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Definition: Tensile testing of TMT bars (thermo-mechanically treated high-strength deformed steel bars for concrete reinforcement) to IS 1786 determines the 0.2 % proof stress or yield stress, the ultimate tensile strength (UTS), the percentage elongation after fracture and, for ductile grades, the total elongation at maximum force, on a full-section bar pulled in a universal testing machine of class 1 accuracy.
1. Grades and the values they must meet
IS 1786 defines grades by minimum yield stress: Fe 415, Fe 500, Fe 550 and Fe 600, with "D" (ductile) variants that demand higher elongation and UTS/YS ratio, and "S" (seismic) variants for Fe 415 and Fe 500 with a capped yield and a UTS/YS ratio of at least 1.25. The table gives the mechanical requirements as commonly cited from IS 1786:2008 with its amendments; confirm against the current edition before writing a specification.
| Grade | 0.2 % proof / yield stress, min (MPa) | UTS, min | Elongation A, min (%) | Total elongation at max. force Agt, min (%) |
|---|---|---|---|---|
| Fe 415 | 415 | 1.10 × actual YS, ≥ 485 MPa | 14.5 | — |
| Fe 415D | 415 | 1.12 × actual YS, ≥ 500 MPa | 18.0 | 5 |
| Fe 415S | 415 (max. 540) | ≥ 1.25 × actual YS | 20.0 | 10 |
| Fe 500 | 500 | 1.08 × actual YS, ≥ 545 MPa | 12.0 | — |
| Fe 500D | 500 | 1.10 × actual YS, ≥ 565 MPa | 16.0 | 5 |
| Fe 500S | 500 (max. 625) | ≥ 1.25 × actual YS | 18.0 | 10 |
| Fe 550 | 550 | 1.06 × actual YS, ≥ 585 MPa | 10.0 | — |
| Fe 550D | 550 | 1.08 × actual YS, ≥ 600 MPa | 14.5 | 5 |
| Fe 600 | 600 | 1.06 × actual YS, ≥ 660 MPa | 10.0 | — |
Elongation is measured on a gauge length of 5.65 √So (five diameters for a round bar). Where the bar shows no defined yield point, the 0.2 % proof stress is reported, which requires an extensometer.
2. Test procedure
- Sampling. One tensile, one bend and one rebend test piece per cast per size, or per lot as agreed; the bar is tested in the as-rolled condition, never machined.
- Cross-sectional area. Because the ribs make a diameter measurement meaningless, the effective area is calculated from mass per metre: A (mm²) = m / (0.00785 × L), with m in grams and L in mm. IS 1786 allows the mass per metre to deviate from nominal by ±7 % for 8–10 mm, ±5 % for 12–16 mm and ±3 % for 20 mm and above.
- Gauge marks. Punch or scribe marks at 5 d (5.65 √So) intervals along the test length so that the fracture can be re-assembled and elongation measured on a length that contains the fracture near its middle.
- Gripping. Wedge grips with serrated jaws matched to the bar diameter; the free length between grips should be at least 5 d + 100 mm so the extensometer and the gauge length are clear of the jaws.
- Loading rate. Follow IS 1608 / ISO 6892-1: a stress rate of 6–60 MPa/s (or the equivalent strain rate) up to yield, then a crosshead rate that keeps the strain rate below 0.008 /s until fracture. A programmable UTM holds these automatically.
- Results. Record ReH (upper yield) or Rp0.2, Rm (UTS), the UTS/YS ratio, percentage elongation after fracture and, for D and S grades, Agt from the extensometer or from marks measured away from the neck.
3. Bend and rebend tests
The bend test (IS 1599) folds the bar through 180° around a mandrel; the outer surface must show no crack visible to the normal eye. The rebend test checks strain-ageing embrittlement: the bar is bent to an included angle of 135°, kept in boiling water at 100 °C for 30 minutes, cooled, then bent back to an included angle of 157.5°; again no cracks are allowed. Mandrel diameters depend on grade and size — for bars up to 22 mm, 3 d (Fe 415), 4 d (Fe 500), 5 d (Fe 550) and 6 d (Fe 600) for the bend test, one size larger above 22 mm, and larger still for the rebend test. Bending is done in a bend fixture on the UTM compression platens or in a dedicated bending machine at ambient temperature.
4. Which UTM capacity for 8–40 mm bars
The peak force is UTS × area. Use a realistic UTS — Fe 500 bars commonly break at 600–650 MPa and Fe 550/600 at 650–750 MPa — and keep the peak force below about 80 % of the machine capacity so that the load cell is not run at full scale on every test.
| Bar size | Area (mm²) | Force at 600 MPa (kN) | Force at 700 MPa (kN) | Recommended capacity |
|---|---|---|---|---|
| 8 mm | 50.3 | 30.2 | 35.2 | 100 kN |
| 10 mm | 78.5 | 47.1 | 55.0 | 100 kN |
| 12 mm | 113.1 | 67.9 | 79.2 | 100 kN (Fe 415/500); 200 kN for Fe 550/600 |
| 16 mm | 201.1 | 120.6 | 140.7 | 200 kN |
| 20 mm | 314.2 | 188.5 | 219.9 | 400 kN |
| 25 mm | 490.9 | 294.5 | 343.6 | 400 kN (Fe 415/500); 600 kN for Fe 550/600 |
| 28 mm | 615.8 | 369.5 | 431.0 | 600 kN |
| 32 mm | 804.2 | 482.5 | 563.0 | 600 kN (Fe 415/500); 1000 kN for Fe 550/600 |
| 36 mm | 1017.9 | 610.7 | 712.5 | 1000 kN |
| 40 mm | 1256.6 | 754.0 | 879.6 | 1000 kN |
For a rolling mill or a construction laboratory testing every size from 8 to 40 mm, a 1000 kN machine covers all sizes, and a 100 kN machine alongside it gives proper resolution on 8–12 mm bars; a 400 kN or 600 kN machine is the usual single-machine compromise for sites that stop at 25 or 32 mm. Grip jaws must be ordered for each diameter band (e.g. 8–12, 12–20, 20–32, 32–40 mm).
5. What a class-1 machine must provide
- Force verification to IS 1828 Part 1 / ISO 7500-1 class 1 over the ranges used, with a certificate traceable to national standards; for 8 mm bars on a 1000 kN machine, verify a lower range or use a second load cell.
- Rate control that follows IS 1608 through the yield region; a sudden speed change at yield changes the measured ReH.
- An extensometer of class 1 (IS 12872 / ISO 9513) for Rp0.2 and Agt, with a gauge length of at least 50 mm or 5 d.
- Software that computes YS, UTS, ratio, elongation and pass/fail against the selected grade and prints a report in the IS 1786 format with heat number, size, mass per metre and area.
Frequently asked questions
What is the difference between Fe 500 and Fe 500D?
Both need a minimum yield of 500 MPa. Fe 500D must additionally reach 16 % elongation (vs 12 %), a UTS at least 1.10 times the actual yield (vs 1.08) and at least 565 MPa (vs 545 MPa), and a total elongation at maximum force of 5 %. D grades also have tighter limits on carbon, sulphur and phosphorus. They are specified for seismic zones and critical structures.
How is the yield stress of a TMT bar measured?
If the stress–strain curve shows a clear yield plateau, the upper yield stress ReH is reported. TMT bars of higher grades often show no plateau, so the 0.2 % proof stress Rp0.2 is determined from the extensometer curve — this needs an extensometer, not crosshead travel.
Can I use the nominal area instead of mass per metre?
IS 1786 bases the stress calculation on the effective cross-sectional area obtained from mass per metre, because the deformed profile cannot be measured accurately with callipers. Using the nominal area is acceptable only when the measured mass per metre equals the nominal value; otherwise the stress is in error by the same percentage.
What UTM capacity do I need for 32 mm Fe 500 bars?
Area 804 mm² × 600–650 MPa gives 480–525 kN, so a 600 kN machine works at 80–88 % of capacity; a 1000 kN machine is the comfortable choice, and mandatory if you also test Fe 550 or Fe 600 in 32 mm.
How long should the test piece be?
Allow the gauge length (5 d), an extensometer length, a margin of at least one diameter each side of the gauge marks, and the grip length at both ends: for 25 mm bar with 150 mm jaws, about 600 mm. Bend and rebend pieces need roughly 8 d plus the mandrel circumference; 400–500 mm is typical.
Which ETS machine
- UTE 10, 100 kNFor 8–12 mm bars with proper resolution; also machined coupons and wire.
- UTE 40, 400 kNThe common single-machine choice for construction laboratories testing up to 25 mm.
- UTE 60, 600 kNCovers 8–32 mm Fe 415/500 with hydraulic wedge grips and bend attachment.
- UTE 100, 1000 kNAll sizes to 40 mm in every grade, for rolling mills and NABL laboratories.
- All universal testing machinesCapacities, grips, extensometers and IS 1786 report software.
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