Determination of Percentage Elongation After Fracture Using Extensometers: Testing Methods, Standard

Percentage elongation after fracture (A) is one of the most important ductility indicators obtained from tensile testing of metallic materials and a mandatory item in most material test reports. This article explains how elongation after fracture is determined using extensometers, covering applicable standards such as ISO 6892-1, ASTM E8/E8M, GB/T 228.1 and JIS Z 2241, specimen requirements, step-by-step test procedures, data reduction, validity criteria, and typical industrial applications.

Technical Overview

Percentage elongation after fracture, designated A in ISO 6892-1, GB/T 228.1 and JIS Z 2241, and commonly reported simply as "elongation" under ASTM E8/E8M practice, quantifies the permanent plastic deformation a tensile specimen sustains up to fracture. It is calculated from the original gauge length L0 and the final gauge length Lu measured after the fractured specimen halves are carefully fitted back together:

A = (Lu − L0) / L0 × 100 %

Conventional practice determines Lu by manually measuring the distance between pre-marked gauge points. However, manual methods become unreliable for small specimens, fine wires, thin strips, low-ductility high-strength materials, and automated or high-throughput test lines. In these situations the extensometer, a calibrated displacement-measuring device that is either mechanically attached to or optically tracks the specimen, provides an accurate, traceable, and often fully automated alternative. Extensometers used for this purpose must be calibrated in accordance with ISO 9513 or verified in accordance with ASTM E83.

An important distinction must be made between two quantities. Elongation at fracture (Ae) is what an extensometer records continuously and includes the elastic strain present at the instant of fracture. Elongation after fracture (A) is conventionally measured after the specimen is unloaded and the elastic strain has been recovered. When the extensometer gauge length is set equal to L0 and fracture occurs within that gauge length, the applicable standards permit A to be derived from Ae by subtracting the elastic contribution, which is typically estimated from the tensile strength Rm and the elastic modulus E.

Types of Extensometers

• Clip-on mechanical extensometers with knife-edge or point contact on the specimen parallel length.
• Averaging axial extensometers that measure strain on opposite sides of the specimen to compensate for bending.
• Automatic contacting extensometers that engage and disengage under machine control for high-volume testing.
• Non-contact video extensometers that track painted or speckled gauge markers with cameras.
• Laser and optical reflection extensometers suitable for wires, foils, and high-temperature or corrosive environments.
• Digital image correlation (DIC) systems that provide full-field strain maps and virtual gauge lengths.

Accuracy Classes

• ISO 9513 defines extensometer system classes 0.2, 0.5, 1 and 2 based on gauge length error, resolution, and indication error.
• ASTM E83 defines classes A, B-1, B-2, C, D and E with equivalent verification requirements.
• For determination of elongation after fracture, an ISO 9513 class 1 or better system, or an ASTM E83 class B-1 or better system, is generally recommended; class 0.5 / B-1 or higher is preferred when the elastic modulus or proof strength is measured simultaneously.

Testing Samples

Metallic Materials

Extensometer-based elongation testing is applicable to essentially all metallic materials that exhibit measurable ductility, including:

• Carbon steels, low-alloy steels, and stainless steels in all delivery conditions.
• Aluminum and aluminum alloys, copper and copper alloys, titanium and titanium alloys.
• Nickel-based superalloys, zinc alloys, and ductile cast irons.
• Welded joints, heat-affected zones, and additive-manufactured metal specimens.
• Metallic wires, fasteners, and tubular products where manual gauge marking is impractical.

Product Forms and Specimen Geometry

Specimens may be machined from or tested as full sections of sheet, strip, plate, bar, rod, wire, tube, profiles, forgings, and castings. Two categories of gauge length are used:

• Proportional specimens, where L0 = k × √S0 with k = 5.65 (short proportional length, approximately 5d for round bars) or k = 11.3 (long proportional length, approximately 10d). Results are reported as A or A11.3.
• Non-proportional specimens, where L0 is a fixed length such as 50 mm or 80 mm independent of cross-section, reported as A50mm or A80mm.

Extensometer-based determination is particularly valuable when L0 is small (for example 25 mm or less), when specimens are too small to mark with punches or scribes, or when the expected elongation is low and manual reading error becomes significant relative to the measured value.

Specimen Preparation

• Machining must avoid heating or cold-working that alters material properties; surface finish shall meet the requirements of the governing standard.
• Straightening or flattening of specimens is prohibited where it would affect properties, unless permitted and documented.
• Original dimensions shall be measured with the accuracy stipulated in the standard so that S0 and L0 are determined within allowable tolerances.
• Where permitted, gauge marks may still be applied as a backup for manual verification of Lu.

Testing Items

The primary measured quantity is percentage elongation after fracture (A), together with elongation at fracture (Ae) when extensometer data are reported directly. Because the extensometer is mounted for the entire test, the same run typically yields the complete uniaxial tensile data set:

• Percentage elongation after fracture (A, A11.3, A50mm, A80mm) and elongation at fracture (Ae).
• Upper and lower yield strengths (ReH, ReL).
• Proof strengths (Rp0.2, Rt0.5) and, if required, the elastic modulus E.
• Tensile strength (Rm).
• Percentage total extension at maximum force (Agt) and uniform elongation (Ag).
• Reduction of area (Z), determined by complementary manual measurement of the fractured cross-section.

The test report must clearly state whether A or Ae is reported, the gauge length designation, the specimen orientation, the temperature, the strain-rate control method, and the extensometer accuracy class.

Testing Methods

Apparatus and Calibration

• A universal testing machine verified to ISO 7500-1 (class 1 or better) or ASTM E4, preferably with closed-loop control for Method A strain-rate control.
• An extensometer calibrated to ISO 9513 (class 1 or better for elongation; class 0.5 or 0.2 for modulus determination) or verified to ASTM E83 (class B-1 or better), with the gauge length set equal to L0.
• Testing performed at room temperature between 10 °C and 35 °C unless otherwise specified, with temperature-sensitive materials conditioned and recorded accordingly.

Test Procedure

• Measure the specimen dimensions, calculate the original cross-sectional area S0, and determine L0 according to the specified proportional or non-proportional rule.
• Attach or set up the extensometer symmetrically on the parallel length with gauge length equal to L0, and verify zero balance and stable contact or marker tracking.
• Grip the specimen with care to minimize bending; any initial preload shall be kept as low as practicable.
• Apply loading under the strain rates of ISO 6892-1 Method A (for example 0.00025/s through the elastic and yield range, then 0.0067/s to 0.008/s in the plastic range) or Method B stress-rate control, as specified.
• Continue loading to complete fracture while continuously recording force and extension; monitor and note the fracture location.
• Contacting extensometers shall either remain attached to fracture using break-away or auto-release knife edges, or be removed only after maximum force and before fracture, in which case elongation after fracture is completed by manual measurement of Lu.
• Non-contact video or laser extensometers may track deformation all the way to fracture without risk of damage and record the extension at the fracture instant.

Calculation and Elastic Correction

When fracture occurs within the extensometer gauge length, which is equal to L0, the percentage elongation after fracture may be calculated from the recorded elongation at fracture by removing the elastic extension recovered on unloading. The elastic component is estimated from the stress at fracture; a widely used approximation is:

A ≈ Ae − (Rm / E) × 100

The elastic correction becomes significant for high-strength materials and short gauge lengths, and shall be applied and documented in accordance with the governing standard. If the product specification accepts Ae directly, the report shall state this explicitly because A and Ae are not interchangeable.

Validity Criteria and Retesting

• The result is valid only if fracture occurs within the gauge length; if fracture occurs in the transition fillet or within a distance from the grips smaller than permitted by the standard, the elongation value is invalid and a retest is required.
• Retesting shall be performed on a duplicate specimen from the same sample in accordance with the product standard or prior agreement.
• Because post-necking strain localizes in a narrow region, the measured A depends on the position of the neck within L0; comparative evaluations must therefore use identical gauge lengths and specimen geometry.
• Measurement uncertainty contributions include gauge length setting error, extensometer class, elastic correction, temperature, and fracture location, and should be evaluated for conformity decisions near specification limits.

Applications

Extensometer-based determination of elongation after fracture is routinely used by accredited third-party laboratories, manufacturers, and research institutes for material qualification, product release, and failure analysis across many industries:

• Steel and non-ferrous metal producers verifying ductility of plate, strip, bar, rod, wire, and tube against EN, GB, JIS, and ASTM product specifications.
• Construction and infrastructure projects for reinforcing steel and structural steel acceptance testing.
• Automotive and aerospace supply chains requiring full tensile characterization of sheet metals, fasteners, and castings with strict traceability.
• Oil and gas pipelines and pressure vessel fabrication, including welding procedure qualification of weld metal and cross-weld specimens.
• Additive manufacturing research and production, where small machined specimens make extensometer measurement the only practical approach.
• Incoming material inspection, benchmarking studies, and root-cause investigations of brittle or unexpected fractures.

FAQ

Why use an extensometer instead of manual measurement of Lu?

An extensometer removes operator reading error, enables testing of specimens too small or too hard to mark, permits full automation, records the entire strain history for additional properties, and provides traceable calibration to ISO 9513 or ASTM E83, which improves reproducibility and lowers measurement uncertainty.

What is the difference between elongation after fracture (A) and elongation at fracture (Ae)?

A is determined after unloading, so the elastic strain has been recovered, and is conventionally obtained from Lu and L0. Ae is the total extension at the fracture instant recorded by the extensometer and includes elastic strain. A can be derived from Ae by subtracting the elastic contribution, commonly approximated by Rm divided by E, when fracture occurs within the extensometer gauge length.

Which extensometer accuracy class is required?

ISO 9513 class 1 or better, or ASTM E83 class B-1 or better, is generally sufficient for elongation after fracture. Class 0.5 / B-1 or higher accuracy is recommended when proof strength or elastic modulus is determined in the same test.

What happens if the specimen fractures outside the gauge length?

The elongation value is considered invalid because the necking strain is not fully contained within the measured length. The test is repeated on a duplicate specimen unless the product standard explicitly defines alternative acceptance criteria.

Can non-contact video extensometers be used?

Yes. Video, laser, and DIC-based extensometers are increasingly preferred because they track the specimen to fracture without mechanical interference, suit small and delicate specimens, allow virtual gauge lengths, and can be calibrated to the same ISO 9513 or ASTM E83 class requirements as contact devices.

Which standards govern this test?

The principal standards are ISO 6892-1 and its equivalents GB/T 228.1 and JIS Z 2241, ASTM E8/E8M, EN ISO 6892-1, along with ISO 9513 and ASTM E83 for extensometer calibration and verification, and ISO 7500-1 and ASTM E4 for force verification of the testing machine.