Experimental Assessment and Computational Modeling of Adhesive, Self Piercing Rivets (SPR), and Hybrid (Adhesive-SPR) Joints: Enhancing Joint Performance for Aluminum Sheet Material

Abstract

The implementation of aluminum alloys in future vehicle construction (e.g., body-in-white) can reduce weight by 30-45%, improving the fuel economy of internal combustion vehicles and increasing the battery range of electric and hybrid variants. Critical to enabling the adoption of aluminum alloys and multi-material structures in the automotive industry are robust joining methods that play a crucial role in structural performance; crashworthiness; durability; and noise, vibration and harshness. Structural adhesives and self-piercing rivets (SPRs) have demonstrated aluminum and multi-material joining capability with promising mechanical performance, relative to traditional and emerging joining technologies. Hybrid joining, augmenting adhesives with SPRs, has been explored to address the limitations of individual joining methods and to enable further weight reduction opportunities. However, the limited mechanical response data on hybrid joints and lack of validated computational models often lead to expensive and time-consuming experimental testing and over-design of the joint. In the first phase of this research, experimental studies were undertaken to assess the mechanical response of adhesive, SPR and hybrid joints made with aluminum sheet material at specimen (coupon) and component (vehicle-scale structure) levels. First, seven aluminum surface preparation methods were investigated, and the method that achieved cohesive failure within the adhesive layer and maximized single-lap shear joint (SLJ) strength was adopted throughout the study. Next, the specimen-level experiments investigated the influence of loading mode, joint-level morphology, aluminum sheet thickness, and alloy type on joint strength, stiffness and energy absorption. Shear (SLJ) and tension (H-specimens) test specimens were created with adhesive, SPR, and hybrid joints using two aluminum alloys (AA6061-T6 and AA5052-H32) with three sheet metal thicknesses (1, 2 and 3 mm), commonly used in the automotive industry. All test specimens were fabricated in symmetrical configurations (the same alloy type and equal sheet thickness) for a total of 108 specimens (36 specimens per joining method), with three repeats for each test condition. The morphology of the adhesive, SPR and hybrid joints was quantified to ensure joint consistency and link the effect of joint attributes (e.g., bond line thickness and SPR mechanical interlock) to the mechanical response. Hybrid joining process variations were assessed to enhance joint strength and stiffness of hybrid joints made with thick sheet material, and the process with improved joint-level morphology was evaluated in subsequent specimen and component level tests. Lastly, vehicle-scale structural components were created from two hat sections made with 3 mm thick AA6061-T6 alloy and joined together to create a tube. The Caiman tubes were joined using adhesive, SPR, and hybrid joining, and then tested under Mode I loading with three repeats for each joining method. SLJ and H-specimens with adhesive joining exhibited higher strength (up to 360%) and stiffness (up to 422%) compared to SPR joints, while SPR joints demonstrated higher energy absorption (up to 352%) co-depending on the loading mode, aluminum sheet thickness, and alloy type. Hybrid joints with 1 and 2 mm thick sheets enhanced the performance of the individual joining methods, demonstrating strength and stiffness comparable to or higher than the individual joints, and energy absorption substantially higher (up to 336% higher than adhesive and up to 53.5% higher than SPR). Hybrid joints made with 3 mm sheets exhibited reduced strength and stiffness relative to adhesive joints; however, a statistically significant performance improvement was realized using the hybrid joining process variation proposed in this study for thick sheet material. Importantly, hybrid joining substantially increased the peak load and energy of the Caiman components relative to adhesive joining (244% and 1461%, respectively), highlighting the importance of hybrid joining relative to adhesive for bonded structures under Mode I loading. In the second phase of this research, finite element (FE) models of the specimen configurations (SLJ and H-specimen), and joining methods were created using a cohesive zone model (CZM) with material-level properties for adhesives, constraint and CZM models with parameters calibrated in this work for SPR rivets, and an integrated CZM-CZM and CZM-Constraint models for hybrid joints. The specimen-specific FE models were verified and validated using the experimental data for each sheet thickness (22 models in total). Next, FE models of the Caiman test were developed to validate the joining models at a component level (5 models in total), using the experimental load-displacement response and optical measurements of failure progression and joint separation. Lastly, FE investigations were conducted to assess the influence of key joint attributes, relevant to automotive, on the mechanical response of the joints. The FE models of the adhesive, SPR and hybrid joints were able to predict joint response for varying test specimen geometry, adherend thickness, and modes of loading. The hybrid CZM-CZM model demonstrated a high level of accuracy and excellent computational efficiency, predicting the Caiman test peak load within 9.5%, and with reduced simulation runtime compared to the CZM-Constraint model. The results of this research study highlighted important parameters in terms of automotive structure design trade-offs (e.g., joining method, joint morphology, sheet thickness and alloy type), while statistical analysis provided evidence that hybrid joining can enhance the strength, stiffness and energy absorption relative to adhesive or SPR joining. The demonstrated multi-scale approach to develop, verify and validate joining models allowed for predicting the mechanical response of the individual and hybrid joints under different modes of loading. The results of this research study provide an experimental and computational basis for research and design of structural-scale joining methods for lightweight vehicles.