⚙️ Tensile Test of Steel
Static tensile test of metallic materials at room temperature. Determination of yield strength, tensile strength and elongation.
Overview
The static tensile test is the most important and frequently performed method for testing mechanical properties of metals. It involves uniaxial tension of a specimen with standardized dimensions at constant strain rate until fracture. From the recorded force-elongation curve, key strength parameters are determined: yield strength (Re or Rp0.2), tensile strength (Rm), elongation (A) and reduction of area (Z).
Standard PN-EN ISO 6892-1 specifies test method at room temperature (10–35°C) and defines three rate control procedures: A1 (with feedback loop on strain rate), A2 (with constant crosshead speed) and B (with constant stress rate in elastic range). Method A is recommended as more precise and reproducible.
Tensile test results form the basis for qualifying steel into appropriate grades (e.g., S235, S355 per PN-EN 10025) and assessing material suitability for specific structural applications. Testing is mandatory in mill certificates (certificates 3.1 and 3.2 per PN-EN 10204) and in quality control of reinforcing bars (PN-EN 10080), bolts, sheets and tubes.
Test specimens can be flat (from sheets) or round (from bars), with standardized dimensions depending on the product. Most commonly proportional specimens with gauge length L₀ = 5.65√S₀ (short) or L₀ = 11.3√S₀ (long) are used.
Method principle
A metal specimen of standardized dimensions is gripped in the jaws of a testing machine and subjected to uniaxial tension at controlled strain rate. The machine continuously records force (load cell) and elongation (extensometer), generating a tensile curve σ-ε. From the curve, the following are determined: upper and lower yield strength (ReH, ReL) or 0.2% proof strength Rp0.2 (at 0.2% permanent strain), tensile strength Rm (maximum stress), elongation A (percentage change in gauge length after fracture) and reduction of area Z (percentage change in cross-sectional area at fracture location).
Applications
- Qualification of structural steel into grades (S235, S355, S460)
- Mill certificates — acceptance certificates 3.1/3.2 per PN-EN 10204
- Quality control of reinforcing bars (PN-EN 10080)
- Testing of bolts, nuts and steel fasteners
- Acceptance control of sheets, tubes, sections
- Qualification of welding procedures (specimens from welded joints)
- Research and development of new alloys
- Failure analysis — investigation of fracture and failure causes
Key parameters
| Parameter | Value |
|---|---|
| Test temperature | 10–35°C (room temperature) |
| Strain rate (method A2) | 0.00007 s⁻¹ (to Re) → 0.00025 s⁻¹ (after Re) |
| Extensometer class | Class 1 per ISO 9513 (accuracy ±1 µm) |
| Gauge length L₀ | 5.65√S₀ (short) or 11.3√S₀ (long) |
| Machine force range | 50–600 kN (typical for steel testing) |
| Rm repeatability | ±1% (within same material) |
Standard
- Standard number
- PN-EN ISO 6892-1:2020
- Title (PL)
- Metale — Próba rozciągania — Część 1: Metoda badania w temperaturze pokojowej
- Title (EN)
- Metallic materials — Tensile testing — Part 1: Method of test at room temperature
Step-by-step procedure
1. Specimen preparation
Take specimen from product per product standard. Machine to standardized dimensions. Apply gauge length L₀ marks.
2. Specimen dimension measurement
Measure width and thickness (flat specimen) or diameter (round) at 3 locations. Calculate cross-sectional area S₀.
3. Specimen gripping
Grip specimen in testing machine jaws. Ensure axiality (no eccentricity). Attach extensometer.
4. Test parameter setup
Select rate control procedure (A1, A2 or B). Set strain rate: 0.00007 s⁻¹ to yield strength, then 0.00025 s⁻¹.
5. Preload application
Apply small preload (max 5% of expected Re) to eliminate slack in grips.
6. Tension — elastic zone
Start tensioning at constant strain rate. Record force and elongation. Observe plotted σ-ε curve.
7. Transition through yield strength
Observe yield plateau (ReH/ReL) or determine Rp0.2 from curve. After yield strength, increase rate to 0.00025 s⁻¹.
8. Tension to fracture
Continue until specimen fracture. Record maximum force (→ Rm) and force at fracture.
9. Post-fracture elongation measurement
Join both specimen parts. Measure final length Lu. Calculate: A = (Lu - L₀)/L₀ × 100%.
10. Reduction of area measurement
Measure cross-sectional dimensions at fracture location. Calculate: Z = (S₀ - Su)/S₀ × 100%.
11. Calculations and report
Calculate Re (or Rp0.2), Rm, A, Z. Compare with product standard requirements. Prepare report with tensile diagram.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Universal testing machine (tensile tester) | ZwickRoell Z100/Z250, Instron 5985/8802, MTS Criterion C45 | 150 000–600 000 PLN |
| Contact extensometer | ZwickRoell makroXtens, Instron 2630-series, MTS 634 | 15 000–50 000 PLN |
| Grips and specimen holders | Wedge hydraulic or mechanical, adapted to specimen cross-section | 5 000–25 000 PLN/set |
| Caliper / micrometer | Mitutoyo 293-240, accuracy 0.001 mm — for measuring specimen dimensions | 200–2 000 PLN |
| Testing software | ZwickRoell testXpert III, Instron Bluehill Universal | 10 000–40 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Standardized specimens | — | Proportional flat or round specimens, machined per PN-EN ISO 6892-1 annex B |
| Reference material (CRM) | — | Certified reference material for machine verification (e.g., steel with known Rm) |
| Marking medium | — | Scriber or marker for applying gauge length L₀ marks |
Health and safety (OHS)
- Testing machine — crushing hazard from hydraulic grips, use guards
- Specimen fracture — elastic energy, fragment ejection risk, use protective shield
- Specimen machining — sharp metal edges, protective gloves
- Noise during fracture — use hearing protection for large cross-sections
- Safety glasses mandatory during entire test