Displacement rate controlled tensile testing is a fundamental mechanical testing method in which the crosshead of a universal testing machine is driven at a constant, programmed speed to determine the tensile properties of metallic and polymeric materials. Because yield strength, tensile strength, and elongation are all sensitive to loading rate, precise control of displacement rate is essential for producing accurate, repeatable, and standards-compliant test data. This article explains the principles, applicable standards, specimen types, test procedures, and typical industrial applications of displacement rate controlled tensile testing.
Technical Overview
Displacement rate controlled tensile testing is a control strategy for uniaxial tension testing in which the moving crosshead of a servo-controlled universal testing machine travels at a constant, pre-programmed velocity while the force and extension of the specimen are continuously recorded. It is one of three classical control regimes used in tensile testing, alongside stress rate control and closed-loop strain rate control, and it remains the most widely implemented method in routine production and quality-control laboratories worldwide.
In the framework of ISO 6892-1 and its Chinese counterpart GB/T 228.1, displacement rate control corresponds to Method B, in which the estimated strain rate is derived from the crosshead separation speed divided by the parallel length of the specimen. ASTM E8/E8M similarly permits crosshead-speed-based testing, with allowable speeds related to the length of the reduced section, while JIS Z 2241 prescribes equivalent provisions for the Japanese market. The principal advantage of displacement rate control is its robustness: the control loop does not depend on extensometer feedback, which makes the method simple to automate, easy to validate, and highly reproducible between laboratories.
Because most engineering materials are strain-rate sensitive, the test speed has a direct influence on the measured results. Higher displacement rates generally raise yield strength and tensile strength and reduce uniform elongation, particularly in mild steels and polymers. Consequently, disciplined selection and tolerancing of the displacement rate is not a matter of convenience but a precondition for valid, intercomparable material data.
Testing Samples
Metallic Materials
Displacement rate controlled testing applies to virtually all metallic product forms, and specimen geometry is selected according to the source material and the governing standard.
- Flat products: specimens machined from sheet, strip, and plate, with proportional gauge lengths of 5.65√S0 or non-proportional gauge lengths of 50 mm or 80 mm.
- Long products: round, square, and hexagonal bars, rod, and wire, tested as machined cylindrical specimens or in full section.
- Tubes: full-section tube specimens or longitudinal and transverse strips taken from the pipe wall.
- Castings, forgings, and weldments: specimens removed from designated locations, including cross-weld specimens for welded joint evaluation.
Plastics and Composites
Rigid and semi-rigid plastics are tested using machined or molded dumbbell specimens, such as types 1A and 1B per ISO 527-2 or types I through V per ASTM D638. Fiber-reinforced composites are typically tested with rectangular straight-sided coupons at low displacement rates to avoid dynamic effects.
Elastomers and Flexible Materials
Rubber and elastomeric materials are tested with die-cut dumbbells or ring specimens per ASTM D412 and ISO 37, usually at comparatively high crosshead speeds such as 500 mm/min to reflect service-relevant loading conditions.
Special Components
Fasteners, threaded rods, structural connectors, spring wire, and additively manufactured coupons are also evaluated under displacement rate control, either as full-size components or as representative machined specimens.
Testing Items
When conducted under controlled displacement rates, the tensile test yields a complete engineering stress-strain curve from which the following properties are determined:
- Upper yield strength (ReH) and lower yield strength (ReL) for materials exhibiting a yield plateau.
- Proof strengths, including Rp0.2, Rp1.0, and Rt0.5, for materials with continuous transition behavior.
- Tensile strength (Rm).
- Elongation after fracture (A, A50mm, A80mm) and total extension at fracture (At).
- Reduction of area (Z) for cylindrical specimens.
- Young's modulus (E), determined with a Class 1 or better extensometer at reduced crosshead speed.
- Strain hardening exponent (n) and plastic strain ratio (r) for sheet metal formability assessment, using the slow rate stages permitted by the standards.
- Complete force-displacement and stress-strain datasets suitable for material card generation and finite element analysis.
Testing Methods
Testing is performed on electromechanical or servo-hydraulic universal testing machines equipped with load cells verified per ISO 7500-1 or ASTM E4 and extensometers classified per ISO 9513 or ASTM E83. Grip selection, which may include wedge, hydraulic, or pneumatic grips, and careful axial alignment are essential to prevent bending stresses that distort yield measurements.
Rate Specification in Major Standards
- ISO 6892-1 / GB/T 228.1, Method B: the crosshead speed is calculated as the target estimated strain rate multiplied by the parallel length. Commonly applied rates correspond to estimated strain rates of 0.00007/s, 0.00025/s, 0.0025/s, and 0.0067/s, and multi-stage profiles, such as a slow rate through yield followed by a faster rate during strain hardening, are permitted within defined tolerances.
- ASTM E8/E8M: rate of stressing is limited during elastic loading when yield properties are determined, and free-running crosshead speeds for tensile strength determination are proportioned to the length of the reduced section.
- ISO 527-1 / ASTM D638 for plastics: standard crosshead speeds from 1 to 500 mm/min, with modulus determination normally performed at 1 mm/min.
- ASTM D412 / ISO 37 for elastomers: a typical test speed of 500 mm/min.
- ISO 6892-2 for elevated-temperature testing of metallic materials, applying the same rate principles with furnace and thermal-stability requirements.
Typical Test Procedure
- Verify machine calibration, grip condition, and alignment before testing.
- Measure and record specimen dimensions to determine the original cross-sectional area S0.
- Mount the specimen with appropriate gripping force and apply a small preload to remove slack.
- Attach the extensometer to the gauge length if direct strain measurement is required.
- Execute the programmed displacement rate profile, switching between stages as permitted by the standard.
- Record force and extension continuously until complete fracture.
- Reassemble the fractured halves to measure final gauge length and, for round specimens, final diameter for reduction of area.
- Calculate the reported properties and evaluate test validity, including fracture location within the gauge length.
Data Evaluation and Validity
Results are assessed against the validity criteria of the governing standard, such as fracture position relative to the gauge marks and adherence to rate tolerances. Key contributors to measurement uncertainty include rate tolerance, machine compliance, grip slippage, alignment error, and temperature, all of which accredited third-party laboratories control through validated procedures and interlaboratory verification.
Applications
Displacement rate controlled tensile testing underpins material qualification and quality assurance across essentially every engineering sector:
- Automotive: incoming inspection of mild and advanced high-strength steel sheets, validation of n and r values for forming simulation.
- Aerospace: qualification of aluminum alloys, titanium alloys, and superalloys, including statistically based allowables development.
- Construction and infrastructure: mill testing of reinforcing bar and structural steel sections for compliance with EN and GB product standards.
- Energy and pipelines: characterization of line pipe and pressure vessel steels, including weld seam and heat-affected zone evaluation.
- Fasteners and hardware: full-scale tensile verification of bolts, studs, and anchors per ISO 898-1.
- Medical devices and additive manufacturing: mechanical property verification of implant alloys and printed coupons.
- Research and failure analysis: rate-dependence studies and generation of input curves for material modeling.
Accredited third-party laboratories provide these tests for supplier qualification, certification of conformity, dispute resolution, and research programs, ensuring that reported properties are traceable to internationally recognized standards.
FAQ
What is the difference between displacement rate, strain rate, and stress rate control?
Displacement rate control fixes the crosshead speed, stress rate control fixes the rate of increase of stress during elastic loading, and strain rate control uses extensometer feedback to hold the actual specimen strain rate constant. Displacement rate control is the simplest and most robust of the three, although the actual strain rate in the specimen can deviate from the nominal value due to machine compliance and grip effects.
Why does the displacement rate affect tensile test results?
Most materials exhibit rate-dependent deformation behavior. Increasing the displacement rate raises the measured yield and tensile strengths and can reduce ductility values, because dislocation motion, viscoelastic relaxation, and other time-dependent mechanisms respond to the loading speed. Standardized rates therefore ensure comparability between laboratories.
How is crosshead speed converted into an estimated strain rate?
The estimated strain rate equals the crosshead separation speed divided by the parallel length of the specimen, following Method B of ISO 6892-1 and GB/T 228.1. This estimate is approximate because it does not account for elastic deflection of the machine and grips.
Can Young's modulus be measured under displacement rate control?
Yes, provided a calibrated extensometer of suitable accuracy class is attached directly to the specimen and the test is run at the reduced speed prescribed for modulus determination, for example 1 mm/min for plastics. Crosshead displacement alone should never be used for modulus calculation because machine compliance introduces significant error.
When must strain rate control be used instead of displacement rate control?
Closed-loop strain rate control, corresponding to Method A of ISO 6892-1, is preferred or required when the highest accuracy in yield properties is needed, when comparing data across machines of different stiffness, or when a customer specification explicitly designates Method A. Displacement rate control remains fully valid and widely accepted for routine testing when applied within the standard's tolerances.
What tolerances apply to the displacement rate?
ISO 6892-1 and GB/T 228.1 limit the permitted variation of the estimated strain rate, typically to a band around the nominal value during each stage of the test, and these tolerances must be verified by periodic calibration of the crosshead speed. Comparable machine verification requirements appear in ASTM E8/E8M and the associated calibration standards.
