The tensile strain rate applied during a tensile test has a measurable influence on the yield point and yield strength values of metallic materials, making rate control essential for accurate and comparable results. This article explains the principles behind strain rate sensitivity, the standardized test methods used to quantify its effect on the yield point, and how accredited third-party laboratories apply international standards such as ISO 6892-1, ASTM E8/E8M, and GB/T 228.1 in practice.
Technical Overview
The yield point of a metallic material is not a fixed material constant; it is a measured property that depends on the loading conditions under which it is determined. Among these conditions, the tensile strain rate is one of the most significant variables. When a specimen is deformed at a higher rate, dislocation motion becomes more difficult relative to thermally activated relaxation processes, and the measured yield stress generally increases. Conversely, slower rates allow more time for dislocation rearrangement, producing lower measured yield values.
This rate sensitivity is particularly pronounced in body-centered cubic (BCC) materials such as carbon and low-alloy steels, which often exhibit a distinct upper and lower yield point. For materials without a pronounced yield plateau, such as austenitic stainless steels and most aluminum alloys, the 0.2% proof strength is used as the equivalent yield indicator, and it is similarly affected by rate.
Modern tensile testing standards have therefore shifted from stress-rate control to strain-rate control. ISO 6892-1, ASTM E8/E8M, GB/T 228.1, and JIS Z 2241 all define specific strain rate ranges for the determination of yield properties, because two laboratories testing the same material at different rates may report yield strength values that differ by several percent, even though the material itself is identical. Determining the effect of tensile rate on the yield point involves systematically testing replicate specimens at controlled, documented rates and quantifying the resulting variation in yield parameters.
Testing Samples
Carbon and Low-Alloy Steels
Structural steels, ship plate, pressure vessel steels, and reinforcement bars are the classic materials for yield point studies because they typically display a pronounced upper yield strength (ReH) and lower yield strength (ReL). Samples are usually machined or full-section proportional flat and round specimens prepared in accordance with the geometry requirements of ISO 6892-1 or ASTM E8/E8M.
Sheet and Strip Products
Cold-rolled and hot-rolled sheet, automotive advanced high-strength steels (AHSS), and coated strip are tested as rectangular flat specimens. Because sheet formability and crash performance are rate-dependent, yield determination at specified strain rates is directly relevant to design data.
Bars, Rods, and Wire
Round machined specimens or full-section wire samples are used for bar and wire products. Fastener blank rod and prestressing wire are frequently evaluated at multiple strain rates to verify compliance with property clauses that specify the rate condition.
Tubular Products
Linepipe and pressure tube materials are sampled as longitudinal strip specimens, flattened strap specimens, or full-section ring samples depending on diameter and wall thickness, in line with product standards that reference ISO 6892-1 or API-based requirements.
Nonferrous Alloys
Aluminum, copper, titanium, and nickel alloys generally show no distinct yield plateau, so the 0.2% proof strength (Rp0.2) is determined. Although their rate sensitivity is typically lower than that of ferritic steels, it remains measurable and must be controlled when comparing supplier data.
Fasteners and Welded Specimens
Bolts, screws, and stud materials are tested as full-size parts or machined coupons, and cross-weld tensile specimens are used to evaluate the yield behavior of welded joints under rate-controlled conditions.
Testing Items
The principal parameters determined when assessing the influence of tensile rate on the yield point include:
- Upper yield strength (ReH) at each specified strain rate
- Lower yield strength (ReL) at each specified strain rate
- 0.2% proof strength, non-proportional extension (Rp0.2), for materials without a yield plateau
- 0.2% offset yield strength per ASTM E8/E8M
- Yield point elongation (Ae), where applicable
- Stress rate values for comparison with legacy Method B data
- Quantified difference in yield values between test rates, expressed in MPa or as a percentage
- Strain rate sensitivity assessment across two or more rate levels
Testing Methods
Reference Standards and Rate Control Philosophies
Testing is performed in accordance with ISO 6892-1 (Metallic materials — Tensile testing — Part 1: Method of test at room temperature), GB/T 228.1, ASTM E8/E8M, and, where applicable, JIS Z 2241. ISO 6892-1 offers two approaches: Method A, based on closed-loop strain rate control using an extensometer signal, and Method B, the older approach based on stress rate control prior to yielding.
Method A: Strain Rate Control
Method A is the preferred modern approach because it directly controls the parameter that governs yield behavior. For yield determination, the standard defines rates with an estimated error of ±20% in the plastic strain rate, commonly expressed as 0.00007 s⁻¹, 0.00025 s⁻¹, and 0.0025 s⁻¹. The test is typically run in three stages: an elastic stage using crosshead or closed-loop control, a transition stage, and a constant plastic strain rate stage maintained through yield. Method A002, using an estimated crosshead speed based on the measured elastic slope of a precursor specimen, is also permitted when extensometer feedback through yield is impractical.
Method B: Stress Rate Control
Method B specifies stress rates within the elastic range, for example 6 to 60 MPa/s for steels with a conventional modulus and lower ranges for nonferrous alloys. Although still referenced, Method B allows considerable variation in the actual strain rate at yield because the strain rate depends on the material's elastic modulus, which can cause interlaboratory disagreement in yield values.
Rate-Effect Determination Procedure
- Machine verification is confirmed in accordance with ISO 7500-1, and the extensometer is classified per ISO 9513, class 1 or better, with a gauge length appropriate to the specimen.
- Replicate specimens are machined from adjacent locations of the same lot to minimize metallurgical scatter.
- Specimens are tested in sequence at two or more controlled rates, for example 0.00025 s⁻¹ and 0.0025 s⁻¹, keeping all other parameters identical.
- ReH, ReL, or Rp0.2 is evaluated from the force-extension record for each rate, using consistent evaluation criteria for the upper yield peak and the lowest force within the yield plateau.
- Results are tabulated, and the difference in yield values is calculated to quantify the rate effect, together with a statement of the rate actually achieved and its compliance with the ±20% tolerance.
- Temperature, gripping alignment, and specimen preparation are documented, since bending and eccentricity can distort the upper yield point.
Measurement Uncertainty and Reporting
A complete report identifies the standard used, the control method, the specified and achieved rates, the extensometer class and gauge length, individual and mean yield values at each rate, and any deviations. This level of documentation is essential when the data support arbitration, supplier qualification, or finite element input cards that are rate-dependent.
Applications
Rate-controlled yield point determination serves a wide range of industrial and engineering purposes:
- Structural steel qualification and mill certification, where product standards specify the rate condition under which minimum yield values are verified
- Automotive material characterization, where yield data at higher strain rates support crash simulation and forming models
- Linepipe and pressure equipment supply, where yield strength values feed directly into design allowable stresses and must be rate-consistent
- Fastener and prestressing steel verification against property classes that define test rate conditions
- Resolution of interlaboratory disputes by identifying rate differences as the cause of divergent yield results
- Research and development of high strain rate sensitive alloys, bake-hardening steels, and new grades requiring rate sensitivity characterization
- Generation of input parameters for constitutive models used in numerical simulation of plastic deformation
FAQ
Why does the tensile rate change the measured yield point?
Plastic deformation is controlled by dislocation motion, which is partly thermally activated. At higher strain rates, there is less time for thermally assisted dislocation movement, so a higher stress is required to initiate yielding, which raises the measured yield point. The effect is strongest in BCC steels and at room temperature.
What strain rate is specified for yield point determination in ISO 6892-1?
For yield properties under Method A, the plastic strain rate should be maintained within ±20% of one of the standard rates, commonly 0.00025 s⁻¹ as the default for yield determination, with 0.00007 s⁻¹ and 0.0025 s⁻¹ also defined. Product standards or material specifications state which rate applies.
What is the difference between Method A and Method B?
Method A controls strain rate directly using extensometer feedback or a calculated crosshead speed, giving consistent yield values between laboratories. Method B controls stress rate in the elastic region, which results in an uncontrolled and material-dependent strain rate at yield and is retained mainly for continuity with historical data.
How large is the rate effect on yield strength in practice?
The magnitude depends on the material. For common structural steels, changing the strain rate by a factor of ten can shift the lower yield strength by roughly 1 to 5%, while for highly rate-sensitive grades the difference can be larger. This is precisely why documenting the rate is mandatory when reporting yield values.
Can results from different strain rates be compared directly?
They should not be compared without qualification. If two datasets were produced at different rates, the rate condition must be stated and, where necessary, dedicated multi-rate testing should be performed to establish the material's rate sensitivity before drawing conclusions about compliance or differences between suppliers.
What equipment is required for this determination?
A calibrated universal testing machine meeting ISO 7500-1 class 1 or better, a class 1 or class 0.5 extensometer per ISO 9513 with the capability for closed-loop strain rate control, appropriate grips with good alignment, and data acquisition fast enough to resolve the upper yield peak. Accredited third-party laboratories routinely maintain this configuration for rate-controlled yield testing.