The ability of a material to undergo permanent deformation without fracture when stressed beyond the yield point.
Plasticity is irreversible deformation governed by dislocation motion in metals or chain sliding in polymers. Above σ_y, the material flows plastically with a tangent modulus (E_t) much lower than E. In perfectly plastic materials E_t = 0; in strain-hardening materials E_t > 0. The plastic strain is the permanent set remaining after unloading.
Controlled plasticity enables metal forming, press-fitting, and cold working. In 3D-printed thermoplastics, plastic deformation is generally undesirable in structural applications: it signals overloading. However, some plastic deformation before fracture (ductile behaviour) provides visual warning of failure. FDM snap-fits must not enter the plastic zone during assembly or repeated use.
Plastic deformation is often invisible until large: a part may look normal but be permanently overstressed. Strain hardening in thermoplastics is minimal compared to metals; FDM parts approaching yield will show creep-like behaviour. Repeated yielding (ratchetting) under cyclic loading accumulates permanent strain even at stresses below the monotonic yield strength.
Related terms: Elasticity, Yield Strength, Stress-Strain Curve, Creep, Ductility
| Field | mechanics, engineering |
|---|---|
| Also called | Plastic Behaviour, Plastic Deformation |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert