Determination of Fracture Time in Slow Strain Rate Tensile Testing (SSRT): Principles, Methods, and

Slow strain rate tensile testing (SSRT), also known as constant extension rate testing (CERT), is an accelerated laboratory method for evaluating the stress corrosion cracking (SCC) and environmental embrittlement susceptibility of metallic materials. Determination of fracture time under a controlled, slow strain rate in a corrosive environment provides a quantifiable and reproducible measure of material–environment compatibility. This article explains the technical principles, specimen requirements, test procedures, evaluation criteria, and typical applications of fracture time determination in slow strain rate tensile testing.

Technical Overview

Slow strain rate tensile testing is a mechanical–environmental test method in which a tensile specimen is strained to failure at a constant, carefully controlled low strain rate—typically in the range of 10⁻⁵ to 10⁻⁷ s⁻¹—while exposed to a corrosive test environment. Under these conditions, the synergistic action of tensile stress and electrochemical reactions can initiate and propagate stress corrosion cracks far more rapidly than under static loading, allowing susceptibility screening within hours or days rather than the months required by conventional constant-load or constant-deflection tests.

Fracture time (time to failure, t_f or TTF) is one of the primary quantitative outputs of an SSRT test. It is defined as the elapsed time from the start of straining until complete separation of the specimen. When a material is susceptible to environmental cracking, the fracture time in the corrosive medium is significantly shortened compared with the fracture time measured in an inert reference environment (air, inert gas, or oil) at the same strain rate. The relative reduction in fracture time, together with the loss of ductility (elongation and reduction in area) and changes in fractography, forms the basis of susceptibility assessment.

The method is standardized internationally in ASTM G129 (Standard Practice 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), and the corresponding Chinese national standard GB/T 15970.7. Supplementary guidance for specific industries is provided by documents such as NACE TM0198 and ISO 15156 / NACE MR0175 for sour service environments.

Testing Samples

Metallic Materials

SSRT with fracture time determination is applicable to virtually all structural metallic materials, including:

• Austenitic, duplex, ferritic, and martensitic stainless steels
• Carbon steels and low-alloy steels, including pipeline and pressure vessel steels
• Nickel-based corrosion-resistant alloys (e.g., alloy 625, alloy 825, C-276)
• Aluminum and magnesium alloys
• Titanium and zirconium alloys
• Copper alloys and weldments, heat-affected zones, and clad materials

Specimen Geometry and Preparation

Smooth cylindrical tensile specimens are most commonly used, machined in accordance with ASTM E8/E8M, ASTM G129, or ISO 6892-1 dimensional requirements, with a typical gauge diameter of 3–6 mm and a gauge length-to-diameter ratio of approximately 4:1. Notched specimens or pre-cracked (fatigue-cracked) specimens may be used when crack initiation behavior or crack growth resistance is of specific interest.

Surface preparation is critical for reproducibility. Specimens are generally ground to a uniform surface finish (typically 600 grit or finer), degreased, rinsed, dried, and weighed and dimensioned before testing. Welded specimens must be oriented so that the weld metal, fusion line, or heat-affected zone lies within the gauge length, depending on the evaluation objective.

Testing Items

The core deliverable of this test is fracture time, but a complete SSRT evaluation typically reports the following items measured in both the test environment and the inert reference environment:

• Fracture time (time to failure) at the specified strain rate
• Maximum tensile load and nominal ultimate tensile strength in the environment
• Uniform elongation and total elongation to fracture
• Reduction in area at fracture
• Occurrence and density of secondary (secondary/sidewall) cracks on the gauge surface
• Susceptibility indices derived from fracture time and ductility ratios (e.g., t_f,env/t_f,inert, elongation ratio, and reduction-in-area ratio)
• Fracture mode classification (ductile microvoid coalescence versus brittle intergranular or transgranular cracking) by SEM fractography

Testing Methods

Test Apparatus and Environment Control

Testing is performed on a dedicated slow strain rate tensile machine, or a universal testing machine equipped with a slow-strain-rate control module, fitted with an environmental cell. The cell is typically made of glass, PTFE, or another inert material, and is equipped with facilities for temperature control, solution circulation or refreshment, pH monitoring, and, where required, gas purging (e.g., H₂S, CO₂, N₂, or O₂) or autoclave pressurization for high-temperature high-pressure water chemistry. Applied strain rates are verified and held constant within tight tolerances throughout the test.

Reference Test in an Inert Environment

A baseline test is first conducted at the identical strain rate in air, inert gas, or inert oil to establish the inherent mechanical response of the material, including its reference fracture time and ductility values. This reference is essential because fracture time is strongly dependent on strain rate, specimen geometry, and material strength.

Environmental Test Procedure

The specimen is installed in the environmental cell, the test solution is introduced, and the system is allowed to reach thermal and chemical equilibrium (including any required pre-exposure or open-circuit potential stabilization) before straining begins. The machine then extends the specimen at the selected constant strain rate—commonly 10⁻⁶ s⁻¹ as the default screening rate—until complete fracture occurs. Load, displacement, and time are recorded continuously, and the fracture time is captured automatically or from the load–time record.

The test is terminated either upon specimen separation or when the load has dropped to a defined fraction of the maximum load, indicating imminent fracture. If the specimen does not fail within a predefined maximum duration, the test may be stopped and the result reported as "no failure" at that strain rate.

Post-Test Evaluation and Data Interpretation

After fracture, the specimen is examined for secondary cracks along the gauge length, measured for elongation and reduction in area, and analyzed by scanning electron microscopy to identify the fracture mode. Susceptibility to environmentally assisted cracking is judged by comparing the fracture time and ductility in the corrosive environment against the inert reference:

• A pronounced decrease in fracture time and reduction in area, combined with intergranular or brittle transgranular fracture surfaces and secondary cracking, indicates high SCC susceptibility.
• Similar fracture times and ductility, with fully ductile dimpled fracture in both environments, indicate resistance to cracking under the tested conditions.
• Where quantitative thresholds are required, critical strain rates or threshold stresses can be established by testing at multiple strain rates.

Accredited third-party laboratories perform these tests under controlled, traceable conditions and issue reports that include full test parameters, load–time curves, fracture times, calculated susceptibility ratios, and fractographic evidence, supporting material qualification and failure analysis programs.

Applications

Fracture time determination by SSRT is widely used across industries where environmentally assisted cracking threatens structural integrity:

• Oil and gas: qualification of tubulars, valves, and fittings for sour (H₂S) service in accordance with ISO 15156 / NACE MR0175, and evaluation of sulfide stress cracking and hydrogen embrittlement resistance.
• Nuclear power: assessment of irradiation-assisted stress corrosion cracking (IASCC) and SCC susceptibility of stainless steels and nickel alloys in high-temperature, high-purity water.
• Chemical and petrochemical processing: material selection for equipment exposed to chlorides, caustics, polythionic acids, and other aggressive media.
• Pipelines and pressure vessels: validation of linepipe steels and weldments in service-representative ground water, brines, and cathodic protection scenarios.
• Aerospace: evaluation of high-strength aluminum, titanium, and steel alloys in chloride-containing and humid environments, including hydrogen pre-charged conditions.
• Marine and offshore structures: screening of fasteners, anchors, and structural alloys in seawater and sea spray environments.
• Hydrogen energy systems: measurement of hydrogen embrittlement susceptibility of storage and transport alloys in gaseous hydrogen or after electrochemical charging.
• Failure analysis: reproduction of service failures in the laboratory to confirm cracking mechanisms and root causes.

FAQ

What strain rate is typically used for SSRT fracture time measurement?

The most common screening strain rate is 10⁻⁶ s⁻¹, which balances cracking sensitivity against reasonable test duration (typically one to seven days). Depending on the material–environment system, rates between 10⁻⁵ s⁻¹ and 10⁻⁷ s⁻¹ may be used, and some codes recommend testing at several rates to identify the critical rate that maximizes cracking susceptibility.

How is fracture time defined and recorded?

Fracture time is the elapsed time from the commencement of straining to the complete separation of the specimen. It is obtained directly from the machine's time record, usually identified as the time at which the load falls abruptly after the final crack propagation event.

Why compare fracture time with an inert environment test?

Fracture time depends on strain rate, specimen geometry, and material strength. Only by comparing the fracture time in the corrosive environment against the fracture time of an identical specimen tested at the same rate in an inert environment can environmental degradation be isolated and quantified as a susceptibility ratio.

What are the advantages of SSRT over constant-load SCC tests?

SSRT accelerates crack initiation and propagation through continuous rupture of passive films at emerging slip steps, producing results in days rather than the weeks or months needed for static tests, and it yields a definitive endpoint (fracture) even for materials that would not fail under constant load within a practical timeframe.

What limitations should be considered when interpreting fracture time data?

The slow dynamic strain imposed during SSRT is more severe than most service loading conditions, so the method is best regarded as a conservative screening tool rather than a life-prediction technique. Results should be interpreted together with constant-load or fracture-mechanics-based threshold data when making final fitness-for-service decisions.

Which standards govern this test?

The principal standards are ASTM G129, ISO 7539-7, and GB/T 15970.7 for the slow strain rate method itself, supported by ASTM E8/E8M and ISO 6892-1 for tensile specimen and mechanical measurement requirements, and industry-specific documents such as NACE TM0198 and ISO 15156 for sour-service applications.