Are these different materials configured as concentric cylinders (copper inside rubber inside plastic etc) or as separate lengths connected to each other like a chain (a length of copper connected to a length of rubber connected to a length of plastic etc)? The calculations for each would be different.
Assuming the concentric configuration, you should also remember that these materials will have different moduli of elasticity. Individually they will stretch different amounts for a given applied load (strain vs stress). But as an assembly of multiple bodies they are all constrained to stretch by the same amount. One of the members will contribute the greatest amount to the load carrying capacity of the cable assembly, probably the copper. I would assume that member carries ALL the load, calculate its resulting strain, and then check to see if the other members can withstand that same amount of strain without failing.