Circular membrane tension calculation

Hi, I am a new PhD student working on the vibration behavior of a thin polymer film. Specifically the film is an elastomer VHB 4910. The question that I have specifically is that initially the film is 500 microns thick. Upon the application of prestress, the film is stretched (clamped) and the film thickness decreases to 20 microns. Assuming that the thickness is uniform, how can I calculate the tension in the membrane? 

Thanks for any suggestions that you can provide, and happy new year!