The slow, time-dependent plastic deformation of a material under constant stress, especially at elevated temperatures.
Creep occurs in three stages: primary (decreasing rate), secondary (steady-state), and tertiary (accelerating rate leading to rupture). It is thermally activated: significant for polymers above 0.5 × Tg (in Kelvin) and for metals above 0.4 × T_melting. The steady-state creep rate is ε̇ = A × σ^n × exp(−Q/RT), where Q is activation energy and n the stress exponent.
PLA's low Tg (~55-60°C) makes it highly susceptible to creep even at room temperature under sustained load. Shelf brackets, clamps, and load-bearing mounts printed in PLA will sag over weeks to months. PETG, ABS, and nylon have better creep resistance. Enclosures for electronics generate heat that accelerates polymer creep. For sustained loads, design with generous safety factors or switch to HTPLA, ABS, or PA.
Creep is time-dependent: a part that passes short-duration testing may fail months later. PLA begins creeping measurably at temperatures as low as 40°C under moderate stress. Creep strain is not recoverable (unlike elastic strain): the part does not spring back when unloaded after creep. Accelerated creep tests at elevated temperature may not accurately predict room-temperature long-term behaviour.
Related terms: Stress, Elasticity, Plasticity, Residual Stress
| Field | mechanics, engineering |
|---|---|
| Also called | Creep Deformation, Time-dependent Deformation |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert