Characterization of the Structural Response of Adhesively Bonded Ultra-High Strength Steel Tubes Under a Range of Loading Conditions and Assessment of a Rate Dependent Cohesive Zone Model

Weight reduction through the use of adhesive joining in multi-material lightweight structures requires material characterization and substructure level model validation to support CAE design. In this study, automotive-scale structural tubes were created by adhesively joining tailored hot stamped ultra-high strength steel hat sections using a two-part toughened epoxy adhesive applied to the flanges. A custom fixturing method was developed to achieve consistent bond line thickness for the adhesive joint. The physical tubes were tested in three-point bend, axial crush, and Mode I loading at quasi-static and dynamic loading rates, from which the structural response and failure characteristics were established. The experiments were modeled numerically using a previously developed cohesive zone method (CZM) that had been validated for coupon level tests. In the current work, the CZM model is assessed under structural loading conditions, based on predictions of load-displacement response, peak load, energy absorption, displacement-to-failure, and deformation pattern. In addition, crack extension along the adhesive joint was assessed for the Mode I loading condition. The novel bonding procedure developed for this study resulted in consistent experimental loading response. Generally, the predicted results agreed with experimental results, particularly for the Mode I loading and crack extension behavior. However, the CZM model was not able to accurately predict displacement-to-failure for the three-point bend tests, owing to out-of-plane buckling observed in the experiments. With a few exceptions, the CZM adhesive model based on coupon-level data was able to predict the peak force, displacement-to-failure, and energy absorption of the bonded structural assemblies to within 16% of the average experimental responses.