Hi Erich
to the Surface Engineering Forum.
Interesting, but not really my area of expertise, but here's my twopence worth of thought anyway:
Assuming your coating will have a lower strain than your substrate. One problem will be observing the first signs of failure in the coating, which probably will occur in a multitude of places along the filament length, rather than one place as you would expect for the substrate. So, I think you will probably require a microscope to view the coating surface to detect the onset of failure. Without going too "high tech" I can only think of making up a small jig so that you can strain the filament ( I assume it will be a bit like trying to test a human hair, scale wise
) in small increments, while microscopically observing the coating surface for failure at each small step in strain. This should give you a good idea of the relative strain between substrate and coating in combination. Remember, you are trying to test two materials joined together, one will effect the results of the other, so be careful not to interpret results in isolation. To do that you would need to test the coating alone without the substrate.
Hope that helps.