Yield Strength vs Tensile Strength Explained

## The Tensile Test Both yield strength and ultimate tensile strength (UTS) are measured in a uniaxial tensile test per ASTM E8. The stress-strain curve reveals the proportional limit, yield strength (0.2% offset), UTS (peak stress), and fracture. ## Yield Strength Yield strength is the stress at which permanent deformation begins, defined by the 0.2% offset method for most metals. It is the primary design parameter for structural applications. | Code | Allowable Stress Basis | |------|----------------------| | ASME Section VIII, Div. 1 | 2/3 of yield at temperature | | AISC (structural steel) | 0.6 x yield (ASD) | | Eurocode 3 | yield / gamma_M | ### Representative Values | Alloy | Yield Strength (MPa) | |-------|---------------------| | Pure aluminum (1100-O) | 35 | | Mild steel (A36) | 250 | | 6061-T6 aluminum | 276 | | Ti-6Al-4V (annealed) | 860 | | Inconel 718 (aged) | 1036 | ## Ultimate Tensile Strength UTS is the maximum engineering stress during the tensile test. It governs fracture prevention, fastener proof loads, and provides fatigue estimation (endurance limit is roughly 0.5 x UTS for steels below 1400 MPa). ## The Yield-to-Tensile Ratio | Y/T Ratio | Interpretation | |-----------|---------------| | 0.50-0.65 | Large plastic deformation before fracture | | 0.65-0.80 | Balanced strength and ductility | | 0.80-0.93 | High strength, limited plastic reserve | | 0.93-1.00 | Near-brittle failure | Pipeline codes (API 5L) often limit Y/T ratio to maximum 0.93 for strain-based design. ## Practical Implications **Bolts**: Designed on yield strength to maintain clamp load. Overtorqued past yield causes permanent stretch. **Crash structures**: Low Y/T ratio materials absorb energy through plastic deformation before fracture. **Pressure vessels**: ASME limits allowable stress to the lesser of UTS/3.5 or 2/3 of yield.