P 1299 – Design method for adhesively bonded compounds with adherends subjected to cathodic-dip paint coating and fiber reinforced plastics


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P 1299 – Design method for adhesively bonded compounds with adherends subjected to cathodic-dip paint coating and fiber reinforced plastics

Cathodic dip coating (CDC) has been the state of the art in automotive engineering for years to protect the car body from corrosion. Often, fiber-reinforced composites (FRC), other temperature-sensitive materials or metallic materials are bonded after the CDC to the already coated surface. In principle, the cathodic dip coating provides an excellent adhesive base. However, it is problematic that such adhesive joints represent the strength-limiting factor for the joint in certain load cases. For this reason, a reliable design of an adhesive bond between CDC-coated and temperature-sensitive components is not possible for SMEs according to the current state of the art. This is where the present research project comes in.
In the present research project, the basic requirements for achieving reproducible cathodic dip coating strengths were first determined. Tests were carried out with different adhesive systems as well as cathodic dip coatings and substrates in order to identify suitable materials. Subsequently, the effects of various influencing factors on the failure behavior of the adhesive adhesive bonded joints were investigated, depending on the adhesive and substrate. The strength and stiffness of the respective adhesive system as well as the geometric design of the adhesive layer played an important role. Based on this, it was examined whether the findings can also be transferred to FRC mixed joints.
In the next step, a formulaic relationship for the mathematical description of the transition from cohesive adhesive failure to cathodic dip coating failure was derived on the basis of the data collected in the project. This led to the hypothesis that a high proportion of normal stresses significantly leads to cathodic dip coating failure. Based on the calculated stress states, it became clear that it is not the absolute values of the individual stresses that cause cathodic dip coating failure, but the ratio of normal to shear stresses that is critical. It was found that for bondings with high overlap lengths and high adhesive layer thicknesses, the proportions of normal stresses increased in relation to the shear stresses, which correlates with an increased tendency to cathodic dip coating failure.
In conclusion, it was shown that the mechanisms of action on cathodic dip coating failure identified in the project can also be applied to component-like specimens. With the help of the design method developed, SMEs can optimize existing products and apply it to the development of new products. This provides the designer with decision-making aids for the optimum design of the connection at an early stage of the development phase.

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Prof. Dr.-Ing. G. Meschut