The tensile test is the most widely used mechanical test for metals. In India, the method is set out in IS 1608 (Part 1), which is harmonised with the international standard ISO 6892-1 for testing at room temperature. This guide walks through the main parts of the method so that you know what is expected of the specimen, the machine, the operator and the report. It is a practical summary, not a substitute for the standard itself, so keep the current edition at hand when you write your lab procedure.
What IS 1608 Part 1 covers
The standard defines the terms, specimen types, test conditions and calculation methods for tensile testing of metallic materials at ambient temperature. The properties it addresses include:
- Upper and lower yield strength (ReH, ReL) for materials with a distinct yield point
- Proof strength at a specified plastic extension, such as Rp0.2
- Tensile strength (Rm)
- Percentage elongation after fracture (A)
- Percentage reduction of area (Z)
Other parts of the IS 1608 series cover testing at elevated temperature and other conditions.
Specimens and gauge length
Specimens may be machined test pieces, or full sections such as bars, wires and tubes tested without machining. Machined specimens have a parallel length with transition radii to grip ends; their dimensions and tolerances are given in the annexes of the standard for each product form.
The original gauge length (L0) is marked on the specimen before testing. For proportional test pieces, the gauge length is related to the original cross-sectional area S0 by L0 = k√S0, where the coefficient k is internationally 5.65. For round specimens this works out to five times the diameter. Non-proportional gauge lengths are used for some products, such as sheet, and the elongation must then be reported with the gauge length used, because elongation values from different gauge lengths are not directly comparable.
Measure the cross-section accurately at several points along the parallel length and use the smallest area, or the value the standard specifies, in calculations.
Test steps on a universal testing machine
- Check the machine: it should be verified to IS 1828 (Part 1) / ISO 7500-1 in the force range you will use, and the class should meet your requirements.
- Prepare and measure the specimen: mark the gauge length and record dimensions.
- Grip the specimen: align it on the machine axis so that it is loaded in pure tension. Poor alignment introduces bending and lowers results.
- Attach an extensometer if you need proof strength or modulus. It should meet the class required by the standard for the property measured.
- Apply load at the specified rate: the current standard prefers strain-rate control in defined ranges for the elastic, yield and plastic parts of the test, with stress-rate control allowed as an alternative method. Use the method your specification calls for and state it in the report.
- Continue to fracture and record the maximum force.
- Fit the broken pieces together carefully to measure the final gauge length and the minimum cross-section at the fracture.
Calculating the properties
| Property | How it is calculated |
|---|---|
| Tensile strength Rm | Maximum force ÷ original cross-sectional area |
| Proof strength Rp0.2 | Stress at the intersection of the curve with a line parallel to the elastic part, offset by 0.2% strain |
| Elongation after fracture A | (Final gauge length − original gauge length) ÷ original gauge length × 100 |
| Reduction of area Z | (Original area − minimum area after fracture) ÷ original area × 100 |
The standard also sets out when a result is invalid, for example when fracture occurs outside the gauge length in a way that affects elongation, and how to handle such cases.
What the test report should contain
- Reference to the standard used, including the part and edition
- Identification of the material and specimen
- Type of specimen and its dimensions
- Gauge length, and whether the specimen was proportional
- Testing rate or control method
- Test results, rounded as the standard specifies
- Any deviation from the standard or anomalies observed
Testing software on a computerized machine can generate most of this automatically, but the operator remains responsible for checking dimensions, the validity of the fracture and the method stated.
Equipment checklist
- A computerized universal testing machine or an electronic universal testing machine with capacity suited to your strongest specimens
- Grips and jaw faces to suit round, flat and wire specimens
- An electronic extensometer for proof strength and modulus
- Calibrated micrometer or vernier and a gauge-marking device
- Calibration certificates for the machine and extensometer
For a more general overview, see our earlier post on tensile test procedure, specimen and report, and browse the tensile testing machine collection.
Rounding and uncertainty
The standard specifies how results are rounded, for example strength values to the nearest whole N/mm² and elongation values to a defined precision. Accredited labs may also need to estimate measurement uncertainty for each property, considering force measurement, dimensions, extensometer accuracy and test rate. Building these steps into your procedure from the start avoids rework when a customer or assessor asks for them.
Frequently asked questions
Is IS 1608 the same as ISO 6892-1?
IS 1608 (Part 1) is harmonised with ISO 6892-1, so the method is technically equivalent. Always quote the standard your customer or specification asks for.
Why must the gauge length be reported with elongation?
Elongation after fracture depends on gauge length, because necking is concentrated in a small region. Values from different gauge lengths cannot be compared directly.
Can crosshead movement be used instead of an extensometer?
For tensile strength and elongation after fracture measured on the broken pieces, an extensometer is not essential. For proof strength and modulus, it is.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.