Experimental and numerical characterization of adhesive joints with composite substrates

The use of adhesives in the last decade has grown substantially in various industrial fields, particularly in the marine, aerospace and automotive sectors. The constant research for lightweight commonly leads the industries to choose composite materials for developing more efficient products, due to their good mechanical properties and very low weight per unit of volume. Although composite materials present many advantages, they cannot be joined with traditional techniques, such as bolted joints, which involve drilled holes that are harmful to these materials, then adhesive joints are preferred. The adoption of adhesive joints leads also to a more uniform stress condition along the joint, moreover can prevent the corrosion of substrates with their sealant properties. In this thesis work, an experimental and numerical investigation on adhesive joint subjected to different loading conditions has been carried out. The aim of the thesis is to analyze the behavior of two different adhesives, used in the automotive industries, by means of U-shaped specimens made of carbon fibers. The adhesives are tested through the Arcan test, equipment widely used for testing welded joints, that allows testing the adhesive at different loading conditions. The test leads to the characterization of the adhesive joints under different loading angles, which in particular are 0° (pure shear loading condition), 45° (combined shear-tensile loading condition) and 90° (pure tensile loading condition). This is very important in the automotive industry since the adhesive joints are mainly designed to support shear loads while in real working cases, such as impacts, they can be subjected to combined loads. The tests aim to find the mechanical behavior of the adhesive joints in relation to the loading angles. Two adhesives are used to bond two sets of substrates (2 layers and 4 layers) with different thicknesses to show the correlation between the adhesive behavior and the stiffness of the substrates. The adhesives are a 2-component polyurethane adhesive which is characterized by a ductile behavior and a 2-component methacrylate that presents a fragile behavior compared to the polyurethane. In the first part of the activity, the substrates were prepared in the lab by using prepreg made of carbon fiber layers cured in the oven under vacuum. In the second part of the thesis adhesive joints were prepared and tested. Finally, a Finite Element Model was developed to study the mechanical properties of the joints. The numerical simulation has been carried out using the software LS-Dyna and was validated through iterative procedures comparing the numerical results with the experimental ones to obtain important information such as peel and shear stress in the inner section of the adhesive joints. The aim of the thesis (ongoing) is to assess whether and how the adhesive type and the stiffness of the substrates can influence the mechanical properties of the adhesive joints and define a failure envelope in a stress space based on the tested mixed-stress conditions.