simulations of weld-bonded coach-peel, lap-shear and tube axial impact tests, serving as validation of the characterization capability and computational efficiency of the modeling techniques.
Based on the modeling of weld-bonded coupons, the influence of steel gage, metal grade and adhesive thickness on the failure of adhesive was studied, the crack-arresting effect of spot weld to the failure of adhesive was investigated, and the impact on the spot weld failure mode due to the involvement of adhesive was analyzed. Then the weld pitch effect was studied by modeling weld-boned structures. The simulations of weld-boned tubes under axial impact loading and impact bending, and weld-boned T-joints under impact loading were performed with different design variables, e.g., steel gage, steel type, adhesive thickness, adhesive type, to investigate the influence of the variables on the load-carrying capability of the structures. Finally a lightweight design scheme was put forwads for vehicle frontal rail and B-pillar. It is demonstrated that, by using weld-bonding, structural weight can be reduced without sacrificing the load-carrying capability.
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