Study Of Modeling Techniques Of Weld-Bonded Joints Under Impact Loading

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.