Slow Strain Rate Tension Testing: Principles, Methods, and Applications in Stress Corrosion Cracking

Slow strain rate tension testing (SSRT), also known as constant extension rate testing (CERT), is the most widely recognized laboratory method for evaluating the susceptibility of metallic materials to stress corrosion cracking and environmentally assisted cracking. Performed in accordance with ISO 7539-7, ASTM G129, and GB/T 15970.7, this accelerated technique provides quantitative data on strength, ductility, and time to failure, enabling manufacturers to qualify materials for aggressive service environments. This article explains the testing principle, specimen requirements, evaluation parameters, procedures, and typical industrial applications of slow strain rate tension testing.

Technical Overview

Slow strain rate tension testing (SSRT) is an accelerated laboratory technique used to determine the susceptibility of metallic materials to stress corrosion cracking (SCC), hydrogen embrittlement, and other forms of environmentally assisted cracking (EAC). During the test, a uniaxial tensile specimen is pulled to failure at a constant, deliberately low extension rate—typically producing nominal strain rates in the range of 10⁻⁵ to 10⁻⁷ s⁻¹—while fully immersed in a corrosive test environment. The slow deformation continuously ruptures protective surface films and maintains the material at a freshly exposed, actively straining condition, which maximizes the interaction between mechanical stress and the aggressive medium, thereby promoting crack initiation and propagation.

The core of the method is comparative: the mechanical response of the specimen in the corrosive environment is compared against the response of an identical specimen tested in an inert reference environment, such as air, argon, or inert oil, at the same temperature and strain rate. A significant loss of ductility, strength, or time to failure in the corrosive medium—confirmed by characteristic brittle fracture features—indicates susceptibility to environmentally assisted cracking. Because a single test can be completed in one to three days, SSRT is one of the most efficient screening tools available for materials selection and qualification, complementing longer-duration constant load and fracture mechanics tests.

Testing Samples

Metallic Materials and Product Forms

Slow strain rate tension testing is applicable to virtually all structural metals, including:

• Carbon steels and low-alloy steels, including pipeline and pressure vessel steels
• Austenitic, duplex, and super-austenitic stainless steels
• Nickel-base and cobalt-base alloys
• Aluminum and magnesium alloys
• Titanium and zirconium alloys
• Copper alloys and high-strength fastener steels
• Welded joints, heat-affected zones, and clad or overlay materials

Acceptable product forms include plate, sheet, strip, pipe and tube, bar, rod, forging, casting, and welded fabrications. Specimens are usually oriented relative to the working direction (longitudinal or transverse) to reflect the stressing condition of concern in service.

Specimen Geometry

The standard specimen is a smooth cylindrical tensile test piece with a reduced gauge section machined in accordance with ISO 6892-1 or ASTM E8 dimensions, with proportional gauge lengths of 4D or 5D. Flat rectangular specimens may be used for sheet products where round bars cannot be extracted. Notched specimens or specimens containing a transverse weld centered in the gauge section may also be tested when localized susceptibility must be assessed. Miniature specimens can be accommodated where the available material is limited, such as in failure analysis work on removed components.

Specimen Preparation

Proper preparation is essential to obtain reproducible results:

• Machine the gauge section with low-stress grinding or fine turning to avoid introducing residual tensile stresses and machining tears
• Achieve a surface finish fine enough (typically Ra 0.8 µm or better) that machining marks do not act as crack initiation sites unrelated to the environment
• Mark specimens for identification outside the gauge length only
• Measure and record the gauge diameter or width and thickness at multiple positions to calculate the original cross-sectional area
• Degrease, rinse, and dry the specimens immediately before testing to remove oils and contaminants

Testing Items

Slow strain rate tension testing delivers a complete set of mechanical and environmental cracking data. Typical reported items include:

• Tensile strength in the test environment and in the inert reference environment
• Yield strength, where the strain rate and load–displacement record permit determination
• Elongation after fracture in both environments
• Reduction of area at fracture in both environments
• Time to failure (time to rupture) from the start of loading
• Average strain rate over the test and the maximum load achieved
• Ratio parameters expressing susceptibility, such as the reduction of area ratio (RA environment / RA inert), elongation ratio, and tensile strength ratio
• Number and distribution of secondary stress corrosion cracks on the gauge surface
• Fracture mode classification from scanning electron microscopy, distinguishing ductile dimple fracture from intergranular, transgranular cleavage, or mixed brittle fracture characteristic of SCC or hydrogen embrittlement
• Optional electrochemical measurements such as corrosion potential during the test

Testing Methods

Apparatus and Test Setup

Testing is performed on a constant extension rate or slow strain rate tensile machine equipped with a calibrated load cell and displacement control capable of maintaining stable, very low crosshead speeds. The specimen is mounted in an environmental cell that fully immerses the gauge section; cells range from simple glass vessels for ambient-pressure aqueous solutions to pressure vessels and autoclaves for high-temperature, high-pressure water or gaseous hydrogen service. Temperature control, solution circulation or refreshment, and gas purging or saturation (for example, deaeration with nitrogen or saturation with hydrogen sulfide) are provided as required by the simulated service environment.

Test Environments

Two environments are always required: an inert reference environment and the aggressive environment of interest. Common aggressive environments include:

• Chloride-containing aqueous solutions, including concentrated boiling magnesium chloride solution used for screening austenitic stainless steels
• Acidified, chloride-rich brine saturated with hydrogen sulfide for sour service evaluation
• Simulated formation water, produced water, or process chemicals
• High-temperature, high-purity water with controlled dissolved oxygen and chemistry for light water reactor applications
• High-pressure gaseous hydrogen for hydrogen energy and storage applications
• Caustic solutions, polythionic acid environments, and other media relevant to refinery and chemical plant service

Test Procedure

The general procedure, aligned with ISO 7539-7 and ASTM G129, proceeds as follows:

• Mount the specimen in the machine, attach the environmental cell, and verify the loading train alignment
• Introduce the test solution, establish the required temperature, gas saturation, and chemistry, and allow the specimen to reach a stable pre-immersion period
• Apply extension at a constant crosshead speed corresponding to the selected nominal strain rate, typically 10⁻⁶ s⁻¹, and continue until complete fracture of the specimen
• Continuously record load, displacement, and time; monitor and log environmental parameters throughout the test
• After fracture, remove the specimen, measure the final gauge dimensions, and compute elongation and reduction of area
• Examine the gauge surface for secondary cracking, then perform fractographic examination of the fracture surface by scanning electron microscopy and metallographic cross-sectioning where confirmation of cracking morphology is needed
• Repeat the test on an identical specimen in the inert reference environment at the same strain rate and temperature

Evaluation Criteria

Susceptibility is quantified by comparing the two data sets. Ductility and strength ratios close to 1.0 indicate resistance to environmentally assisted cracking in the given environment, while significantly depressed ratios—commonly associated with values below roughly 0.8, with acceptance limits set by the applicable material specification or end-user requirement—indicate susceptibility. The diagnosis is confirmed when SEM fractography reveals brittle intergranular or cleavage features and gauge-surface secondary cracks that are absent from the inert-environment specimen. When the susceptibility of a material is unknown, testing at several strain rates (for example, 10⁻⁶ and 10⁻⁷ s⁻¹) is recommended because cracking susceptibility is strongly strain-rate dependent.

Applications

Slow strain rate tension testing is applied across every major industry in which metals operate under tensile stress in aggressive media:

• Oil and gas: qualification of tubulars, valves, and components for sour (H₂S) service and material ranking for corrosion-resistant alloys
• Nuclear power: evaluation of stress corrosion cracking susceptibility of reactor internals, piping, and cladding materials in high-temperature water
• Refining and petrochemicals: assessment of stainless steels and nickel alloys in polythionic acid, caustic, and chloride process environments
• Marine and offshore: screening of structural and fastener materials in seawater and splash-zone chemistry
• Aerospace: hydrogen embrittlement evaluation of high-strength steels and susceptibility ranking of aluminum alloys
• Hydrogen energy: testing of steels for high-pressure gaseous hydrogen storage, transport, and refueling infrastructure
• Pipeline integrity: assessment of linepipe steels in simulated groundwater with cathodic protection chemistry
• Failure analysis and research: rapid diagnosis of service failures involving cracking, and development of new corrosion-resistant alloys and coatings

Accredited third-party laboratories routinely perform SSRT to support material qualification dossiers, vendor selection, fitness-for-service studies, and research and development programs.

FAQ

What strain rate is typically used in slow strain rate tension testing?

A nominal strain rate of 10⁻⁶ s⁻¹ is the most common starting point, and testing is normally performed within the range of 10⁻⁵ to 10⁻⁷ s⁻¹. Because cracking susceptibility depends strongly on strain rate, testing at two or more rates is recommended when the behavior of a material in a given environment has not been previously characterized.

How long does a typical test take?

Depending on the selected strain rate and the ductility of the material, a single test typically runs from about 24 hours to 3 days until fracture. Including the parallel inert-reference test and post-test fractography, a complete evaluation is usually delivered within one to two weeks.

Which standards govern this test?

The principal standards are ISO 7539-7 (Slow strain rate testing), ASTM G129 (Slow strain rate testing to evaluate the susceptibility of metallic materials to environmentally assisted cracking), and GB/T 15970.7, which is the corresponding national standard within the GB/T 15990-series on stress corrosion testing. Specimen dimensional requirements reference ISO 6892-1 and ASTM E8.

Why is an inert reference test necessary?

The mechanical properties of the material in an aggressive environment can only be judged relative to its intrinsic behavior. Testing an identical specimen in air or an inert medium at the same strain rate and temperature isolates the environmental contribution to any loss of ductility, strength, or time to failure.

How are results interpreted?

Engineers compare ductility and strength ratios between the corrosive and inert tests. Ratios near unity demonstrate resistance, while markedly reduced ratios indicate susceptibility. Scanning electron fractography and gauge-surface crack inspection provide the definitive confirmation of a stress corrosion or hydrogen-assisted cracking mechanism.

How does SSRT differ from constant load testing?

SSRT is an accelerated, severe screening test that drives the specimen to fracture in days, making it ideal for material ranking and qualification. Constant load threshold tests more closely simulate service stressing but require much longer durations. The two methods are complementary and are frequently specified together in material qualification programs.