Elastic-Plastic Response of Polymer Coating Under Repetitive Impact

A polymer changes its shape when subjected to a sufficient load. Such a change in shape is called deformation. Elastic deformation takes place when the material returns to its original shape upon removal of the external force. However, when the applied load surpasses the elastic/plastic threshold, the material starts to deform plastically and will not fully recover its original shape. As a result, such plastic deformation is irreversible.

The combination of versatile characteristics, cost-effectiveness and highly tailored production makes polymers excellent candidates for extensive use in a variety of industries, both as structural materials¹, such as bus windows and eyeglasses, and as protective coatings, such as automotive and floor paints and leather coatings. In this study, protective polymer coatings are applied on automotive leathers to shield the leather surface from harmful direct sunlight and make them easy to clean and maintain. Such coatings are subjected to repeated high deformations during daily usage by passengers or vehicle vibration due to harsh road conditions. Moreover, objects with sharp edges such as keys in pockets can also potentially damage the surface. Consequently, the resistance to long-term repetitive impact becomes critical to the service lifetime of the coating and the leather underneath. As a result, a monitored, controlled and repeatable technique for evaluating the elastic/plastic response of polymer coatings under long-term repetitive punch impact is needed.

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The evolution of the elastic/plastic response of the polymer coatings was studied under long-term repetitive punch impact. The polymer film (~1.6 mm thickness) was mounted in a custom sample holder, as shown in Fig. 1. The edge of the film was fixed and the center hole has a diameter of 20 mm. As illustrated in Fig. 2, linear load/unload cycles were applied to the polymer film by a stainless steel ball of 10 mm diameter. The test conditions are summarized in Table 1.

The deformation displacement vs. time and the load-displacement curves are displayed in Fig. 3 and Fig. 4, respectively, and the corresponding results are summarized in Table 2. As the 4 N loading-unloading cycles were applied, the maximum deformation displacement of the tested membrane progressively increased from 1468 to 2825 µm, and the sample did not return to its original shape. This indicates that permanent (plastic) deformation was created by the repeated punching motion of the steel ball. The deformation at the maximum load of 4 N during each cycle increased at a high rate in the first few cycles and then progressively slowed down and reached a relatively constant value. This can be related to the creep behavior of the tested polymer membrane.

It is also observed that the oscillation amplitude of the deformation displacement decreased from 836 to 404 µm, indicating that the polymer membrane exhibits increased stiffness and a loss of flexibility after the load cycles. The flexibility of the protective coating for leather seats is an important characteristic, the deterioration of which may result in reduced comfort for passengers and possible delamination from the leather surface.

In summary, the repetitive punch impact test in this study allows us to simulate the usage of the materials under such working conditions and evaluate the evolution of their elastic/plastic behavior in a monitored and controlled manner. Such an experimental setup could also be used to investigate the fatigue behavior of thin films or membranes.

In this study, we developed an experimental method to assess the effect of repeated external impacts on the elastic/plastic behavior of polymer coatings in a controlled and monitored manner. The tested polymer membrane exhibits increased stiffness and a loss of flexibility after the loading cycles. The deterioration of flexibility can lead to reduced comfort for passengers and possible delamination and fracture of the protective layer on the leather. Such an experimental setup also provides a potential solution for investigating the fatigue behavior of flexible thin films or membranes.

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1. Mulliken, A. D.; Boyce, M. C., Mechanics of the rate-dependent elastic–plastic deformation of glassy polymers from low to high strain rates, International Journal of Solids and Structures, Volume 43, Issue 5, March 2006, pp. 1331–1356.