DefinitionTensile strength is the maximum pulling (tensile) stress a material can withstand before it breaks, measured in a tensile test and reported in MPa or psi; it is also called ultimate tensile strength (UTS).
Explained
How Is Tensile Strength Measured?
In a tensile test, a specimen is pulled slowly while the machine records applied force and elongation. Stress is force divided by the original cross-sectional area, and strain is stretch divided by original length. Plotting stress against strain produces the stress-strain curve, and tensile strength is read from the peak.
- Metals: stress rises through the elastic region, past yield strength, to a maximum where the specimen starts to neck before fracture. That maximum is the tensile strength. ASTM E8/E8M covers yield strength, tensile strength, elongation and reduction of area.
- Plastics: tested to ASTM D638 (or ISO 527-1) using dumbbell-shaped specimens up to 14 mm (0.55 in) thick. Plastics may show a peak at yield and a separate stress at break, so datasheets often report tensile strength at yield and at break separately.
Conversion: 1 MPa ≈ 145 psi.
Tensile Strength vs Yield Strength
| Aspect | Yield strength | Tensile strength |
|---|---|---|
| What it marks | Where permanent (plastic) deformation begins | The maximum stress reached before fracture |
| Where on the stress-strain curve | Where the curve leaves its straight elastic line; for metals usually found by the 0.2% offset method | The highest point, where necking begins (metals) |
| What happens beyond it | The part no longer springs back; it stays bent or stretched | The part necks down and breaks |
| Metals | Lower than tensile strength | The higher of the two |
| Plastics | Often the peak stress just after the elastic region | Stress at break, which may be above or below yield |
| Used for | Sizing parts that must keep their shape | Comparing materials and judging margin against breaking |
How to Use Tensile Strength in Material Selection
- Don’t design to it. For most parts, a permanent bend is already a failure, so working stresses are kept below the yield strength with a safety margin. Tensile strength tells you how much reserve remains before fracture.
- Know the material class. The tensile strength of some polymers can rival some ceramics, but it is no match for even the softest metals. A plastic part replacing a metal one usually needs thicker sections or ribs.
- Treat plastic data with care. ASTM D638 notes that plastic tensile results vary with specimen preparation, test speed and environment, and that data are not valid for loading times or environments very different from the test. Compare only values from the same standard and conditions, and allow for temperature and long-term load.
- Ask for proof when it matters. For regulated or load-critical parts, request material certifications with the delivery.
Key takeaways
- Tensile strength is the maximum tensile stress a material can withstand before fracture.
- It is determined from the highest point on a stress-strain curve recorded during a tensile test.
- ASTM E8/E8M is the common test standard for metals; ASTM D638 or ISO 527-1 is used for plastics.
- Yield strength marks the start of permanent deformation and is usually lower than tensile strength in metals.
- Designers keep working stresses below yield strength and use tensile strength as a reserve against breaking.
- Plastic tensile results depend on specimen preparation, test speed, temperature and environment.
Example
In simple terms, tensile strength is how hard you can pull on a material, per unit of cross-section, before it snaps; a higher tensile strength means a bar of the same size can carry a larger pulling load before breaking.
Related terms
References
- MATSE 81: Materials in Today's World, Lesson 4 "Stress-Strain Testing" — Penn State University
- MATSE 81, Lesson 4 "Plastic Deformation" — Penn State University
- MATSE 81, Lesson 4 "Mechanical Behavior of Polymers" — Penn State University
- ASTM E8/E8M-24, Standard Test Methods for Tension Testing of Metallic Materials — ASTM International
- ASTM D638-22, Standard Test Method for Tensile Properties of Plastics — ASTM International