On local panel distortions due to hot-curing adhesives

For many joining applications, adhesive bonding is the favoured method. It provides the ability to join dissimilar materials such as metals and plastics. Adhesive bonds can be formed over large flange areas subsequently increasing the overall stiffness of the assembly. In some cases, however, the bonding process can lead to distortions, i.e. unwanted visible deformations, especially if thin-walled structures such as car body panels are involved. These deviations from the intent shape might not affect the structural integrity of a part, but, if visible to customers on a product’s surface, they can be considered an aesthetic flaw. Distortions can be divided in local ones, which occur close to the bond line, and global ones, which affect the geometry of the entire assembly. In this thesis, the development of local panel distortions that occur in a hot-cure cycle of an adhesive is studied. A laboratory sample is exposed to temperature cycles with different heating rates and cure temperatures; the displacement of a steel strip is monitored. The panel curvatures are measured after cure cycles with and without a dwelling step at elevated temperature before the cure temperature is reached. A simulationmodel is developed that takes thermal and chemical volume changes, the cure evolution, the liquid-solid transition and stress relaxation of the adhesive into account. Mechanical properties and expansion coefficients were measured and integrated in the simulation model. Simulation results are compared with data from the experiments. The sensitivity of the predictions of distortions to changingmaterial properties is investigated. Distortions can arise early in the cure cycle. Even during the heating phase, panels can start to deform. The model predicts well the development of distortions. The influence of the temperature cycle on distortions is also reproduced by the model due to the hypo-elastic formulation of the stress-strain relation during cure. A pre-cure step can be used to reduce distortions. The model can reproduce that. Improvements, however, might be in a range in which also other effects play a role, such as deviations of adherends from an intended shape before the bonding process. Predictions showed different sensitivities to changing material constants. In a set-up where the adhesive is not confined, the bulk modulus did not show any significant influence on the predictions of distortions, so that an estimate for it may suffice. Chemical shrinkage, however, needs to be measured in-situ or should be corrected from measurements at roomtemperature to fit the real behaviour at elevated temperatures, since the influence on distortions is strong. The gel point, as the transition point from liquid to solid, has also a significant influence on predictions of distortions and should be determined with sufficient accuracy.Aerospace Structures and MaterialsAerospace Engineerin