I have a 1.5 mm thick plastic sheet part which hast to be bent at a joint 0.3 mm thick by ~ 10 Deg. I know (found from design drawing) compressed and stretched lengths of the joint. How can I find the length of the joint in non-bent (flat) position so that when it is bent the compressed and stretched joint lengths have the initial known lengths ?
If we can assume that the material behaves in an elastic linear way and that the joint is only loaded by bending, then as you commented the neutral line lies in the middle and that deformation on the two extreme fibres is equal but in opposite directions. Then, original length is just the average of both deformed lengths.
Of course, to get this result we must assume too that the conditions for the Euler–Bernoulli beam theory holds, and according to your diagram we might approaching its limits (short span compared to width, not very small deformation...) but I think it's still a good approximation.
However, if the material is not linear elastic, location of neutral fibre depends on the constitutive equation of the material (the stress-strain diagram), and the original length may not be the average of both deformed lengths, specially if the constitutive equation is not symmetrical.