Polymers　and　composites　have　been　widely　applied　in　many　industries,　especially　in　aerospace.　There　are　growing　demands　for　understanding　their　natural　viscoelastic　performance,　especially　in　aerospace　applications,　where　precise　dimensional　control　and　prediction　of　the　residual　strength　and　modulus　are　vital　for　the　success　of　an　aerospace　vehicle.　The　existing　superposition　principles　mainly　focus　on　the　well-known　WLF-based　horizontal　shift　to　predict　the　long-term　behaviour　through　short-term　experimental　tests　in　terms　of　creep　or　stress　relaxation.　Whilst　the　intrinsic　microstructural　changes　or　damages　within　a　polymeric　material　due　to　viscoelastic　deformation　are　not　considered,　which　may　lead　to　large　differences　for　long-term　predictions.　Here,　we　look　into　the　very　fundamentals　of　the　existing　superposition　principles,　aiming　to　develop　towards　a　unified　theory　to　better　predict　the　long-term　relaxation　modulus　or　creep　of　a　polymeric　solid.　This　is　achieved　by　considering　both　the　free　volume　theory-based　horizontal　shift　factors　and　activation　energy-based　shift　factors;　microstructural　changes　or　damages　induced　vertical　shift　factors　are　then　coupled　to　improve　the　prediction　accuracy　of　the　superposition　methods.　These　will　facilitate　long-term　predictions　and　accelerated　aging　tests　for　viscoelastic　solids.　©　The　Author(s),　under　exclusive　license　to　Springer　Nature　Switzerland　AG　2024.