Tensile test speed adjustment method testing evaluates how the loading rate of a universal testing machine is controlled, verified, and documented to ensure accurate, reproducible tensile results. Because most engineering materials are strain-rate sensitive, standards such as ISO 6892-1, ASTM E8/E8M, GB/T 228.1, and ISO 527 define precise requirements for stress-rate, strain-rate, and crosshead-speed control. This article explains the main rate control methods, the verification procedures used by accredited third-party laboratories, and how test speed influences measured mechanical properties.
Technical Overview
Tensile test speed adjustment method testing refers to the systematic selection, control, and verification of the loading rate applied during a tensile test. The measured values of yield strength, tensile strength, elongation, and modulus of elasticity are not purely material constants; they depend on the rate at which the specimen is deformed. Faster testing generally raises strength values and lowers ductility values, while slower testing produces the opposite effect. Without disciplined rate control, results from different machines or laboratories cannot be meaningfully compared.
Modern tensile testing standards therefore define several distinct rate control modes. ISO 6892-1, the principal standard for metallic materials at room temperature, specifies Method A based on closed-loop strain rate control and Method B based on stress rate control combined with a nominal crosshead speed. The corresponding Chinese national standard GB/T 228.1 and the Japanese standard JIS Z 2241 follow the same framework, while ASTM E8/E8M defines its own stressing rate and crosshead separation rate limits. For plastics, ISO 527 and ASTM D638 specify a series of preferred constant crosshead speeds, and for elastomers ISO 37 and ASTM D412 define standard test speeds.
Rate adjustment method testing confirms that a testing system can actually achieve and maintain the commanded rate within the tolerance bands of the applicable standard. It involves verifying crosshead speed accuracy, stress-rate accuracy, and closed-loop strain-rate accuracy, and it is typically performed alongside force verification per ISO 7500-1 or ASTM E4 and extensometer verification per ISO 9513 or ASTM E83. Machine speed verification can be conducted in accordance with ASTM E2658, which establishes procedures for verifying and classifying testing machine speed.
Testing Samples
Metallic Materials
Metallic test pieces are the most rate-sensitive category and represent the majority of rate adjustment method testing. Typical specimen forms include machined round bars with proportional or non-proportional gauge lengths, flat rectangular coupons from sheet and plate, wire and rod samples, tubular sections, castings, forgings, and fastener blanks. Welded joint specimens and rebar samples are also frequently tested under rate-controlled conditions.
Plastics and Composites
Thermoplastics, thermosets, and fiber-reinforced composites are tested using dumbbell-shaped specimens such as Type 1A and 1B defined in ISO 527-2 or Types I through V defined in ASTM D638. Because polymers are viscoelastic, their tensile response is strongly dependent on deformation speed, making strict speed control essential.
Rubber and Elastomers
Vulcanized rubber and thermoplastic elastomer samples are typically cut into dumbbell forms per ISO 37 or ASTM D412 and tested at elevated crosshead speeds, commonly 500 mm/min, to reflect service-relevant loading conditions.
Films, Textiles, and Adhesives
Thin films, geotextiles, yarns, bonded lap-shear specimens, and sealant test pieces are tested at speeds defined by product-specific ISO, ASTM, or EN standards. Although the materials differ, the principle is identical: the applied rate must be specified, controlled, and recorded.
Testing Items
Rate adjustment method testing covers two families of testing items: verification of the rate control capability itself, and determination of material properties at controlled rates.
- Crosshead speed accuracy: comparison of commanded speed versus measured speed across the working range, including short-term stability and repeatability.
- Stress rate accuracy: verification that the stress increase per unit time during the elastic loading segment stays within the permitted range of the standard.
- Closed-loop strain rate accuracy: verification that real-time strain rate control, based on extensometer feedback or parallel length estimation, holds the target rate with acceptable deviation and minimal overshoot.
- Rate transition performance: evaluation of the smoothness of switching between elastic-segment stress rate control and plastic-segment strain rate control.
- Yield properties: upper and lower yield strength (ReH, ReL) and proof strength (Rp0.2, Rp1.0) determined at specified rates.
- Tensile strength (Rm), percentage elongation after fracture (A), percentage total extension at maximum force (Agt), and reduction of area (Z).
- Modulus of elasticity, typically determined at a low strain rate in the early elastic region.
- Strain rate sensitivity studies: comparative testing at multiple rates to characterize rate dependence for research or dispute resolution.
Testing Methods
Strain Rate Control (ISO 6892-1 Method A)
Method A uses closed-loop control in which the machine continuously adjusts crosshead speed to keep the actual strain rate at the commanded value. Method A1 calculates strain rate from the extensometer gauge length, giving the highest accuracy, while Method A2 estimates strain rate over the parallel length, which simplifies operation when an extensometer is not used throughout the test. Standard strain rate ranges include, for example, 0.00007 s⁻¹, 0.00025 s⁻¹, 0.0025 s⁻¹, and 0.0067 s⁻¹, with 0.00025 s⁻¹ commonly selected as the reference rate for metallic materials. Closed-loop control is the preferred approach because it compensates automatically for machine compliance and stiffness differences between specimens.
Stress Rate and Crosshead Speed Control (ISO 6892-1 Method B)
Method B controls the test through the stress rate during the elastic segment, followed by a constant crosshead separation speed chosen to approximate the target strain rate. The permitted stress-rate ranges depend on whether the material modulus is below or above 150 000 MPa, as tabulated in the standard. Method B is widely used on conventional machines, but because it is open-loop, the achieved strain rate depends on machine stiffness and specimen geometry, so reproducibility must be confirmed through verification testing.
ASTM E8/E8M Rate Requirements
ASTM E8/E8M for metallic materials defines rate limits in terms of stressing rate during yield determination and crosshead separation rate based on specimen gauge length, with relaxed free-running speed permitted after yield is complete. Laboratories testing to both ISO and ASTM regimes must document which rate convention governs each result, because measured yield values may differ legitimately between the two systems.
Constant Crosshead Speed for Polymers and Elastomers
ISO 527-1 specifies preferred test speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min, with the tensile modulus of plastics conventionally determined at 1 mm/min. ASTM D638 uses an equivalent series of speeds. Rubber testing per ISO 37 and ASTM D412 is usually performed at 500 mm/min. Verification confirms that the machine holds these speeds constant over the full test duration.
Verification and Test Procedure
A typical rate adjustment method test follows these steps:
- Confirm force verification per ISO 7500-1 or ASTM E4 and extensometer classification per ISO 9513 or ASTM E83 before rate testing begins.
- Select the governing standard and control mode (Method A1, A2, B, or constant crosshead speed) based on the material specification.
- Measure and enter the specimen gauge length and parallel length so that speed and strain rate setpoints are computed correctly.
- Run reference speed checks across the machine range, comparing commanded and achieved rates, for example in accordance with ASTM E2658.
- Perform the tensile test while logging commanded rate, achieved rate, force, and extension continuously.
- Evaluate rate stability, overshoot, and transition behavior, and record the actual rates used in the test report, as required by ISO 6892-1.
- Report all mechanical properties together with the rate control mode and the numerical rates applied, ensuring full traceability.
Applications
Rate adjustment method testing supports quality control and materials qualification across every industry that relies on tensile data:
- Metal production and processing: mill certificate verification, incoming inspection of sheet, plate, bar, tube, and wire, and release testing of structural steel and aluminum products.
- Automotive and aerospace: material qualification, supplier approval, and failure investigation where rate-dependent yield behavior must be documented precisely.
- Fastener and component testing: verifying proof load and tensile strength of bolts, screws, and formed parts under standardized rates.
- Plastics, packaging, and composites: grade comparison, material data sheet generation, and design input, where viscoelastic rate sensitivity is pronounced.
- Accredited third-party laboratories: demonstrating machine capability during accreditation assessments and interlaboratory comparisons, where rate discrepancies are a leading cause of inconsistent results.
- Arbitration and dispute resolution: providing defensible tensile data with fully documented rate control when parties disagree on material conformance.
- Research and development: strain rate sensitivity characterization for simulation input, crash modeling, and high-rate application screening.
FAQ
Why does tensile test speed change the test results?
Plastic deformation in metals and viscoelastic flow in polymers are time-dependent processes. At higher rates, dislocation motion and molecular relaxation cannot keep pace with the applied deformation, so strength values increase and elongation values usually decrease. Standardized rate control removes this variability so that results reflect the material rather than the machine settings.
What is the difference between stress rate and strain rate control?
Stress rate control regulates how quickly the force per unit area increases, typically during the elastic segment, and is expressed in MPa/s. Strain rate control regulates how quickly the specimen elongates per unit length per second, expressed in s⁻¹, and can be maintained throughout yield and work hardening using closed-loop feedback. Strain rate control is generally more accurate and more reproducible because it compensates for machine compliance.
Should Method A or Method B be used for metallic materials?
ISO 6892-1 recommends Method A, particularly Method A1 with extensometer feedback, whenever the equipment supports closed-loop control, because it delivers the best interlaboratory agreement. Method B remains acceptable for conventional machines and legacy procedures, but the method used must be stated in the test report since the two approaches can produce slightly different yield values.
How often should machine speed and rate control be verified?
Force verification is normally performed annually per ISO 7500-1 or ASTM E4, and speed verification following the principles of ASTM E2658 should follow a similar schedule or be performed after relocation, major maintenance, control system updates, or whenever rate-dependent results appear inconsistent. Daily or weekly operational checks with reference specimens supplement formal verification.
Does test speed affect elongation as well as strength?
Yes. Higher test speeds commonly reduce uniform elongation and total elongation after fracture and can shift the measured uniform elongation point. Ductility values are therefore only comparable when tests are run at the same specified rate, which is why standards require the actual rates used to be recorded.
Is strain rate control required for plastics testing?
No. ISO 527 and ASTM D638 control the test through constant crosshead speed rather than closed-loop strain rate, because polymer specimens deform non-uniformly outside the gauge region. However, strict adherence to the specified crosshead speed remains essential, and the speed used must always be reported alongside the property values.