What Is Ultimate Tensile Strength (UTS)?
Ultimate tensile strength (UTS), also called tensile strength, is the maximum engineering stress a material can withstand before fracture under uniaxial tension. It is determined by dividing the maximum load sustained during a tensile test by the original cross-sectional area of the specimen. UTS is reported in megapascals (MPa) or kilopounds per square inch (ksi) and is a mandatory property on every mill test certificate.
UTS on the Stress-Strain Curve
During a standard tensile test (ASTM E8 / E8M), the specimen undergoes three distinct stages:
| Stage | Behavior | Key Point |
|---|---|---|
| Elastic region | Stress proportional to strain; material returns to original shape if unloaded | Proportional limit |
| Yielding and strain hardening | Permanent deformation begins; stress continues to rise as the material work-hardens | Yield strength |
| Necking and fracture | Localized thinning occurs; engineering stress drops after reaching maximum | UTS (maximum stress) |
UTS occurs at the onset of necking. After this point, the actual (true) stress in the necked region continues to increase, but the engineering stress (calculated using the original area) decreases until fracture.
UTS Values for Common Piping Materials
| Material | ASTM Spec | UTS (min) MPa | UTS (min) ksi |
|---|---|---|---|
| Carbon steel (A106 Gr. B) | A106 | 415 | 60 |
| Carbon steel forgings (A105) | A105 | 485 | 70 |
| 1.25Cr-0.5Mo (A335 P11) | A335 | 415 | 60 |
| 2.25Cr-1Mo (A335 P22) | A335 | 415 | 60 |
| 304L Stainless | A312 | 485 | 70 |
| 316L Stainless | A312 | 485 | 70 |
| Duplex 2205 (S31803) | A790 | 620 | 90 |
| Super Duplex 2507 (S32750) | A790 | 795 | 116 |
| Inconel 625 (N06625) | B444 | 827 | 120 |
| Alloy 20 (N08020) | B729 | 551 | 80 |
UTS vs. Yield Strength in Design
| Parameter | UTS | Yield Strength |
|---|---|---|
| Definition | Maximum stress before fracture | Stress at onset of permanent deformation |
| Design basis (ASME B31.3) | Allowable stress = UTS / 3 (at room temperature) | Allowable stress = YS / 1.5 |
| Governing value | Lower of UTS/3 or YS/1.5 governs the design | Lower of UTS/3 or YS/1.5 governs the design |
| Typical UTS/YS ratio | - | Carbon steel: 1.5-1.8; Austenitic SS: 2.2-2.8; Duplex: 1.3-1.5 |
Testing Requirements
Tensile testing per ASTM E8/E8M requires a standard specimen machined from the product (pipe, plate, forging, or casting). The test report must include:
- Ultimate tensile strength (minimum per specification)
- Yield strength at 0.2% offset
- Elongation in 2” or 50 mm gauge length
- Reduction of area (for round specimens)
All four values must meet the ASTM minimum requirements for the heat to be certified. The results appear on the mill test certificate (MTR), and any value below the minimum means the material is rejected.
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