Slow strain rate testing (SSRT), also known as constant extension rate testing (CERT), is a widely recognized accelerated laboratory method for evaluating the susceptibility of metallic materials to stress corrosion cracking (SCC) and environmentally assisted cracking (EAC). By applying a controlled, slow dynamic strain to a tensile specimen immersed in a corrosive environment, SSRT produces rapid, reproducible, and quantitative results that support material selection, qualification, and failure analysis across the oil and gas, nuclear power, chemical processing, aerospace, and marine industries. This article explains the principles, sample types, measured parameters, test procedures, and typical applications of slow strain rate stress corrosion testing in accordance with international standards such as ASTM G129 and ISO 7539-7.
Technical Overview
Stress corrosion cracking (SCC) is a failure mechanism driven by the combined action of a tensile stress and a specific corrosive environment acting on a susceptible material. Under service conditions, SCC may take months or years to initiate and propagate, which makes conventional constant-load or static exposure tests slow and costly for material screening. Slow strain rate testing (SSRT) overcomes this limitation by imposing a slow, constant extension rate on a tensile specimen, typically producing nominal strain rates in the range of 10⁻⁷ to 10⁻⁵ s⁻¹, while the specimen is fully exposed to the test environment.
At these slow deformation rates, the protective surface film on the metal is repeatedly ruptured faster than it can repassivate in a susceptible environment. This exposes bare metal at emerging slip steps, enabling crack initiation through either anodic dissolution mechanisms (slip-dissolution) or hydrogen-assisted mechanisms such as hydrogen embrittlement and sulfide stress cracking. Because the specimen is strained continuously until complete fracture, a single SSRT experiment normally concludes within one to three days, dramatically accelerating the evaluation compared with static exposure testing.
The core logic of SSRT is comparative. Identical specimens are tested to failure in the corrosive environment of interest and in an inert reference environment such as air, argon, nitrogen, or oil at the same temperature. Degradation of ductility, strength, or fracture energy in the corrosive environment relative to the inert control indicates susceptibility to environmentally assisted cracking. The method is governed by recognized standards including ASTM G129, ISO 7539-7, and GB/T 15970.7, with NACE TM0198 addressing the specific use of SSRT for screening corrosion-resistant alloys in sour oilfield service.
Key advantages of SSRT include short test duration, high reproducibility, quantitative embrittlement indices, conservative screening capability, and compatibility with a wide range of environments from ambient glass cells to high-temperature, high-pressure autoclaves. Its main limitation is that the imposed dynamic strain is more severe than most service loading conditions, so SSRT is generally regarded as a screening and ranking tool rather than a direct life-prediction method.
Testing Samples
Smooth Round Tensile Specimens
Standard cylindrical tensile bars with threaded ends or shoulder loading are the most common SSRT specimens. Subsize specimens with a gauge diameter of approximately 4 mm and a gauge length of about 25 mm are widely used, particularly when testing expensive alloys or limited material from components. The smooth gauge section provides a uniform stress state and allows evaluation of the intrinsic SCC susceptibility of the base metal.
Flat Sheet and Plate Specimens
Reduced-gauge-section dogbone specimens are prepared from plate, sheet, strip, or thin-wall components. Specimen orientation relative to the rolling direction (longitudinal or transverse) is recorded because SCC susceptibility can be strongly anisotropic in wrought products.
Notched Specimens
Circumferentially notched tensile specimens concentrate stress and strain at the notch root, promoting crack initiation at a defined location. Notched specimens are used to evaluate notch sensitivity, to study local microstructures, and to generate higher triaxial stress states relevant to hydrogen-assisted cracking.
Welded Joint Specimens
Specimens with the weld metal, heat-affected zone, and adjacent base metal all contained within the gauge length are tested to identify the weakest region of a welded assembly. This is essential for qualifying welding procedures in aggressive environments such as sour gas fields or high-temperature water reactor circuits.
Full-Section and Component Samples
When machining a standard blank is impractical, small-scale specimens can be extracted from bars, wires, tubes, fasteners, or actual service components. Surface preparation typically involves polishing the gauge section to a consistent finish, degreasing, and precise measurement of gauge dimensions and reference marks before testing.
Testing Items
SSRT generates a complete mechanical data set for each environment, and susceptibility is quantified by comparing the results obtained in the corrosive medium with those obtained in the inert reference environment. Typical measured and derived quantities include:
- Time to failure in the test environment and in the inert reference environment
- Ultimate tensile strength and maximum load sustained during the test
- Total elongation to fracture and uniform elongation
- Reduction in cross-sectional area at the fracture surface
- Fracture energy, calculated as the area under the load-extension curve
- Embrittlement indices, such as the reduction-in-area ratio, elongation ratio, time-to-failure ratio, and fracture energy ratio, obtained by dividing the value measured in the corrosive environment by the corresponding value in the inert environment
- Number and distribution of secondary cracks along the gauge surface, reported as cracks per unit length
- Fracture surface morphology, distinguishing ductile dimpled fracture from brittle intergranular, transgranular cleavage, or quasi-cleavage modes
- Crack path relative to microstructural features such as weld beads, heat-affected zones, phase boundaries, and stringer inclusions
- Optional electrochemical measurements, including open-circuit potential monitoring, potentiostatic polarization during straining, and cathodic hydrogen charging
As a general interpretive convention, ratios approaching 1.0 indicate resistance to environmentally assisted cracking, while substantially reduced ratios, for example a reduction-in-area ratio below approximately 0.8, are commonly treated as evidence of susceptibility, with specific acceptance thresholds defined by the applicable material qualification standard or project specification.
Testing Methods
SSRT is performed using a stiff-frame testing machine capable of extremely slow, stable crosshead speeds, combined with an environmental enclosure. Ambient-pressure tests use glass or polymer corrosion cells equipped with temperature control, solution circulation, and gas sparging. High-temperature and high-pressure tests, such as simulations of nuclear primary water or deep sour wells, are conducted in autoclaves, often with titanium or nickel-alloy liners, refresh loops, and precise control of dissolved gases. The general test procedure follows these steps:
- Machining, surface preparation, cleaning, and dimensional inspection of specimens, with gauge sections finished to the roughness specified by ASTM G129 or ISO 7539-7
- Preparation of the test solution, for example acidified brine saturated with hydrogen sulfide and carbon dioxide for sour service simulation, chloride-bearing solutions for stainless steel evaluation, caustic solutions, or high-purity water with controlled oxygen and hydrogen content
- Mounting the specimen, sealing the environmental cell or autoclave, and stabilizing the chemistry and temperature of the environment before loading begins
- Selecting the strain rate, most commonly 1 × 10⁻⁶ s⁻¹, with exploratory programs sometimes spanning 10⁻⁵ to 10⁻⁷ s⁻¹ to identify the strain-rate window of maximum susceptibility
- Running the test at constant extension rate until complete fracture of the specimen while continuously recording the load-extension curve
- Retrieving the specimen, cleaning corrosion products following ASTM G1 practices, and measuring the fractured cross section for reduction in area
- Conducting metallographic examination of longitudinal sections to locate secondary cracks, and fractographic examination by scanning electron microscopy to classify the fracture mode
- Calculating embrittlement ratios against the inert-environment control test and issuing a susceptibility assessment
Duplicate or triplicate testing in both the corrosive and inert environments is standard practice to ensure statistical confidence. Where required by the study objectives, applied potentials, solution pH, temperature, chloride concentration, or hydrogen sulfide partial pressure are systematically varied to map the environmental window of cracking susceptibility. Accredited third-party laboratories routinely perform these tests under controlled conditions with full traceability of material pedigree, solution chemistry, and machine calibration.
Applications
SSRT is applied across a broad spectrum of industries wherever the combination of stress and environment threatens mechanical integrity:
- Oil and gas: screening and qualification of carbon steels, low-alloy steels, martensitic stainless steels, duplex stainless steels, and nickel alloys for sour service, supporting compliance with materials selection frameworks such as ISO 15156 and NACE MR0175
- Nuclear power: evaluation of stress corrosion cracking of austenitic stainless steels and nickel-base alloys in simulated boiling water reactor and pressurized water reactor environments, including irradiation-assisted cracking research
- Chemical and petrochemical processing: assessment of chloride stress corrosion cracking of austenitic stainless steels, caustic cracking of carbon steel, and compatibility of equipment alloys with process streams
- Pipelines and midstream infrastructure: study of near-neutral pH and high-pH stress corrosion cracking of line pipe steels, including the influence of cathodic protection potentials and soil chemistry
- Aerospace: qualification of high-strength aluminum alloys, landing gear steels, and titanium alloys against hydrogen embrittlement and hot-salt stress corrosion
- Marine and offshore: ranking of alloys and coatings for seawater and splash-zone service, including evaluation under imposed cathodic protection
- Failure analysis and research and development: root-cause confirmation of in-service cracking, optimization of heat treatments and cold-work levels, evaluation of inhibitors and surface treatments, and comparative ranking of candidate materials for new projects
FAQ
What strain rate is typically used in SSRT?
The standard strain rate is 1 × 10⁻⁶ s⁻¹, which provides sufficient time for film rupture and environmental attack while keeping the test duration practical. Depending on the material-environment system, susceptibility windows may shift, so strain rates between 10⁻⁷ and 10⁻⁵ s⁻¹ are sometimes explored to capture the worst-case condition.
How long does a slow strain rate test take?
Because the specimen is strained continuously until fracture, a typical SSRT test is completed within one to three days, and often within 24 to 48 hours at the standard strain rate. This is dramatically faster than constant-load or U-bend exposure tests, which may require weeks or months to produce failures.
How does SSRT differ from constant-load and U-bend tests?
Constant-load and U-bend tests apply static stress and rely on natural crack initiation, whereas SSRT imposes continuous dynamic strain that repeatedly ruptures protective films and forces crack initiation. SSRT therefore provides a severe, accelerated, and quantitative screening result but does not directly predict service life, so it is frequently complemented by static tests for critical applications.
How is SCC susceptibility judged from the results?
Susceptibility is judged by the loss of ductility and strength in the corrosive environment relative to an inert control, expressed as the reduction-in-area ratio, elongation ratio, time-to-failure ratio, or fracture energy ratio, together with fractographic evidence of brittle intergranular or transgranular cracking and the presence of secondary cracks along the gauge length.
Can SSRT be performed at high temperature and high pressure?
Yes. Autoclave-based SSRT systems routinely simulate environments such as high-temperature deoxygenated or oxygenated water for nuclear applications and high-pressure brines containing hydrogen sulfide and carbon dioxide for deep oil and gas wells, with full control over temperature, pressure, and dissolved gas chemistry.
Can welded joints be evaluated by SSRT?
Yes. Specimens are machined so that the weld metal and heat-affected zone lie within the gauge length, allowing the weakest link of the welded assembly to be identified. This approach is widely used for welding procedure qualification in sour service and other aggressive environments.
Which standards govern slow strain rate testing?
The principal standards are ASTM G129, ISO 7539-7, and GB/T 15970.7 for general SSRT methodology, NACE TM0198 for screening corrosion-resistant alloys in sour oilfield service, and ASTM G1 for the cleaning and preparation of corroded specimens after testing.