Slow strain rate testing (SSRT), also known as slow extension rate testing, is a widely accepted accelerated laboratory method for evaluating the susceptibility of metallic materials to stress corrosion cracking (SCC) in aggressive environments. By slowly straining a tensile specimen to failure in a corrosive medium and comparing its behavior with an inert reference environment, SSRT provides rapid and reproducible assessment of environmentally assisted cracking within days rather than months. This article covers the technical principles, specimen requirements, test procedures, evaluation criteria, relevant standards such as ASTM G129, ISO 7539-7, and NACE TM0198, and typical industrial applications of slow strain rate stress corrosion testing.
Technical Overview
Slow strain rate testing (SSRT) is an accelerated constant-extension-rate test used to evaluate the susceptibility of metallic materials to stress corrosion cracking (SCC) and other forms of environmentally assisted cracking (EAC). In this method, a uniaxial tensile specimen is pulled to fracture at a slow, constant extension rate—typically in the range of 10⁻⁴ to 10⁻⁷ s⁻¹, most commonly around 10⁻⁶ s⁻¹—while fully exposed to a corrosive test environment. Because the applied plastic strain continually ruptures protective passive films and exposes fresh metal to the aggressive medium, cracking processes that may take months to develop under constant load can be initiated and propagated within one to several days.
The fundamental principle of SSRT is comparison. A specimen tested in the corrosive environment of interest is evaluated against an identical specimen tested in an inert reference environment, such as dry air or inert oil, at the same strain rate. If the material is susceptible to SCC, the specimen tested in the corrosive medium will exhibit reduced ductility (lower elongation and reduction of area), a shorter time to failure, a lower maximum load, and characteristic brittle fracture features such as intergranular or transgranular cracking. If the material is resistant, the mechanical response in both environments will be essentially identical.
SSRT is recognized as a conservative screening technique. It deliberately imposes severe plastic deformation that would not normally occur in service, which makes it highly sensitive for detecting susceptibility but also means that results should be interpreted carefully when extrapolating to actual operating conditions. The principal international standards governing this method include ASTM G129, ISO 7539-7, NACE TM0198 for sour oilfield service, and the corresponding national standards such as GB/T 15970.7. Compared with conventional constant-load or U-bend tests, which may require 30 days to several months of exposure, SSRT delivers results in a fraction of the time, making it a valuable tool for material screening, qualification, and research.
Testing Samples
Material Types
SSRT can be applied to virtually any metallic engineering material, including:
- Carbon and low-alloy steels used in pipelines, pressure vessels, and refinery equipment
- Austenitic, martensitic, precipitation-hardening, and duplex stainless steels
- Nickel-based corrosion-resistant alloys such as Alloy 625, Alloy 825, C-276, and related grades
- Aluminum alloys used in aerospace and marine structures
- Titanium and zirconium alloys for chemical, nuclear, and marine applications
- Copper alloys, magnesium alloys, and welded joints of the above materials
Specimen Geometry
The most common specimen is a smooth cylindrical tensile specimen with a reduced gauge section, machined in accordance with the dimensions and tolerances of ASTM E8/E8M, ASTM G129, or ISO 7539-7. Typical gauge diameters range from 3 mm to 6.4 mm (0.125 to 0.25 in), with a gauge length of four times the diameter. Flat rectangular tensile specimens may be used when product forms are too thin for round bars, for example sheet and strip materials. Notched specimens or specimens containing fatigue pre-cracks can also be employed when fracture-mechanics-based evaluation of crack growth behavior is required.
Product Forms and Sampling
Specimens may be machined from a wide range of product forms, including plate, sheet, pipe and tube wall, bar, forging, casting, wire, and weld metal or heat-affected zone (HAZ) of welded joints. Sampling orientation must be documented because SCC susceptibility is often anisotropic; specimens are typically taken in the longitudinal, transverse, or through-thickness direction depending on the direction of service stress and the relevant specification.
Surface Preparation
Surface condition has a strong influence on SSRT results. The gauge section is usually ground and polished to a consistent finish, commonly 320 to 600 grit or finer, with the final grinding direction parallel to the specimen axis. Specimens must be degreased, rinsed, and dried before testing, and their gauge dimensions are measured accurately before mounting in the test frame. Machining residual stresses should be minimized to avoid introducing artifacts into the test results.
Testing Items
The following parameters and indicators are typically measured and reported in an SSRT program:
- Time to failure in the test environment and in the inert reference environment
- Percentage elongation to failure in both environments
- Reduction of area at fracture in both environments
- Maximum tensile load and engineering stress–strain curves
- Ratio of time to failure, elongation ratio, and reduction-of-area ratio (environment versus inert reference)
- SCC susceptibility index derived from the ductility loss ratios
- Secondary cracking on the gauge surface (presence, density, and morphology)
- Fracture surface morphology examined by scanning electron microscopy (SEM), distinguishing ductile dimpled fracture from brittle intergranular or transgranular facets
- Average crack growth rate, where applicable, estimated from the maximum crack depth divided by the exposure time
Testing Methods
Test Apparatus
A typical SSRT system consists of a constant extension rate (slow strain rate) testing machine with a stiff load frame and a closed-loop or geared drive capable of stable crosshead speeds in the micrometer-per-hour range, an environmental cell or autoclave surrounding the specimen gauge section, a calibrated load cell, and instrumentation for recording load and displacement continuously. For high-temperature, high-pressure testing, a pressurized autoclave with a pull-rod penetration, recirculation loop, and precise control of temperature, pressure, and water chemistry (dissolved oxygen, pH, chloride concentration, or H₂S partial pressure) is used. A reference test in inert air or oil is conducted on the same machine under identical mechanical conditions.
Test Environments
The test environment is selected to simulate the service condition of interest. Common environments include:
- Neutral 3.5% sodium chloride solution for marine and offshore materials
- Acidified brine saturated with hydrogen sulfide (NACE TM0177 Solution A or solutions defined in NACE TM0198) for sour oilfield service
- High-temperature high-pressure water (for example, 288–360 °C) for nuclear power plant materials
- Caustic hydroxide solutions for chemical plant equipment subject to caustic cracking
- Boiling magnesium chloride or lithium chloride solutions for screening austenitic stainless steels
- Custom process liquors, acidic condensates, or buffered solutions representing specific plant chemistry
Test Procedure
A standard test sequence comprises the following steps:
- Machine, polish, clean, and dimension the specimens; document material identity, heat treatment, and orientation.
- Prepare and characterize the test solution, deaerate or gas-saturate as required, and stabilize the environmental cell at the target temperature and pressure.
- Mount the specimen, apply the corrosive environment, and allow a pre-exposure period (commonly 30 minutes to several hours) for the corrosion potential to stabilize.
- Apply the selected constant extension rate, typically 1 × 10⁻⁶ s⁻¹, and record load versus time until complete fracture of the specimen.
- Run a duplicate test in the inert reference environment at the identical strain rate.
- After fracture, clean the specimen, measure final dimensions, calculate elongation and reduction of area, and examine the fracture surface and gauge surface by optical microscopy and SEM.
- Compare the environmental and inert results and calculate the susceptibility ratios.
Evaluation Criteria
Susceptibility is quantified through normalized ratios, for example the reduction-of-area ratio R(A) = RA(corrosive) / RA(inert), the elongation ratio, and the time-to-failure ratio. Ratios approaching 1.0 indicate resistance to SCC, while significantly reduced ratios—commonly below 0.8 to 0.9, or as defined by the applicable acceptance specification—indicate susceptibility. SEM fractography showing intergranular or cleavage-like brittle fracture facets, together with secondary stress corrosion cracks along the gauge length, provides confirmatory evidence of environmentally assisted cracking. Where required by specifications such as NACE TM0198, a minimum number of replicate tests is performed and the most conservative result is used for assessment.
Applicable Standards
- ASTM G129 – Standard Guide for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking
- ISO 7539-7 – Corrosion of metals and alloys, stress corrosion testing, Part 7: Method for slow strain rate testing
- NACE TM0198 – Slow Strain Rate Test Method for Screening Corrosion-Resistant Alloys for Stress Corrosion Cracking in Sour Oilfield Service
- GB/T 15970.7 – Chinese national standard corresponding to the slow strain rate test method
- NACE TM0177 and NACE MR0175 / ISO 15156 – related standards for sulfide stress cracking and materials for sour service that frequently define acceptance environments
Applications
Slow strain rate testing is used across many industries wherever the combination of tensile stress and a specific corrosive environment may threaten structural integrity:
- Oil and gas: qualification and screening of corrosion-resistant alloys and low-alloy steels for sour service containing H₂S, in support of ISO 15156 / NACE MR0175 material selection
- Nuclear power: evaluation of austenitic stainless steels and nickel alloys for stress corrosion cracking in high-temperature, high-purity or oxygenated reactor water, including irradiated material studies
- Refining and petrochemicals: assessment of carbon steel, stainless steel, and nickel alloy equipment exposed to chlorides, polythionic acids, caustics, and hydrogen-bearing process streams
- Pipelines and subsea hardware: screening of linepipe steels and weldments for cracking in brine and produced water environments
- Chemical processing: verification of alloy performance in hot caustic, chloride, acidic, and specialty chemical media
- Aerospace and marine: susceptibility assessment of high-strength aluminum alloys and structural materials exposed to seawater and marine atmospheres
- Research and development: ranking new alloys, evaluating the effect of heat treatment, cold work, microstructure, and weld procedures on SCC resistance, and studying cracking mechanisms
Accredited third-party laboratories routinely perform SSRT to support material qualification, failure analysis, vendor qualification, and independent verification for engineering projects, providing test reports that are widely accepted for regulatory and contractual purposes.
FAQ
What is the difference between SSRT and constant-load SCC testing?
Constant-load (static) SCC tests apply a fixed stress, typically a percentage of the yield strength, and may require exposure periods of 30 days or longer with no guarantee that cracking will initiate. SSRT instead imposes a continuous slow extension that accelerates film rupture and crack initiation, producing a definitive result in one to seven days. SSRT is therefore an excellent screening tool, while constant-load testing better reproduces true service loading when longer-term confirmation is required.
Why is the strain rate so important in SSRT?
Stress corrosion cracking only occurs within a characteristic range of strain rates for a given material–environment system. If the strain rate is too high, mechanical rupture outpaces the electrochemical reactions; if it is too low, repassivation may suppress crack propagation. For most systems the critical range lies between 10⁻⁵ and 10⁻⁷ s⁻¹, and 10⁻⁶ s⁻¹ is the most widely used default. When the susceptibility of an unknown system is being evaluated, testing at several strain rates is recommended to identify the rate that maximizes cracking response.
How long does a typical SSRT test take?
At a strain rate of 10⁻⁶ s⁻¹, a specimen with approximately 20% elongation in an inert environment typically fractures in roughly two to three days, including a short pre-exposure period. High-temperature autoclave tests or materials with limited ductility may fracture sooner. Because an inert reference test is also required, a complete SSRT program for one condition generally takes about one to two weeks, including specimen preparation and fractographic examination.
Can welded joints be evaluated by SSRT?
Yes. Specimens can be machined so that the gauge section contains the weld metal, the fusion line, or the heat-affected zone, either in the cross-weld or longitudinal orientation. This is one of the most common uses of SSRT for pipeline and pressure vessel weld qualification, since the heterogeneous microstructure and residual stresses in weldments often make them the most SCC-susceptible region of a structure.
Is SSRT accepted for sour service qualification?
SSRT performed in accordance with NACE TM0198 is a recognized screening method for corrosion-resistant alloys intended for sour oilfield environments. It is frequently used alongside or ahead of other tests such as NACE TM0177 Method A constant-load testing, with acceptance criteria defined by the applicable project specification or by the user in combination with ISO 15156 / NACE MR0175 requirements.
What factors can influence the reproducibility of SSRT results?
Key variables include specimen surface finish and machining quality, strain rate accuracy, environmental chemistry control (pH, chloride, H₂S partial pressure, dissolved oxygen), temperature stability, and the metallurgical condition of the material. Because SSRT is deliberately aggressive, small deviations in these parameters can affect ductility ratios. Best practice requires calibrated equipment, documented environmental monitoring, inert reference tests under identical mechanical conditions, and an adequate number of replicate specimens as specified in ASTM G129, ISO 7539-7, or NACE TM0198.