Viscoelastic & Viscoplastic Behaviour of flex printed circuit boards under bending loads

Hello,

I am currently working on my M.Sc Thesis project of fatigue lifcycle of Flex printed circuit boards which is composite material of Polymer & Metal.Want some help who has worked on this topic which deals with Viscoelastic & Plastic behaviour of materials under action of pure bending loads.Help & Guidance would be appreciated.Need to come up with an equation which has stress & strain relating to lifecycle of Flex printed circuit boards.

Awaiting your reply,

Regards,

Nishant