Slow strain rate testing (SSRT), also known as constant extension rate testing (CERT), is the most widely used accelerated method for evaluating the stress corrosion cracking and hydrogen embrittlement susceptibility of metallic materials, and the accurate determination of the tensile strain rate—typically within 10⁻⁴ to 10⁻⁷ s⁻¹—is the single most critical parameter governing test validity. This article explains the principles, governing standards (ISO 7539-7, ASTM G129, GB/T 35079), specimen types, strain rate determination procedures, data interpretation, and typical industrial applications of slow strain rate tensile testing performed in accredited third-party laboratories.
Technical Overview
Slow Strain Rate Testing (SSRT), also called Constant Extension Rate Testing (CERT), is an accelerated laboratory technique used to evaluate the susceptibility of metallic materials to environmentally assisted cracking (EAC), primarily stress corrosion cracking (SCC) and hydrogen embrittlement (HE). A tensile specimen is strained monotonically to failure at a deliberately slow, constant extension rate—corresponding to a nominal strain rate typically between 10⁻⁴ and 10⁻⁷ s⁻¹, most commonly 10⁻⁶ s⁻¹—while fully immersed in the corrosive environment of interest. The results are compared with an identical test performed in an inert reference environment (air, inert gas, or oil) to quantify the degradation caused by the environment.
The determination of the strain rate is the defining technical feature of the method, because cracking susceptibility is strongly rate-dependent. At conventional tensile testing speeds, mechanical rupture occurs before any significant environmental interaction takes place; at extremely slow rates, passive films repassivate faster than they rupture and hydrogen may diffuse away before accumulating at stress concentrators. Between these extremes lies a critical strain rate window in which film rupture, anodic dissolution at the crack tip, and hydrogen uptake proceed in step with plastic deformation, maximizing the apparent susceptibility. This window is material- and environment-specific: for example, carbon and low-alloy steels in high-pH carbonate/bicarbonate solutions and austenitic stainless steels in hot chloride media are typically most susceptible at rates on the order of 10⁻⁶ s⁻¹, while some high-strength steels in hydrogen-bearing media respond at even slower rates. For this reason, strain rate determination is not a trivial machine setting but a structured part of test design.
The principal standardization documents are ISO 7539-7 (slow strain rate testing of tension specimens), ASTM G129 (slow strain rate testing to evaluate susceptibility to environmentally assisted cracking), NACE TM0198 (SSRT screening of corrosion-resistant alloys in sour oilfield service), and GB/T 35079 (the Chinese national standard for slow strain rate stress corrosion testing). Reference tensile tests in inert media are generally performed according to ISO 6892-1, ASTM E8/E8M, or GB/T 228.1, while load frame verification follows ISO 7500-1 or ASTM E4 and extensometer calibration follows ISO 9513 or ASTM E83.
Testing Samples
Smooth Cylindrical Tensile Specimens
The standard specimen for slow strain rate testing is a smooth cylindrical tensile bar, typically with a gauge diameter of 4 to 6 mm and a gauge length of 25 to 30 mm (or five times the diameter), machined with threaded or shouldered grip ends. The gauge section is ground and polished longitudinally to a surface roughness of Ra 0.4 to 0.8 μm to eliminate circumferential machining marks that could act as crack initiation sites. Specimens are identified only outside the gauge length and are degreased and weighed before testing.
Notched Tensile Specimens
Circumferentially notched specimens, usually with a 45° to 60° V-notch and an elastic stress concentration factor (Kt) of approximately 3 to 5, are used when a localized triaxial stress state is required. Notched geometry is preferred for hydrogen embrittlement evaluations of high-strength steels, threaded fastener materials, and weldments, because it concentrates hydrogen-assisted cracking in a well-defined location and shortens test duration.
Welded and Cross-Weld Specimens
For weldment qualification, specimens are machined with the weld metal and heat-affected zone located within the gauge length so that the test captures the least resistant region of the joint. Cross-weld slow strain rate testing is widely applied to pipeline girth welds, clad and overlay materials, and repair welds in sour or high-temperature aqueous service.
Product-Form Coupons
Specimens can be extracted from actual products—line pipe and tube wall sections, fastener blanks, bar, forging, wire, and sheet—machined in the orientation specified by the applicable product standard. Subsize flat specimens are used when product thickness is insufficient for full-size cylindrical bars.
Testing Items
A complete slow strain rate test program reports both the mechanical response and the environmental degradation indicators. Typical testing items include:
- Determination and verification of the nominal initial strain rate (crosshead displacement rate setting divided by gauge length), typically 10⁻⁴ to 10⁻⁷ s⁻¹
- Time to failure in the corrosive environment and in the inert reference environment
- Ultimate tensile strength and yield strength in both environments
- Elongation to failure and reduction of area after fracture
- Normalized susceptibility ratios: time-to-failure ratio, elongation ratio, reduction-of-area ratio, and strength ratio (environment value divided by inert value)
- Hydrogen embrittlement index derived from the loss of reduction of area or elongation
- Density and morphology of secondary cracks on the gauge surface
- Quantitative fractography by scanning electron microscopy (SEM), including classification of fracture mode and measurement of brittle intergranular or transgranular fracture area fraction
Testing Methods
Apparatus and Calibration
Slow strain rate testing is performed on dedicated constant extension rate machines—gear-reduced electromechanical or servo-controlled frames capable of stable crosshead speeds from roughly 10⁻⁴ to 10⁻¹ mm/min. Load measurement is verified to ISO 7500-1 or ASTM E4, and displacement and extensometry are calibrated to ISO 9513 or ASTM E83. The specimen is housed in an environmental containment ranging from a glass or polymer cell for ambient aqueous service to a titanium-lined or stainless autoclave with refresh loop for high-temperature, high-pressure water chemistry (up to approximately 350 °C and 20 MPa). Test solutions are controlled for temperature (typically within ±2 °C), dissolved oxygen content (deaeration with nitrogen or argon, or controlled oxygen dosing), and aggressive species such as chloride, H₂S, or CO₂; optional potentiostatic control allows testing at fixed electrochemical potentials.
Strain Rate Selection and Determination
The nominal initial strain rate is determined as the crosshead displacement rate divided by the gauge length. For example, a 25 mm gauge length tested at a target rate of 1×10⁻⁶ s⁻¹ requires a crosshead speed of 1.5×10⁻³ mm/min; a specimen failing at 20% elongation will then fail after approximately 55 hours, which is why tests commonly run two to five days. When the susceptibility window of a new material–environment pair is unknown, a screening matrix of rates—for instance 1×10⁻⁵, 1×10⁻⁶, and 1×10⁻⁷ s⁻¹—is executed to locate the rate of maximum sensitivity. Practitioners should note that the constant crosshead speed convention produces a decreasing true strain rate once plastic instability and necking begin, so ISO 7539-7 and ASTM G129 express the test rate as the initial nominal value; frame compliance and grip settling must also be considered when converting machine speed to specimen strain, ideally verified with a calibrated extensometer on the gauge section.
Test Procedure
The standard sequence comprises: (1) a baseline tensile test in an inert environment at the selected strain rate to establish reference mechanical properties; (2) exposure of a replicate specimen to the aggressive environment, allowing sufficient stabilization time for the environment to reach the specified chemistry and temperature before loading begins; (3) straining to failure, with continuous recording of load, displacement, time, and (where relevant) corrosion potential; and (4) termination at fracture, at a defined load drop below the maximum load (commonly 20 to 50%), or after a specified extension limit. Each condition is normally tested with at least two or three replicate specimens to manage scatter. After the test, fracture surfaces are examined by SEM, the gauge surface is inspected for secondary cracking, and reduction of area is measured fractographically or by matched halves.
Data Interpretation and Acceptance
Susceptibility is judged from the ratios of properties in the aggressive environment to those in the inert reference. Reduction-of-area and time-to-failure ratios are the most sensitive indicators; ratios close to 1.0 indicate negligible susceptibility, while decreasing ratios, extensive secondary cracking, and brittle intergranular or transgranular facets on a predominantly ductile fracture surface confirm environmentally assisted cracking. Many specifications apply a threshold—for example, a reduction-of-area ratio of 0.8 or higher—to define acceptable behavior, although the exact criterion is a contractual or qualification decision. Because SSRT deliberately accelerates degradation through dynamic straining, results can overstate service susceptibility; critical applications therefore often require confirmation with constant-load tests (ISO 7539-2), constant-deformation tests (ISO 7539-3), or fracture-mechanics-based crack growth testing before final material qualification by accredited third-party laboratories.
Applications
Slow strain rate tensile testing with rigorously determined strain rates is applied across virtually every industry in which metals operate under tensile stress in aggressive environments:
- Oil and gas: screening of corrosion-resistant alloys for sour (H₂S) service per NACE TM0198 and material selection programs aligned with ISO 15156 environments
- Refining and petrochemicals: chloride stress corrosion cracking of austenitic stainless steels, caustic cracking of carbon steel, and polythionic acid cracking of sensitized equipment
- Nuclear power: evaluation of alloy 600/690, stainless steels, and low-alloy steels in simulated primary and secondary water chemistries at elevated temperature and pressure
- Pipelines: assessment of line pipe steels and girth welds for high-pH and near-neutral-pH stress corrosion cracking susceptibility
- Aerospace and automotive: hydrogen embrittlement evaluation of high-strength steels, plated fasteners, and landing gear components
- Hydrogen energy: compatibility testing of steels for hydrogen production, storage, pipelines, and fueling systems, including testing directly in pressurized hydrogen gas
- Marine and offshore: qualification of mooring chain, riser, and offshore structure alloys in seawater and splash-zone environments
- Research and development: ranking of new alloys, heat treatments, coatings, and welding procedures during material development and qualification
FAQ
What strain rate is normally used in slow strain rate tensile testing?
A nominal initial strain rate of 1×10⁻⁶ s⁻¹ is the most common default because it places most engineering alloy–environment systems within their susceptibility window. The general working range is 10⁻⁴ to 10⁻⁷ s⁻¹, and screening at several rates is recommended whenever the material–environment combination is not well characterized.
How is the tensile strain rate actually determined in the laboratory?
The strain rate is determined as the crosshead displacement rate divided by the specimen gauge length. For a 25 mm gauge length, a rate of 1×10⁻⁶ s⁻¹ corresponds to a crosshead speed of 0.0015 mm/min. The setting is verified through calibrated displacement systems and, where practical, with a calibrated extensometer mounted on the gauge section, with machine compliance taken into account.
Why cannot a standard tensile test speed be used?
Conventional tensile rates (10⁻³ to 10⁻² s⁻¹) are far too fast for electrochemical processes at the crack tip—film rupture, localized dissolution, and hydrogen absorption and diffusion—to keep pace with deformation. The specimen simply fails ductilely before the environment can contribute, giving a false indication of immunity.
Which standards govern slow strain rate testing?
The primary standards are ISO 7539-7, ASTM G129, NACE TM0198 for sour service screening, and GB/T 35079. Baseline tensile testing in inert media follows ISO 6892-1, ASTM E8/E8M, or GB/T 228.1, and machine and extensometer verification follows ISO 7500-1/ASTM E4 and ISO 9513/ASTM E83 respectively.
How is susceptibility quantified?
Through normalized ratios—the reduction-of-area, elongation, time-to-failure, and strength ratios of the aggressive-environment test to the inert-reference test—supported by secondary crack counts and SEM fractography that quantifies the brittle fracture area fraction. Ratios significantly below unity demonstrate environmentally assisted cracking susceptibility.
How long does a typical test take?
At 1×10⁻⁶ s⁻¹, failure of a 25 mm gauge specimen typically occurs after roughly two to five days depending on material ductility. Tests at 10⁻⁷ s⁻¹ may extend beyond a week, and notched or pre-fatigued configurations are sometimes used to shorten duration when justified.
Can welded joints be tested?
Yes. Cross-weld specimens with the weld metal and heat-affected zone inside the gauge length are routinely tested to qualify welding procedures for sour, chloride, and high-temperature aqueous service, often in combination with hardness mapping and microstructural examination.
Do slow strain rate results directly predict service life?
No. SSRT is a highly accelerated comparative method in which constant dynamic straining intensifies environmental attack. It is excellent for ranking materials and screening susceptibility, but quantitative life prediction normally requires supplementary constant-load, constant-deflection, or crack-growth testing under conditions closer to actual service.