| Using lightweight material and selecting suitable joining process are of both scientific and practical significance for ensuring joint strength, improving carbody manufacturing and further achieving automotive light-weighting. With the rapid development of structural adhesive, adhesive bonding is attracting increasing attention among domestic and foreign auto makers and has been practically used in many mainstream vehicles. However, there is still a mutual concern about the adhesive bonding process among automakers and researchers that how to make the structural adhesive strength meet the quality requirements, and to establish a unified evaluation standard of adhesive bonded joint strength.Currently, the joint strength is evaluated by its lap shear strength. However, there are lots of factors which would strongly affect the stress distribution within the adhesive layer and the ultimate strength of the joint, including adhesive type/thickness/width and steel grade/thickness and etc. Unfortunately, most of the published researches focused on the influence of a single factor on joint strength. Yet, it is still lacking of an effective model to quantitatively evaluate the comprehensive effect of both adhesive and steel properties on joint strength.To solve the aforementioned problems, this paper focuses on adhesive boned lap joint of dual phase steel. By making use of finite element analysis, the patterns of stress distribution in the joint during the lap-shearing process were investigated. The effect of adhesive / steel properties and process parameters on stress distribution and joint strength were analyzed as well. Experiments were carried out to verify the modeling results. Based on the finite element model, experimental design and response surface methodology, a mathematical model between the adhesive / steel properties / process parameters and joint strength was built. Effect of process parameters on joint strength was studied and an optimal set of process parameters was obtained. The main content in this dissertation contains three parts:(1) Finite element modeling of the lap-shearing process of adhesive bonded jointA finite element model of a single adhesive bonded lap joint was constructed. Based on that model, the stress distribution within the adhesive layer during lap shearing test was analyzed. The effect of process parameters on stress distribution and joint strength was investigated as well. The results showed that the adhesive layer wassubjected to both shear stress and peel stress during the lap shearing process. The properties of both adhesive and base metal would affect the lap-shear strength of the joint. While the lap-shear strength increased with steel gauge and bondline width, it decreased instead when the steel grade decreased or bondline thickness increased.(2) Experimental analysis of the lap-shear strength of adhesive bonded jointThe adhesive bonding experimental system were built to study the effects of different factors on the lap-shear strength of adhesive bonded joints, which included the width, thickness, yield strength of adhesive and the thickness, yield strength of base metal. It turned out that the adhesive width affected the lap-shear strength most significantly. The increasing of adhesive width would bring higher lap-shear strength, smaller hackle vein and less obvious Z-shaped fracture. Increasing of adhesive thickness would lead to lower lap-shear strength and bigger hackle vein. The base metal yield strength also showed a positive influence of the lap-shear strength.(3) Optimization of the process parameters for adhesive bondingUniform experiment design, response surface methodology was used to develop a mathematical model to analyze the interaction of the processing parameters and predict the strength of the joint. The critical values of the parameters were also calculated by the sequentialquadratic programming(SQP) method. Results indicated that the adhesive width, thickness and the base metal thickness were the main factors. According to the SQP method, optimum parameters were determined as adhesive width 18 mm, adhesive yield strength 29.4MPa, adhesive thickness 0.125 mm, base metal yield strength 897 MPa, thickness 2.112 mm. |
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