Slow Strain Rate Tensile Curve Analysis: Principles, Standards, and Industrial Applications

Slow strain rate tensile (SSRT) curve analysis is an accelerated laboratory technique for evaluating the susceptibility of metallic materials to environmentally assisted cracking, including stress corrosion cracking and hydrogen embrittlement. By comparing load–elongation curves recorded in an aggressive corrosive medium with those obtained in an inert reference environment, engineers can quantify the loss of strength, ductility, and fracture energy that signals premature component failure. This article outlines the principles, specimen types, measured parameters, evaluation methods, and industrial applications of SSRT curve analysis in accordance with ISO 7539-7, ASTM G129, and GB/T 15970.7.

Technical Overview

Slow strain rate tensile curve analysis, also known as constant extension rate testing (CERT), is a widely recognized method for assessing the environmental cracking susceptibility of metals and alloys. A tensile specimen is pulled to failure at a slow, constant strain rate—typically in the range of 1×10⁻⁷ s⁻¹ to 1×10⁻⁵ s⁻¹—while fully exposed to the corrosive test environment. At these low strain rates, the kinetics of anodic dissolution, passive film rupture, and hydrogen absorption are given sufficient time to interact with the applied plastic deformation, so that cracking mechanisms that may take months to appear under constant load can be revealed within days.

The core of the method is comparative curve analysis. A load–elongation (or stress–strain) curve generated in the corrosive medium is superimposed on a reference curve produced in an inert environment such as air or inert gas under identical mechanical conditions. A susceptible material exhibits a characteristic degradation signature in the corrosive environment:

• A lower peak load or reduced ultimate tensile strength
• A markedly shorter elongation to fracture and reduced uniform deformation
• A smaller reduction in area at the fracture surface
• A reduced area under the curve, indicating lower absorbed fracture energy
• Secondary circumferential or branched cracks along the gauge length
• A transition from ductile dimpled fracture to brittle intergranular or transgranular cleavage features

The technique is standardized in ISO 7539-7 (Corrosion of metals and alloys — Stress corrosion testing — Part 7: Method for slow strain rate testing), ASTM G129 (Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking), and the corresponding Chinese national standard GB/T 15970.7. Because of its speed and reproducibility, SSRT curve analysis is routinely performed by accredited third-party laboratories for material qualification, failure investigation, and research and development programs.

Testing Samples

Smooth Round Tensile Specimens

Standard cylindrical specimens with threaded or shoulder-loaded ends 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, following the geometry recommendations of ISO 7539-7 and ASTM G129. Sub-size specimens may be machined when material availability is limited, such as from thin-wall components or small forged blocks.

Flat and Sheet Specimens

Rectangular flat specimens are used for plate, sheet, strip, and clad materials. They allow evaluation of cracking sensitivity in different orientations relative to the rolling direction, which is critical for anisotropic alloys such as high-strength aluminum and titanium grades.

Notched Specimens

Circumferentially notched specimens concentrate stress and strain at the notch root, promoting crack initiation in materials with high inherent resistance or in mildly aggressive environments. Notched geometries are also preferred when hydrogen pre-charged specimens are tested, because hydrogen accumulates in the triaxial stress field ahead of the notch.

Welded Joint and Component-Specific Specimens

Specimens containing a transverse weld can be tested so that the gauge length spans the base metal, heat-affected zone, and weld metal in a single test. Special geometries can be extracted from pipes, fittings, castings, additively manufactured parts, and fasteners to evaluate the actual metallurgical condition of the component.

Testing Items

SSRT curve analysis delivers a complete set of mechanical and fracture-mechanical parameters for each test condition, from which environmental cracking susceptibility is quantified:

• Yield strength and ultimate tensile strength in the test and reference environments
• Elongation to fracture and uniform elongation
• Reduction in area at the fracture surface
• Time to failure under the selected strain rate
• Area under the load–elongation curve (absorbed energy to fracture)
• Normalized susceptibility ratios: strength ratio, plastic elongation ratio, reduction-in-area ratio, and time-to-failure ratio (test environment divided by inert environment)
• Secondary crack population along the gauge length
• Fractography by scanning electron microscopy to classify ductile versus brittle fracture modes
• Cross-sectional metallography to confirm crack path morphology (intergranular, transgranular, or mixed)

Testing Methods

Test Environment Control

Specimens are tested in a corrosion cell or a high-pressure autoclave containing the service-representative medium, for example chloride-bearing solutions, acidified brine saturated with hydrogen sulfide per the screening philosophies of ISO 15156, caustic solutions, or high-temperature high-purity water simulating nuclear coolant chemistry. A matched specimen is tested in an inert environment (air, oil, or inert gas) to generate the reference curve. Key controlled variables include solution chemistry, pH, temperature, dissolved gases, and flow condition, all of which are documented in the test report.

Loading Procedure and Strain Rate Selection

The specimen is loaded onto a dedicated slow strain rate machine equipped with a servo-controlled actuator and a calibrated load cell. The applied crosshead speed is selected to produce the target nominal strain rate, with 1×10⁻⁶ s⁻¹ being the most common screening value. When the first test shows borderline behavior, additional strain rates (for example 1×10⁻⁷ s⁻¹ or 5×10⁻⁷ s⁻¹) are examined because cracking susceptibility is strongly strain-rate dependent. The test continues until complete fracture of the specimen, with load and extension recorded continuously.

Curve Acquisition and Data Reduction

Load–extension data are converted into engineering stress–strain curves, and the characteristic values are extracted for both environments. Post-test measurements include final gauge length for elongation, fracture diameter for reduction in area, and image-based or gravimetric determination of the area under the curve for fracture energy.

Susceptibility Evaluation from the Curves

The environmental degradation of each ductility parameter is expressed as a normalized ratio, for example the reduction-in-area ratio RRA = (RA in corrosive environment) / (RA in inert environment). Values approaching 1.0 indicate high resistance, whereas significantly depressed ratios, together with a truncated curve, low secondary-necking, and brittle fractography, confirm susceptibility. Many evaluation schemes also compute a composite susceptibility index such as I_SSCC = 1 − RRA or 1 − (plastic elongation ratio), providing a single figure of merit for material ranking and specification acceptance.

Applications

SSRT curve analysis supports critical decisions across many industries where metals operate under the combined action of tensile stress and aggressive environments:

• Oil and gas: qualification of carbon and low-alloy steels, corrosion-resistant alloys, and welded joints for sour service containing hydrogen sulfide, supporting material selection under the framework of ISO 15156
• Nuclear power: screening of stainless steels and nickel-base alloys for irradiation-assisted stress corrosion cracking and primary water stress corrosion cracking in simulated light water reactor coolants
• Chemical and petrochemical processing: evaluation of austenitic stainless steels and duplex alloys in chloride, caustic, and polythionic acid environments
• Aerospace: assessment of high-strength aluminum alloys, landing gear steels, and titanium alloys exposed to marine atmospheres and chlorides
• Hydrogen energy: measurement of hydrogen embrittlement sensitivity in pipelines, storage vessels, valve steels, and additively manufactured components under gaseous or electrochemically introduced hydrogen
• Failure analysis: reproduction of in-service cracking in the laboratory to confirm root cause and validate corrective material or process changes

FAQ

What strain rate is used in slow strain rate tensile testing?

The nominal strain rate is normally set between 1×10⁻⁶ s⁻¹ and 1×10⁻⁵ s⁻¹, with 1×10⁻⁶ s⁻¹ serving as the default screening rate in ISO 7539-7 and ASTM G129. For systems that crack only under very slow deformation, such as some hydrogen embrittlement and low-temperature water chemistry conditions, rates down to 1×10⁻⁷ s⁻¹ may be applied.

How long does a typical SSRT test take?

Depending on the strain rate and gauge length, a single test usually runs from one to three days until fracture. Because both a corrosive-environment specimen and an inert-environment reference specimen are required, a complete evaluation typically takes one to two weeks including specimen preparation and fractographic examination.

Why must the curve be compared with an inert-environment reference?

The absolute curve shape depends on the material strength level and specimen geometry. Only by normalizing the parameters obtained in the corrosive medium against an identical test in air or inert gas can the true environmental contribution to the loss of ductility, strength, and fracture energy be isolated and quantified.

How is susceptibility quantified from the curves?

Susceptibility is expressed through dimensionless ratios—time to failure ratio, plastic elongation ratio, reduction-in-area ratio, and tensile strength ratio—or through composite indices derived from them. Low ratios combined with secondary cracking and brittle fracture morphology are the accepted evidence of environmentally assisted cracking susceptibility.

Can welded components and real parts be evaluated?

Yes. Specimens can be machined so that the gauge length contains the weld metal and heat-affected zone, and custom geometries can be extracted from pipes, forgings, castings, and fasteners, allowing the actual product metallurgy and residual stress condition to be assessed.

Is SSRT a replacement for constant-load or fracture-mechanics cracking tests?

SSRT is an accelerated screening and ranking method. Because the imposed strain rate exceeds many service conditions, borderline results are often confirmed with constant-load threshold tests or pre-cracked fracture-mechanics specimens before final material qualification decisions are made.