Investigation on the failure behaviour of adhesively bonded composite joints by using the Arcan test

The aim of this work is to study the behaviour of two different adhesives, used in the automotive industries, by means of U-shaped substrates made of carbon fibres. Such a geometry is taken from a configuration widely used for testing spot-welded joint specimens. These adhesive joints were loaded until failure by means of the Arcan device, that allows testing the adhesive under different loading conditions. In particular, three loading angles were considered, namely 0° (pure shear loading), 45° (combined shear-tensile loading), and 90° (pure tensile loading). This is very important in the automotive industry since the adhesive joints are mainly designed to support shear loads while in real service conditions, such as impacts, they can be subjected to combined loads. Two adhesives were used to bond two sets of substrates (2 layers and 4 layers) with different thicknesses to show the correlation between the adhesive behaviour and the stiffness of the substrates. The adhesives were a 2-component polyurethane, exhibiting ductile behaviour, and a 2-component methacrylate, brittle if compared to the polyurethane. The substrates were prepared in the lab by using prepregs made of carbon fibres layers cured in an oven under vacuum, and then joined. To analyse the mechanical response of the joints, a Finite Element Model was developed using LS-Dyna. It was validated through iterative procedures comparing the numerical results with the experimental ones to obtain suitable information such as peel and shear stress distribution in the joints. The aim of the work is to assess whether and how the adhesive type and the stiffness of the substrates influence the mechanical properties of this type of adhesive joints and define a failure envelope in a stress space based on the tested mixed-stress conditions.