Testing the Effect of Tensile Strain Rate on Percentage Elongation After Fracture

The percentage elongation after fracture of a material is not a fixed property — it varies measurably with the tensile strain rate applied during testing, making strain rate control a critical factor in obtaining comparable and reproducible results. This article explains how accredited third-party laboratories evaluate the effect of tensile speed on elongation after fracture in accordance with ISO 6892-1, ASTM E8/E8M, GB/T 228.1, and ISO 527, covering test specimens, procedures, data evaluation, and practical applications.

Technical Overview

Percentage elongation after fracture (symbol A) is the plastic extension of a tensile specimen expressed as a percentage of its original gauge length L₀ after the specimen has fractured and the two pieces are fitted back together. Because the deformation mechanisms of most engineering materials — dislocation glide, twinning, void growth, and necking propagation — are time-dependent, the rate at which strain is applied during a tensile test directly influences this value. Testing the effect of tensile strain rate on elongation after fracture is therefore an essential verification activity whenever data from different sources, different machines, or different testing regimes must be compared.

In strain-rate-sensitive materials, such as mild steels, austenitic stainless steels, low-carbon deep-drawing grades, many aluminum alloys, copper alloys, and most polymeric materials, a higher loading rate typically raises yield and tensile strength while altering the balance between uniform elongation and post-necking elongation. Faster rates shorten the time available for slip systems and recovery processes to operate, often reducing total ductility, whereas slower rates can allow more extensive uniform deformation. Conversely, some material classes exhibit anomalous behavior — for example, superplastic alloys and certain polymers achieve their maximum elongation only within a narrow, well-defined rate window. A dedicated rate-comparison test program quantifies these effects experimentally rather than assuming them.

The governing standards are ISO 6892-1 (Metallic materials — Tensile testing — Part 1: Method of test at room temperature), ASTM E8/E8M (Standard Test Methods for Tension Testing of Metallic Materials), and GB/T 228.1 for metallic specimens; ISO 527-1/-2 and ASTM D638 govern tensile testing of rigid plastics; ASTM D412 and ISO 37 apply to elastomers. All of these standards define permitted strain rate or stress rate ranges precisely because elongation results are only comparable when the loading rate is controlled and reported.

Testing Samples

Metallic Sheet and Strip Specimens

Flat rectangular specimens with proportional gauge lengths (L₀ = 5.65√S₀ for short proportional gauge, or L₀ = 11.3√S₀ for long proportional gauge) or fixed gauge lengths such as 50 mm are machined or punched from sheet and strip products. Specimen orientation (parallel or transverse to the rolling direction) is documented because anisotropy also interacts with strain rate sensitivity.

Round Bar, Rod, and Wire Specimens

Turned cylindrical specimens with threaded, shoulder, or wedge grips are prepared from bars, rods, and forgings. Wires may be tested full-section with appropriate gripping. Proportional gauge lengths ensure that elongation values from different cross-sections remain comparable.

Tubular and Pipe Segment Specimens

Full-section tube specimens or longitudinal strip specimens cut from pipes are tested with plug supports to prevent crushing at the grips. These products are frequently strain-rate sensitive in the circumferential forming direction, so rate-comparison testing is common for line pipe and boiler tube qualifications.

Polymeric and Elastomeric Specimens

Dumbbell or dog-bone specimens per ISO 527-2 (types 1A/1B) or ASTM D638 (Type I) are used for rigid and semi-rigid plastics, while die-cut rings or dumbbells per ASTM D412 and ISO 37 are used for rubbers. Polymers are strongly viscoelastic, so test speed is one of the dominant variables and must be varied systematically, typically from 1 mm/min to 500 mm/min crosshead speed.

Welded Joint Specimens

Transverse and longitudinal welded specimens are tested to determine whether the weld region changes the strain rate dependence of the joint relative to the base material, which is important for pipeline, pressure vessel, and structural fabrication acceptance.

Testing Items

A rate-effect characterization program typically measures the following items at each prescribed rate level:

• Percentage elongation after fracture (A, A_50mm, A_5.65√S0) — the primary output of the comparison
• Percentage uniform elongation (A_g) — extension at maximum force, before necking begins
• Percentage total elongation at fracture (A_t) — measured with the extensometer attached throughout the test
• Post-necking (local) elongation contribution, derived as A − A_g, which reveals where rate sensitivity acts
• Upper and lower yield strengths (R_eH, R_eL) and 0.2% proof strength (R_p0.2) at each rate
• Tensile strength (R_m) and percentage reduction of area (Z) as correlated parameters
• Strain rate sensitivity exponent (m), calculated from the slope of flow stress versus strain rate on a log-log plot when multiple rates are tested
• Fracture mode characterization (ductile cup-cone versus brittle shear or cleavage-type fractures) at each rate level

Testing Methods

Equipment and Calibration

Testing is performed on servo-hydraulic or electromechanical universal testing machines with load cells calibrated to ISO 7500-1 or ASTM E4, equipped with calibrated extensometers meeting ISO 9513 or ASTM E83 class requirements. For metals, closed-loop control of extensometer strain rate (not merely crosshead speed) is mandatory for Method A of ISO 6892-1. Video extensometers or automatic contact extensometers are preferred when elongation after fracture must be captured without manual reassembly error. Environmental conditions — typically 23 ± 5 °C for metals and 23 ± 2 °C with 50 ± 10% relative humidity for plastics — are recorded, since temperature also shifts ductility.

Strain Rate Control per ISO 6892-1 Method A

Method A specifies closed-loop strain rate control using standard rates of 0.00007 s⁻¹, 0.00025 s⁻¹, or 0.0025 s⁻¹ for the yield determination range, and faster rates (0.0067 s⁻¹, 0.025 s⁻¹, or 0.125 s⁻¹) for post-yield extension up to fracture. In a rate-effect study, identical specimen sets are tested at two or more of these standard rates — for example 0.00025 s⁻¹ versus 0.0067 s⁻¹ — while every other parameter is held constant. Method A2 extends the rate control into the plastic range using the estimated strain over the parallel length after the extensometer is removed, enabling genuine rate comparison up to fracture.

Stress Rate and Crosshead Speed Control (Methods B and C)

For comparison with legacy data, Method B applies controlled stress rates (typically 2–12 MPa/s for determination of proof strength, or up to 60 MPa/s beyond yield), while Method C of GB/T 228.1 uses a constant crosshead separation speed. Because crosshead speed does not equal true specimen strain rate (machine compliance and grip slip intervene), Method C results must be interpreted cautiously when quantifying rate effects. A rigorous study usually combines Method A as the primary basis with Method B/C testing to bracket historical data scatter.

Measurement of Elongation After Fracture

Elongation after fracture is determined by one of two accepted techniques. In the reassembly method, the fractured specimen pieces are fitted together at the fracture faces with light pressure, and the final gauge length L_u is measured with a vernier or optical comparator; A = (L_u − L₀)/L₀ × 100%. Alternatively, an automatic method records extension at the moment of fracture, provided the extensometer gauge span matches the reference gauge length. When fracture occurs outside the gauge length or within a distance of L₀/8 from a gauge mark, the result is invalidated and retested, because such fractures distort the elongation-versus-rate comparison.

Test Matrix Design and Data Evaluation

A typical comparison matrix uses at least three replicate specimens per rate level, with rates spanning at least one decade (for example 0.00025 s⁻¹, 0.0025 s⁻¹, and 0.025 s⁻¹ for metals, or 5, 50, and 500 mm/min for plastics). Results are reported as mean values with standard deviations, plotted as elongation versus logarithm of strain rate, and evaluated using statistical tests to determine whether the observed differences exceed measurement uncertainty. The strain rate sensitivity exponent m is reported where applicable, and the report explicitly states the control method, control rates, gauge length, and specimen orientation so that results remain auditable and reproducible.

Applications

Rate-effect testing of elongation serves a wide range of industrial and research purposes:

• Resolving disputes over mismatched elongation results between a supplier and a purchaser by identifying uncontrolled test speed as the root cause
• Validating production acceptance criteria for automotive sheet steels, where press-forming speeds differ from laboratory test speeds
• Qualifying pipeline and line pipe steels, welds, and pressure vessel materials where dynamic service loading and impact-like events must be considered
• Supporting material model development and finite element simulation input for crash, drop-test, and metal-forming analyses that require strain-rate-dependent ductility data
• Verifying compliance of laboratory test procedures with ISO 6892-1 Method A in accreditation audits and interlaboratory comparisons
• Characterizing polymers, packaging films, and elastomeric components whose service performance depends on loading speed, such as seals, gaskets, and flexible laminates
• Research into superplastic forming, high-strain-rate manufacturing, and additive-manufactured materials with unusual rate sensitivity

FAQ

Why does tensile strain rate change the elongation after fracture?

Deformation in real materials is time-dependent. At higher strain rates, dislocation motion has less time to bypass obstacles, diffusion-assisted relaxation is suppressed, and strain hardening concentrates deformation into the necking region, which often lowers total elongation. At lower rates, more uniform plastic flow can develop, increasing elongation. The magnitude of the effect depends on the material class, crystal structure, and temperature.

How large can the effect be in practice?

For strain-rate-sensitive steels such as interstitial-free or austenitic grades, shifting the test rate across the permitted standard ranges can change measured elongation by several percentage points — frequently more than the entire acceptance tolerance. For polymers, elongation at break can differ by a factor of two or more between the slowest and fastest permitted speeds. This is exactly why the standards now prescribe narrow, controlled rate ranges.

Which strain rate should be used for reporting standard elongation values?

Unless a product specification states otherwise, ISO 6892-1 Method A with a strain rate of 0.00025 s⁻¹ up to yield and 0.0067 s⁻¹ (or the closest specified rate) thereafter is the default for metals, because it gives the best reproducibility between laboratories. The chosen rate must always be stated in the test report.

Is crosshead speed an acceptable substitute for strain rate control?

No, not when quantifying rate effects. Crosshead displacement includes machine deflection and grip movement, so the true specimen strain rate differs from the commanded value, especially at low loads and in the elastic range. Closed-loop extensometer control or verified equivalent methods are required for valid rate comparisons.

How many specimens are needed for a reliable rate comparison?

A minimum of three valid specimens per rate level is recommended, with five or more preferred for materials with high scatter. All specimens should come from the same batch and orientation, and any fracture outside the gauge length must be discarded and replaced so that only geometrically valid results enter the comparison.

Can rate-effect testing be performed at elevated or low temperatures?

Yes. ISO 6892-2 (elevated temperature), ISO 6892-3 (low temperature), and ASTM E21 cover temperature-controlled tensile testing with defined strain rates. Temperature and strain rate interact strongly, so combined programs are valuable for components serving in demanding thermal environments.