Functionally graded adhesive joints using magnetic microparticles with a polyurethane adhesive

The present work focuses on using a recently proposed method to create functionally graded adhesive joints using a polyurethane adhesive and iron microparticles. Functionally graded joints were created with the application of a suitable magnetic field in the centre of the overlap region. This field is able to move the iron microparticles and create a tailored particle distribution. The main goal of this work was to create a particle concentration gradient that changes from particle rich in the middle of the overlap to poor in the ends. Consequentially, the stiffness of the resulting composite is made to vary along the bondline. A numerical simulation was performed to determine the optimal distance between the magnet and the adhesive layer as well as the application time of the external magnetic field. In order to assess the influence of the iron particles on the adhesive, different particle amounts were experimentally considered and two particles distributions were evaluated (uniform and graded distributions). The predicted particles trajectories were validated through a study of the single lap joint (SLJ) fracture surfaces, showing that the magnetic particles follow the magnetic field lines towards the middle of the overlap. Also, by observing the same fracture surfaces, the recently proposed method was validated, a non-uniform particle distribution. Finally, SLJs containing 1% of iron microparticles with a graded distribution were found to exhibit the best mechanical performance. Thus, this method has been demonstrated to be a viable technique to enhance the mechanical properties of bonded joints, leading to a more uniform stress distribution along the bondline