TJ-144
Heavy Duty Hydraulic Grips
Heavy duty hydraulic wedge grips hold a constant clamping force as the specimen necks, which is what prevents late slippage on a high-strength bar.
SpecificationsTesting standard
IS 1608
Metallic materials — Tensile testing — Part 1: Method of test at room temperature
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
IS 1608 is the Indian Standard for tensile testing of metallic materials at ambient temperature. It is harmonised with ISO 6892-1 and specifies the specimen forms, strain rates, and calculations for yield and proof strength, tensile strength, elongation and reduction of area. It is the reference method behind most Indian steel and metal product acceptance.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- IS
- Edition
- 2022
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
What the test does
A specimen of defined form — a machined round or flat coupon, or a full-section product such as reinforcing bar or wire — is gripped in wedge grips and pulled along its axis at a controlled rate until it fractures. An extensometer is fitted across the gauge length where yield or proof strength is required. Force is recorded against extension throughout. After fracture the two halves are fitted back together and the final gauge length and minimum cross-section are measured, giving elongation after fracture and reduction of area. Rate control follows ISO 6892-1 practice, with defined ranges for the elastic and plastic regions.
What it measures, and why it matters
The method yields the properties that virtually every metal product is specified on: yield or proof strength, tensile strength, elongation after fracture and reduction of area. Yield strength is what a structure is designed against; tensile strength is the ultimate capacity; elongation and reduction of area describe ductility, which determines whether a material deforms visibly before it fails or breaks without warning. In Indian practice IS 1608 is the method called up by the product standards behind construction steel, reinforcing bar, structural sections, fasteners and wire, so it sits underneath a very large share of routine material acceptance.
Specimen
The specimen forms follow ISO practice, and the choice between proportional and non-proportional gauge lengths is the one that most affects a reported elongation.
- Proportional gauge length
- L₀ = 5.65 √S₀Ties gauge length to cross-section, so elongation is comparable between specimens of different size. This is the preferred form.
- Non-proportional gauge length
- Fixed, commonly 50 mm or 200 mmUsed where a product standard specifies it. Elongation on a fixed gauge is not comparable with a proportional one.
- Round specimens
- Machined from bar, forgings and thick section
- Flat specimens
- From plate, sheet and strip
- Full-section specimens
- Reinforcing bar and wire tested as manufacturedRebar is not machined — the rib pattern is part of the product, and the nominal area is used.
- Surface finish
- Free of machining marks across the gaugeCircumferential tool marks are notches and initiate fracture.
- Mark the original gauge length before testing
- Fine punch or scribe marksDakElongation after fracture is measured between those marks on the reassembled specimen; without them the figure cannot be obtained at all.
Elongation values are meaningless without the gauge length they were measured over. A 20 % elongation on 5.65√S₀ and a 20 % on a fixed 50 mm gauge describe different material behaviour, and product standards are specific about which applies.
Test rate
- Control mode
- Strain rate, or stress rate in the elastic regionHarmonised with ISO 6892-1, which defines methods A and B for rate control.
- Elastic region
- Controlled so the modulus and proof strength are not rate-distorted
- Plastic region
- A defined strain rate range up to fracture
- Do not speed up after yield to save time
- Unless the method permits itDakTensile strength rises with strain rate in most steels, and an accelerated test quietly flatters the material.
Calculations
Tensile strengthRm
Rm = Fm / S₀
- Fm
- maximum force, N
- S₀
- original cross-sectional area, mm²
Upper yield strengthReH
ReH = FeH / S₀
- FeH
- force at the first peak before the yield drop, N
Applies to materials showing a discontinuous yield, which includes most hot-rolled carbon steels.
Proof strengthRp0.2
Rp0.2 = Fp0.2 / S₀
- Fp0.2
- force at 0.2 % non-proportional extension, N
Used where no clear yield point exists — cold-worked, stainless and most non-ferrous materials. It requires an extensometer; it cannot be derived from crosshead travel.
Percentage elongation after fractureA
A = (Lu − L₀) / L₀ × 100
- Lu
- final gauge length after fitting the fracture together, mm
- L₀
- original gauge length, mm
Percentage reduction of areaZ
Z = (S₀ − Su) / S₀ × 100
- Su
- minimum cross-sectional area after fracture, mm²
How the test runs
- 01Select the specimen form and gauge length the product standard requires.
- 02Machine round or flat specimens, or use full section for rebar and wire.
- 03Measure the cross-section at several points and take the minimum.
- 04Mark the original gauge length with fine punch or scribe marks.
- 05Fit wedge grips with inserts matched to the product form.
- 06Align the specimen so it is loaded along its axis.
- 07Fit the extensometer where proof strength or yield is required.
- 08Load under the specified rate control through the elastic region.
- 09Record the yield or proof strength.
- 10Remove the extensometer if it is not rated to fracture, and continue to failure.
- 11Fit the fracture together and measure the final gauge length and minimum area.
- 12Reject fractures outside the gauge length where the product standard requires it.
Grips and fixtures for this method
Self-tighteningTJ-15
Universal Parallel Wedge Grips
Universal parallel wedge grips with inserts matched to the product — flat for plate and strip, V-grooved for round bar.
Specifications
Axial & transverse
Clip-On Extensometers
A clip-on extensometer is required for proof strength: Rp0.2 is defined against a 0.2 % non-proportional extension and cannot be derived from crosshead travel.
SpecificationsWhat the report has to contain
- Reference to IS 1608 and the part and edition
- Material and product identification, including cast or heat number
- Specimen form, dimensions and original cross-sectional area
- Gauge length used, and whether proportional or non-proportional
- Rate control method and rates applied
- Extensometer class where used
- Yield or proof strength, and which was determined
- Tensile strength
- Percentage elongation after fracture, with the gauge length
- Percentage reduction of area where required
- Fracture location and appearance
- Test temperature
What the machine must be capable of
Force measurement to Class 1 across a range that may run from a few kilonewtons for wire to several hundred for structural sections, together with the frame capacity and grip range to match. Wedge grips need inserts matched to the product form — flat for plate, V-grooved for round bar, and serrated for ribbed rebar, which a smooth face cannot hold. Hydraulic grips maintain clamping force as the specimen necks, which prevents late slippage on high-strength material. A Class 1 extensometer is needed for proof strength, and rate control must be capable of holding the specified strain rates rather than approximating them with a fixed crosshead speed.
What goes wrong in practice
Quoting an elongation without its gauge length is the most common reporting error and makes the figure unusable, since proportional and fixed-gauge results differ systematically. Deriving proof strength from crosshead travel rather than an extensometer is the most common measurement error, and it overstates the strain by including machine compliance. Running faster after yield to save time inflates tensile strength consistently enough that it never looks like scatter. On the bench, slippage in the grips on high-strength bar and fractures falling outside the gauge marks are the two recurring causes of a rejected test.
IS 1608, ISO 6892-1 or ASTM E8/E8M
| IS 1608 | ISO 6892-1 | ASTM E8/E8M | |
|---|---|---|---|
| Gauge length | 5.65√S₀ preferred | 5.65√S₀ preferred | Fixed gauge lengths preferred |
| Rate control | Strain-rate based, ISO-harmonised | Methods A and B | Rate ranges by material and stage |
| Yield convention | ReH / ReL, or Rp0.2 | ReH / ReL, or Rp0.2 | Yield point or 0.2 % offset |
| Used by | Indian product standards and BIS certification | International and European specifications | North American specifications |
IS 1608 is harmonised with ISO 6892-1, so results generally transfer between them. ASTM E8/E8M uses different preferred gauge lengths, which changes the reported elongation — that figure in particular should not be assumed to transfer between the ISO and ASTM families.
Questions we are asked about this test
What is IS 1608?
It is the Indian Standard for tensile testing of metallic materials at ambient temperature, harmonised with ISO 6892-1. It specifies specimen forms, gauge lengths, rate control and the calculations for yield and proof strength, tensile strength, elongation after fracture and reduction of area. Most Indian steel and metal product standards call up IS 1608 as their tensile method.
How does IS 1608 relate to ISO 6892-1?
It is harmonised with it, which means the specimen forms, rate control approach and reported quantities follow ISO practice. In everyday use results transfer between the two, and a laboratory equipped to run one is equipped to run the other. The Indian standard exists so that BIS product standards can call up a national document, and it is the version cited in Indian certification and acceptance work.
Why does gauge length matter so much for elongation?
Because elongation after fracture concentrates in the necked region, so a shorter gauge length contains a larger proportion of that concentrated deformation and reports a higher percentage. The proportional gauge length of 5.65√S₀ ties gauge length to cross-section so that specimens of different sizes are comparable. An elongation figure quoted without its gauge length cannot be used, and product standards are specific about which applies.
When do I report yield strength and when proof strength?
Yield strength where the material shows a discontinuous yield — a clear drop in load at the onset of plastic deformation, which most hot-rolled carbon steels do. Proof strength where it does not, which covers cold-worked steels, stainless grades and most non-ferrous metals. Proof strength at 0.2 % non-proportional extension requires an extensometer, since it is defined against a strain the crosshead cannot measure.
Can reinforcing bar be tested without machining?
Yes, and it should be. Rebar is tested full-section because the rib pattern is part of the product and machining it away would test a different article. The nominal cross-sectional area from the product standard is used in the calculation rather than a measured one. The ribs also mean serrated grip faces are needed — a smooth face has nothing to hold on to.
Why can't I speed up the test after yield?
Because tensile strength rises with strain rate in most steels, so an accelerated test reports a higher strength than the material would give at the specified rate. Unless the method explicitly permits a rate change after yield, running faster to save time quietly flatters the material — and because the effect is consistent, it does not show up as scatter that anyone would investigate.
What do I do if the specimen breaks outside the gauge length?
Check what the product standard says, because practice varies. Many require the test to be repeated if the fracture falls outside the gauge marks or too close to a grip, since the elongation measurement assumes the neck formed within the marked length. Some allow a correction procedure. Either way the fracture location is recorded, because it is the evidence that the elongation figure is valid.
Running IS 1608 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Wide — from a few kilonewtons on small round specimens to over 600 kN on full-section structural products | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | IS 1828-1 / ISO 7500-1 Class 1 | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Strain measurement | An extensometer to IS 12872 / ISO 9513 Class 1 for proof strength; Class 2 acceptable for elongation, gauge length 5.65√S₀ for proportional specimens, commonly 50 or 200 for non-proportional | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Wedge grips — hydraulic or mechanical — sized for the product, with round and flat jaw inserts | Our self-tightening serrated wedge grips, with V-jaws for round specimens, built to the specimen |
| Environment | Ambient, 10 °C to 35 °C unless otherwise specified | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.