Slow Strain Rate Tensile Testing (SSRT), also known as Constant Extension Rate Testing (CERT), is an accelerated laboratory method for evaluating the susceptibility of metallic materials to stress corrosion cracking (SCC), sulfide stress cracking (SSC), and hydrogen embrittlement. By straining a specimen to failure at a controlled low strain rate—typically between 10⁻⁶ and 10⁻⁴ s⁻¹—while exposed to a corrosive environment, the method delivers quantitative, reproducible results within days rather than months. This article explains the technical principles, specimen types, measured parameters, test procedures, standards, and industrial applications of the SSRT method.
Technical Overview
Slow Strain Rate Tensile Testing is a widely accepted accelerated testing technique used to assess the susceptibility of metals and alloys to environmentally assisted cracking (EAC), which includes stress corrosion cracking (SCC), sulfide stress cracking (SSC), hydrogen embrittlement (HE), and liquid metal embrittlement. The test applies a slow, constant extension rate to a tensile specimen immersed in a controlled corrosive environment until complete fracture occurs. The slow deformation continuously ruptures any protective surface film at emerging slip steps, allowing the combined action of mechanical stress, anodic dissolution, and hydrogen absorption to promote crack initiation and propagation.
The defining characteristic of the method is the strain rate, which is typically selected in the range of 10⁻⁶ to 10⁻⁴ s⁻¹. Within this window, the kinetics of film rupture, repassivation, and hydrogen uptake are balanced in such a way that cracking susceptibility is maximally expressed. At conventional tensile testing speeds, the interaction time between the material and the environment is too short for environmental damage to develop; under constant-load conditions, crack initiation may require months or years. SSRT therefore compresses the failure process into a practical timeframe of roughly one to seven days.
Results are evaluated by comparing the mechanical response of specimens tested in the corrosive environment against specimens of the same material tested in an inert reference environment (such as air, inert gas, or silicone oil) at the same temperature. A significant degradation in ductility, time to failure, or fracture energy—often accompanied by a change in fracture mode from ductile dimpling to intergranular or transgranular brittle fracture—indicates susceptibility to environmentally assisted cracking. The method is standardized internationally in ASTM G129, ISO 7539-7, and GB/T 15970.7.
Testing Samples
Smooth Cylindrical Tensile Specimens
Standard round tensile specimens with a reduced gauge section are the most common configuration. Typical gauge diameters range from 3 mm to 6 mm, with a gauge length-to-diameter ratio of approximately 4:1 to 6:1, following the geometry recommendations of ISO 7539-7 and ASTM G129. The gauge surface is usually mechanically polished to a fine finish and thoroughly degreased to eliminate machining marks and contaminants that could serve as artificial crack initiation sites.
Flat Tensile Specimens
Flat rectangular specimens are used for sheet, plate, strip, and thin-walled products where cylindrical blanks cannot be extracted. Sub-sized flat specimens are also common when evaluating localized regions of a component where material volume is limited.
Notched Specimens
Notched specimens introduce a controlled stress concentration and a triaxial stress state in the notch root. This configuration accelerates crack initiation and is useful when evaluating notch sensitivity, hydrogen embrittlement of high-strength steels, or materials with very low baseline cracking susceptibility.
Pre-Cracked Specimens
Fatigue pre-cracked specimens enable fracture-mechanics-based evaluation of crack growth behavior under slow straining. These specimens allow the crack growth rate and threshold stress intensity behavior to be studied in aggressive environments, complementing the ductility-loss metrics obtained from smooth specimens.
Welded Joint Specimens
Cross-weld specimens spanning base metal, heat-affected zone (HAZ), and weld metal are tested to qualify welded joints in pipelines, pressure vessels, and offshore structures. Because the HAZ often exhibits the lowest resistance to environmental cracking, SSRT of welded coupons is frequently specified for sour service qualification programs.
Testing Items
The following parameters are recorded and analyzed in a typical SSRT campaign:
- Time to failure: total elapsed time from the start of loading to complete fracture; a key comparative indicator between inert and corrosive environments.
- Maximum load and ultimate tensile strength: the peak load sustained by the specimen during the test, used to derive the stress ratio Rσ (corrosive environment value divided by inert environment value).
- Elongation to failure: total plastic strain at fracture, used to derive the elongation ratio Rδ; severe loss of elongation is a primary marker of cracking susceptibility.
- Reduction in area: measured at the fracture surface after testing; the reduction-in-area ratio Rψ is one of the most sensitive ductility indices for detecting environmental embrittlement.
- Fracture energy: the area under the load–extension curve, integrated to failure; normalized comparisons quantify overall embrittlement.
- Susceptibility ratios and embrittlement index: normalized ratios such as Rt = tf(env)/tf(inert), Rσ, Rδ, and Rψ, or the embrittlement index I = (1 − R) × 100%, which classify material resistance quantitatively.
- Secondary cracking: the number, distribution, and orientation of cracks along the gauge length outside the fracture plane, examined by optical or scanning electron microscopy.
- Fracture morphology: fractographic analysis distinguishing ductile microvoid coalescence from brittle intergranular, transgranular cleavage, or quasi-cleavage modes indicative of environmental cracking.
- Strain rate dependence: comparison of results across multiple strain rates to identify the strain rate most detrimental to the specific material–environment combination.
Testing Methods
Specimen Preparation
Specimens are machined with careful control of gauge-section dimensions and surface condition. The gauge surface is ground and polished, typically to a fine abrasive finish, then cleaned with suitable solvents and degreased. Dimensions are measured precisely before testing to allow accurate calculation of stress, strain, and reduction in area.
Test Environment Control
The specimen is mounted in a corrosion cell—ranging from a simple glass vessel for ambient-pressure solutions to a high-temperature, high-pressure autoclave for simulated nuclear or reservoir conditions. The test medium is prepared to the specified chemistry, for example sodium chloride solutions, acidified brine saturated with hydrogen sulfide for sour service simulation, caustic solutions, or high-purity water with controlled dissolved oxygen and hydrogen content. Temperature, pH, and—where required—solution redox potential are monitored and held constant throughout the test. Deaeration with nitrogen or carbon dioxide, continuous gas bubbling, or potentiostatic potential control may be applied to maintain stable environmental conditions. An identical specimen is tested in an inert reference environment at the same temperature and strain rate to establish the baseline mechanical behavior.
Strain Rate Selection
The extension rate is chosen based on the material–environment system. Most SCC systems show maximum sensitivity at strain rates near 10⁻⁶ s⁻¹, while systems dominated by hydrogen transport may respond differently. When the critical rate is unknown, a series of tests at rates such as 1 × 10⁻⁴, 1 × 10⁻⁵, 1 × 10⁻⁶, and 1 × 10⁻⁷ s⁻¹ is performed. Excessively slow rates are avoided because uniform corrosion rather than cracking may then dominate and mask the susceptibility assessment.
Test Execution
The specimen is loaded into the testing machine, coupled to the environment cell, and allowed to reach thermal and chemical equilibrium. A constant crosshead extension rate is then applied until the specimen fractures completely. Load and displacement (or strain) are recorded continuously. After fracture, the cell is sampled for pH verification, and the specimen is retrieved, rinsed, dried, and preserved for metallographic and fractographic examination. Duplicate or triplicate tests in each environment are normally performed to confirm reproducibility.
Post-Test Evaluation and Data Reduction
Ductility parameters, time to failure, and fracture energy from the corrosive test are divided by the corresponding values from the inert reference test to obtain the susceptibility ratios. Gauge surfaces are inspected for secondary cracks, and fracture surfaces are examined by scanning electron microscopy to characterize the fracture mode. A material is generally considered susceptible when the ratios fall significantly below unity and brittle fracture features or secondary intergranular cracking are observed. Reporting follows the requirements of ASTM G129 and ISO 7539-7, including full documentation of material condition, specimen geometry, environment, strain rate, and all comparative metrics.
Applications
SSRT is applied across many industries where components operate under tensile stress in aggressive environments:
- Oil and gas: qualification of carbon steels, low-alloy steels, and corrosion-resistant alloys for sour (H₂S-containing) service, supporting material selection consistent with ISO 15156 requirements; evaluation of downhole tubulars, valves, and wellhead components.
- Nuclear power: investigation of primary water stress corrosion cracking (PWSCC) of nickel-based alloys and irradiation-assisted stress corrosion cracking (IASCC) of stainless steels in simulated light water reactor coolant at elevated temperature and pressure.
- Chemical and petrochemical processing: assessment of chloride stress corrosion cracking of austenitic and duplex stainless steels in process streams, cooling waters, and caustic services.
- Marine and offshore engineering: evaluation of structural steels, fasteners, and welded joints in seawater and marine atmospheres.
- Hydrogen energy and storage: screening of pipeline steels, high-strength fasteners, and pressure vessel materials for hydrogen embrittlement in high-pressure gaseous hydrogen or after electrochemical hydrogen charging.
- Aerospace: ranking of high-strength aluminum alloys, titanium alloys, and coated or plated high-strength steels with respect to environmental cracking resistance.
- Surface treatment and coating quality control: verification that electroplated, galvanized, or thermally sprayed components do not exhibit hydrogen-induced embrittlement after processing.
- Weldment qualification: comparative assessment of welding procedures and post-weld heat treatments on environmental cracking resistance of joints in pipelines and pressure equipment.
FAQ
What strain rate is used in slow strain rate tensile testing?
The standard range is 10⁻⁶ to 10⁻⁴ s⁻¹, with 1 × 10⁻⁶ s⁻¹ being the most commonly applied rate for SCC evaluation. The optimum rate depends on the material–environment system, and a rate-sweep is recommended when the system behavior is unknown.
How long does an SSRT test take?
Depending on the strain rate and specimen ductility, a single test typically runs from one day up to about one week. Including inert reference tests and repetitions, a complete evaluation usually requires several days to two weeks of machine time.
How is cracking susceptibility quantified?
Susceptibility is quantified by normalizing the results from the corrosive environment against those from an inert environment, producing ratios for time to failure, elongation, reduction in area, strength, and fracture energy. Values well below 1.0, confirmed by brittle fractographic features, indicate susceptibility.
Why is an inert reference test required?
The inert reference test at the same temperature and strain rate isolates the environmental effect. Any difference in ductility or fracture behavior between the two tests is then attributable to the corrosive environment rather than to the slow strain rate alone or to normal material variability.
Can SSRT be performed at high temperature and high pressure?
Yes. Dedicated autoclave systems with servo-controlled loading allow testing in simulated nuclear primary coolant, high-pressure hydrogen gas, or oilfield brine–H₂S environments at elevated temperatures, following the general procedures of ASTM G129 and ISO 7539-7.
How does SSRT compare with constant-load testing?
Constant-load tests may run for months without failure even for susceptible materials, whereas SSRT actively accelerates crack initiation through continuous plastic deformation. SSRT is therefore the preferred screening and ranking method, while constant-load and threshold-stress determinations are often used to complement it for design purposes.
Which standards govern the method?
The principal standards are ASTM G129, ISO 7539-7, and the Chinese national standard GB/T 15970.7, which specify specimen preparation, environmental control, strain rate selection, test execution, and data reporting for slow strain rate testing of metallic materials. Accredited third-party laboratories routinely perform SSRT in accordance with these standards.