Creep Rupture and Long-Term Load Retention in Printed Photopolymers: Physical Ageing, Environment and Extrapolation Limits

A printed photopolymer that survives a short tensile test can still lose preload, accumulate strain, buckle or rupture under sustained service load. Creep, stress relaxation and creep rupture are distinct endpoints. Their rates depend on temperature, humidity, absorbed chemicals, cure, orientation, specimen age, stress level and geometry. Physical ageing can reduce short-time compliance while embrittlement or environmental damage reduces rupture resistance. Time-temperature superposition and accelerated ageing are powerful only when the same mechanism and material state persist across the shift. This review defines a non-compensatory framework for long-term load retention. It separates constant-stress, constant-load, constant-strain and structural tests; requires true geometry and evolving stress to be tracked; distinguishes recoverable viscoelastic strain, permanent strain, damage and final rupture; and treats censored runouts explicitly. Extrapolation must declare the constitutive form, shift factor, reference state and validation horizon. A short master curve, one apparent power law or one heat-deflection temperature cannot release a long-life claim. Direct creep evidence for printed photopolymers remains limited and formulation-specific. The defensible result is a bounded real-time or validated extrapolated envelope for a defined printed state, environment and failure criterion.