Expansion needs an axis, length and temperature changeRT duroid after etch stress relief: Source & Enginee…. Engineering illustration, not a production photograph or test result.Expansion needs an axis, length and temperature changeRT duroid after etch stress relief: Source & Enginee…Original length LUnconstrained length after ΔTΔ³¢Î”³¢ = CTE × L × ΔTConstraint, stress and warpage require a different model.