Yield Strength (Fy) — Definition, Formula & Steel Grades

Yield strength (Fy) is the stress at which a material transitions from elastic behavior (deformation fully recoverable upon unloading) to plastic behavior (permanent, non-recoverable deformation). It is the single most important material property in structural steel design — every strength limit state in AISC 360, EN 1993, AS 4100, and CSA S16 depends on Fy, either directly or through derived resistances.

Elastic region:  σ = E * ε    (Hooke's Law — fully recoverable)
Plastic region:  σ > Fy       (permanent deformation begins)

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The Yield Phenomenon

For hot-rolled carbon steel, the stress-strain curve exhibits a distinct yield plateau. The upper yield point marks the peak stress just before the sudden drop; the lower yield point is the approximately constant stress level during the yield plateau (Lueders bands propagation). Design standards use the lower yield point as Fy.

For steels that do not exhibit a clear yield plateau — cold-formed sections, high-strength quenched-and-tempered steels, stainless steels — the 0.2% offset method applies:

Draw a line parallel to the elastic slope (E), offset by 0.2% strain (ε = 0.002).
Fy = stress at the intersection of this line with the stress-strain curve.

This is called the 0.2% proof stress and is the basis for cold-formed steel design per AISI S100 and AS/NZS 4600.

Common Yield Strength Values

Note: A992 specifies Fy_max = 65 ksi and Fy/Fu ≤ 0.85, ensuring ductile behavior with a defined yield plateau. A36 allows up to Fy = 36 ksi for plates up to 8 in thick (reduces to 32 ksi above 8 in).

Design Role of Fy

Yield strength is the basis for all cross-section resistance calculations:

The factor 0.6 in shear derives from the von Mises yield criterion: τ_yield = Fy / √3 ≈ 0.577 Fy.

Frequently Asked Questions

What is the yield strength of A36 steel? A36 steel has Fy = 36 ksi (250 MPa) for plates and shapes up to 8 inches thick. Above 8 inches, Fy reduces to 32 ksi. A36 is a legacy structural steel; modern W-shapes use A992 (Fy = 50 ksi).

What is the difference between yield strength and tensile strength? Yield strength Fy is the stress at which permanent (plastic) deformation begins. Tensile strength Fu is the maximum stress the material can withstand before necking and fracture. The Fy/Fu ratio (typically 0.60-0.85 for structural steels) controls ductility — lower ratios indicate greater strain-hardening capacity and ductile behavior.

Does temperature affect yield strength? Yes. Yield strength decreases with increasing temperature. At approximately 400 deg C, Fy reduces to ~70% of ambient; at 600 deg C, to ~30%. Fire design per EN 1993-1-2 and AISC Appendix 4 uses temperature-dependent reduction factors ky,θ for Fy and kE,θ for modulus of elasticity.

International Code References

• AISC 360: Fy per ASTM A6/A6M and AISC Specification Section A3.1. A992 default Fy = 50 ksi for W-shapes.
• EN 1993-1-1: fy per EN 10025 product standards. S355 is the European equivalent to 50 ksi steel, with fy = 355 MPa for t ≤ 40 mm (reduces for thicker plates per Table 3.1).
• AS 4100: fy per AS/NZS 3679.1. G300 (300 MPa) and G350 (350 MPa) are common Australian structural grades.
• CSA S16: Fy per CSA G40.20/G40.21. 350W steel (Fy = 350 MPa) is the Canadian equivalent to A992.

Educational reference only. Yield strength values must be confirmed from certified mill test reports (MTRs) for project-specific design. All structural designs must be independently verified by a licensed Professional Engineer.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.