Polymer coated sheet metal components find diverse applications in many industries. The manufacturing of the components generally involves forming of sheet metal into the desired shape and coating of the formed part with organic coating. An alternative manufacturing route is to coat the sheet metal first before forming. The change in the manufacturing sequence can potentially improve cost and reduce environmental impact. This approach, however, requires the coating to survive the deformation process. Thus, the effect of plastic deformation on coating adhesion is of primary interest to many engineers and researchers. This research aims at developing a methodology to predict the adhesion of coating after metal forming processes. A pull-off apparatus that measures the coating pull-off stress was used to indicate the coating adhesion strength. Several types of specimen were designed to obtain uniaxial tension, biaxial tension, and tension-compression deformation modes on pre-coated sheet by using a uniaxial tensile tester. Experimental results from two selected polymer coated sheet metals show that coating adhesion was affected by plastic deformation. An analytical model based on a virtual interface crack concept was developed to indicate the adhesion potential of the coating-substrate interface. From interfacial fracture mechanics, the initial adhesion potential is defined as the energy release rate characterized by the virtual interface crack and the initial pull-off stress. The analytical model was used to predict coating adhesion loss after deformation in uniaxial tension mode. The analytical model predictions agreed well with experimental results. Finite element analysis tool was applied to simulate more complex deformation modes in stamping of coated sheet meals. The stress field near the interface crack tip was used to calculate the energy release rate and predict the adhesion loss under different deformation modes. The predictions obtained from numerical method are also in good agreements with the experimental results in biaxial tension and tension-compression modes. The research has led to a better understanding of the effects of plastic deformation on coating adhesion. The developed adhesion test methods can be used to generate useful information on coating durability for diverse practical use. It is also expected that the results of the research will facilitate the development of better polymer coated sheet metal to be used in sheet metal forming processes.
