The stress at which a material begins to deform plastically and no longer returns to its original shape on unloading.
Yield strength (σ_y) marks the elastic-plastic boundary. Below it Hooke's law holds; above it permanent set accumulates. Where no sharp yield plateau exists, the 0.2% offset method is used: a line parallel to the elastic slope is drawn from ε = 0.002; the intersection with the curve is σ_y. Typical values: mild steel ~250 MPa, Al alloys 270-500 MPa, PLA 40-55 MPa.
Structural design keeps service stresses below σ_y with a safety factor of 1.5-3×. Knowing σ_y prevents permanent deformation in snap-fits and living hinges during assembly. Composite-filled filaments (CF, GF) significantly raise σ_y compared to unfilled base material.
Designing to UTS instead of yield strength is a critical error for any cyclically loaded part. Thermal cycling inside a printer enclosure can lower σ_y via annealing. Specimens printed flat give very different σ_y to those printed upright: always test in service orientation.
Related terms: Tensile Strength, Stress-Strain Curve, Elasticity, Plasticity
| Field | mechanics, engineering |
|---|---|
| Also called | Yield Point, 0.2% Proof Stress |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert