Formation of plastic creases in thin polyimide films

We present a combined experimental and analytical approach to study the formation of creases in tightly folded Kapton polyimide films. In the experiments, we have developed a robust procedure to create creases with repeatable residual fold angle by compressing initially bent coupons. We then use it to explore the influence of different control parameters, such as the force applied, and the time the film is being pressed. The experimental results are compared with a simplified one-dimensional elastica model, as well as a high fidelity finite element model; both models take into account the elasto-plastic behavior of the film. The models are able to predict the force required to create the crease, as well as the trend in the residual angle of the fold once the force is removed. We non-dimensionalize our results to rationalize the effect of plasticity, and we find robust scalings that extend our findings to other geometries and material properties.