A "strata spring" is not a recognized term in the fields of geology, engineering, or materials science. It's possible that:

* You are referring to a different term. Could you provide more context? Perhaps a description of the spring's function or where you encountered this term?

* It's a specialized term used in a specific industry or field. If so, please provide more information about the context in which you encountered it.

Without more information, it's impossible to define what a "strata spring" might be.