Made in India
How to choose universal testing machine capacity (kN) for tensile testing
Capacity is the first line on every UTM enquiry and the one most often got wrong. Work it out from the specimen, add a margin, and check the lower end of the range as carefully as the top.
Definition: The capacity of a universal testing machine (UTM) is the maximum force its frame and load cell are rated for, in kilonewtons (kN); the correct capacity is the one where the largest specimen you will test breaks at roughly 40–80 % of full scale and the smallest still loads to at least 10–20 % of the range in use.
1. Start from the specimen, not the catalogue
The peak force in a tensile test is simply the ultimate tensile strength (UTS) of the material multiplied by the cross-sectional area of the specimen:
Fmax (N) = Rm (MPa) × A (mm²) → Fmax (kN) = Rm × A / 1000
Use the highest UTS you expect, not the specified minimum. Actual rebar of grade Fe 500 often breaks at 600–650 MPa; a quenched-and-tempered alloy steel can reach 1200 MPa; stainless 304 around 650 MPa; 6061-T6 aluminium about 310 MPa. Then take the largest cross-section you will ever put in the grips — full-section bars and bolts count, not just machined specimens.
2. Apply a working margin
- Upper margin. Choose a capacity at least 20–25 % above Fmax, i.e. capacity ≥ Fmax / 0.8. Running a frame at 95 % of capacity every day shortens grip and load-cell life, and any specimen above nominal UTS will trip the overload.
- Lower limit. A force transducer verified to class 1 of ISO 7500-1 / IS 1828-1 is normally verified from 20 % of each range upward (lower ranges can be verified separately). For a single-range machine, tests that peak below about 10 % of capacity lose resolution — a 0.5 kN wire test on a 600 kN hydraulic UTM is not a meaningful result.
- Spread of work. If your specimens span more than a 20:1 force ratio (say 3 mm wire and 25 mm bar), buy two machines or a machine with an additional low-capacity load cell rather than one oversized frame.
3. Worked examples
| Specimen | Area (mm²) | Design UTS (MPa) | Peak force (kN) | Capacity to specify |
|---|---|---|---|---|
| Ø 1.6 mm steel wire | 2.0 | 1800 | 3.6 | 5–10 kN |
| Plastic ASTM D638 type I (13 × 3.2 mm) | 41.6 | 80 | 3.3 | 5–10 kN |
| Sheet, 20 × 1.5 mm, CRCA steel | 30 | 420 | 12.6 | 25 kN |
| Aluminium 6061-T6, Ø 12.5 mm round | 122.7 | 340 | 41.7 | 50–100 kN |
| Mild steel, Ø 12.5 mm ASTM E8 round | 122.7 | 550 | 67.5 | 100 kN |
| Alloy steel Q&T, Ø 12.5 mm round | 122.7 | 1200 | 147 | 200 kN |
| TMT bar Fe 500, 16 mm full section | 201 | 650 | 131 | 200 kN |
| TMT bar Fe 500, 25 mm full section | 491 | 650 | 319 | 400 kN |
| TMT bar Fe 550, 32 mm full section | 804 | 700 | 563 | 1000 kN |
| M24 bolt, class 10.9 (As 353 mm²) | 353 | 1040 | 367 | 600 kN |
| Concrete cube 150 mm, M40 (compression) | 22 500 | 50 | 1125 | 2000 kN compression machine |
Areas are nominal. Full-section rebar areas come from the IS 1786 nominal diameters; the machined ASTM E8 12.5 mm round is the standard "0.500 in" specimen.
4. Capacity bands and what they are used for
- 5–10 kN: plastics, rubber, films, textiles, paper, adhesives, fine wire, springs, medical devices. Electro-mechanical, with pneumatic or screw grips.
- 25–50 kN: sheet metal coupons, wire up to ~4 mm, small fasteners, aluminium and copper specimens, weld coupons of thin plate. Electro-mechanical.
- 100 kN: the general-purpose laboratory machine — machined steel rounds and flats to ASTM E8 / IS 1608, rebar up to 12 mm, bolts to M12, compression of small blocks. Electro-mechanical or hydraulic.
- 200–400 kN: rebar 16–25 mm, structural steel plate coupons, chain, fasteners to M20, wire rope. Usually hydraulic.
- 600–1000 kN: rebar 28–40 mm, forgings, anchors, heavy plate, brick and block compression, transverse tests on beams. Hydraulic.
- 2000 kN and above: concrete cube and cylinder compression, pre-stressing strand, large structural components. Dedicated compression machines are cheaper than a UTM of the same capacity.
5. Other factors that decide the specification
- Grip range and type. Wedge grips must cover the full-section bar diameter and the flat coupon width you intend to test; on a hydraulic machine check the maximum jaw opening (e.g. up to 40 mm round and 80 × 20 mm flat).
- Daylight and stroke. Rebar test pieces are 500–700 mm long; the space between grips and the crosshead travel must accommodate them plus the elongation (up to 25 %).
- Speed control. ISO 6892-1 and IS 1608 specify strain rates (method A) or stress rates of 6–60 MPa/s for steel (method B). A closed-loop drive that holds the rate through yield gives repeatable yield values.
- Extensometer. For a 0.2 % proof stress you need an extensometer of class 1 (ISO 9513 / ASTM E83 B-2) — the crosshead displacement is not accurate enough.
- Verification class. Specify force verification to ISO 7500-1 / IS 1828-1 class 1 (±1 % of reading) over the ranges you will use, with a calibration certificate traceable to national standards.
- Software. Automatic detection of yield (ReH, ReL, Rp0.2), UTS, elongation, reduction of area, and report formats for IS 1786 / IS 1608 / ASTM E8 save operator time and remove interpretation errors.
Frequently asked questions
Is a bigger UTM always the safer choice?
No. Force accuracy is stated as a percentage of reading only within the verified range; below 10–20 % of a range the reading becomes coarse and the grips are too large for small specimens. Size the machine so that your everyday tests run between 20 % and 80 % of capacity, and add a second range or load cell for occasional small specimens.
What capacity do I need for 8 to 40 mm TMT bars?
A 100 kN machine covers 8–12 mm bars, 200 kN covers up to 16 mm, 400 kN up to 25 mm, 600 kN up to 32 mm at Fe 500 and 1000 kN covers 32–40 mm in every grade. The full table with areas and forces is in our IS 1786 rebar testing guide.
Should I choose kN or tonnes?
Standards and load-cell certificates are in newtons. 1 tonne-force is 9.807 kN, so a "10 t" machine is 100 kN (strictly 98.07 kN). Specify in kN to avoid a 2 % ambiguity and to match ISO 7500-1 verification ranges.
Can one UTM do tension, compression and bending?
Yes — that is the "universal" in the name. Hydraulic UTMs test tension between the upper and middle crosshead and compression or bending between the middle and lower crosshead; electro-mechanical machines use the same crosshead with different fixtures. Check that the compression platens and bend rollers you need are available for the chosen capacity.
What accuracy class should the machine have?
For tensile testing to IS 1608 / ISO 6892-1 the machine must be at least class 1 to ISO 7500-1 (IS 1828 Part 1): force error, repeatability and reversibility within ±1 %, resolution better than 0.5 % of the reading at the lowest verified force.
Which ETS machine
- ETS-TNE-1T, 10 kN and ETS-TNE-2.5T, 25 kNServo ball-screw electro-mechanical machines for plastics, wire, sheet and small metal specimens.
- ETS-TNE-5T, 50 kN and ETS-TNE-10T, 100 kNLaboratory-grade machines for ASTM E8 / IS 1608 metal specimens with extensometer support.
- UTE 10 (100 kN), UTE 20 (200 kN), UTE 40 (400 kN)Hydraulic electronic UTMs for rebar to 25 mm, plate coupons and fasteners.
- UTE 60 (600 kN) and UTE 100 (1000 kN)For 28–40 mm rebar, forgings and heavy structural specimens.
- All universal testing machinesFull range from 500 kg to 1000 kN with grips, fixtures and IS/ASTM report software.
Need help choosing? Tell our engineers what you test — material, size, standard and volume — and we will recommend a machine, scales and test blocks from our range.
Ask an engineer