Low Strain Rate Testing (SSRT)

Low strain rate testing (SSRT) is a method used to determine stress thresholds and susceptibility to stress corrosion cracking by applying a constant elongation rate to materials, particularly in environments such as the nuclear and aerospace industries. Generally described in detail in the ASTM G129 standard, “Standard practice for slow strain rate testing to assess the susceptibility of metallic materials to environmentally induced cracking,” it is a rapid material assessment technique that evaluates susceptibility to stress corrosion cracking or hydrogen embrittlement by applying a very slow elongation rate (typically up to 100 nm) to a sample in a specific environment. This technique ranks material performance in corrosive environments by inducing accelerated failure compared to conventional testing.

Low-stretch rate testing refers to mechanical tests performed at deliberately slow deformation rates, far below those of conventional tensile tests (which typically use high strain rates). These tests are essential in materials science and engineering for two main purposes:

To evaluate how materials behave under slow loading conditions that simulate real-world service (e.g., creep-like behavior or prolonged static loads)
To assess susceptibility to environmentally induced cracking events such as stress corrosion cracking and hydrogen embrittlement.

The most prominent form is the slow strain rate test (SSRT), also known as the constant strain rate tensile test. In this test, a smooth tensile specimen is pulled at a constant low strain rate while exposed to a specific medium (abrasive, high temperature, or gas). Typical strain rates range from ^10⁻⁸ to ^10⁻³ s⁻¹ ^{, and common values for stress corrosion cracking studies are around}^10⁻⁵ to ^10⁻⁷ s⁻¹ ^. The slow rate allows time for environmental interactions (e.g., corrosion, hydrogen diffusion, or passive film degradation) to affect crack formation and propagation, effects that may not be visible in faster tests.

Materials are sensitive to strain rate: their strength, ductility, and failure modes can vary significantly with deformation rate.

At high strain rates (e.g., from impacts or collisions), many metals and polymers become stronger but also more brittle.
At low strain rates, viscous or time-dependent effects become dominant. For example, creep in metals at high temperatures or increased environmental attack in abrasive environments. For instance, playdough stretches enormously when pulled slowly, but breaks when pulled quickly.

In tensile tests performed on mild steel at low strain rates, researchers observed differences in yield strength, ultimate tensile strength, and uniform elongation depending on the exact strain rate selected within the low regime. Even non-standard specimen designs can yield reproducible results for these properties.

Low strain rates for polymers and composites help simulate long-term service conditions (e.g., slow warm-up in a vehicle dashboard), while high speeds are needed for collision modeling.

Slow strain rate testing generally follows these steps:

A standard tensile specimen (flat or sometimes notched) is mounted on a testing machine with precise, slow cross-head control.
The sample is exposed to the relevant medium (e.g., salt water, high-temperature water in nuclear simulations, hydrogen sulfide-containing gas for oilfield applications, or air as a reference).
It is stretched continuously at a constant low rate until it breaks.
The main criteria are as follows:
- Failure time
- Failure prolongation
- Area reduction (measure of ductility)
- Maximum voltage
- Post-test fractography (use of scanning electron microscopy (SEM) to compare ductile pits, brittle splitting, intergranular or intragranular cracking)

Tests are typically performed both in the aggressive medium and in an inert reference medium for comparison (e.g., air or argon). A significant reduction in ductility in the medium indicates susceptibility to stress corrosion cracking or hydrogen embrittlement.

The oscillating slow strain rate test is a variant that repeatedly disrupts protective passive films by adding small load cycles around the yield point, making it more aggressive for evaluating long-term durability in alloys such as stainless steels or nickel alloys.

The standards that guide slow tensile strength testing are as follows:

ASTM G129: General procedure for slow strain rate testing.
ISO 7539-7: Stress corrosion cracking test using slow strain rate.
NACE TM0198: For corrosion-resistant alloys in acidic environments.

The main application areas of the slow strain rate testing method are as follows:

Stress corrosion cracking and hydrogen embrittlement: Widely used in oil and gas (acidic environment pipelines), nuclear energy (pressure water reactors and stainless steels irradiated in boiling water reactor water), aerospace, and chemical processing. It sorts alloys, evaluates environments (pH, temperature, potential), and investigates the mechanisms involved.
Irradiation effects: Slow strain rate testing on irradiated materials reveals hardening, ductility loss, and dislocation channeling leading to brittleness.
General tensile properties: Low strain rate tests on steels or other metals to understand speed-dependent behavior without environmental influences.
Scanning tool: Its relative speed (hours to days) compared to static load testing that can take months or years makes it an excellent method for material qualification and mechanistic studies.

For example, in nuclear research, slow strain rate testing in simulated pressurized water reactor water at approximately 7,4 x 10⁻⁷ ^s⁻¹^has shown irradiation hardening reaching saturation around 10 dPa, decreased ductility at higher doses, and occasional splitting or intergranular cracking.

The advantages of this method are as follows:

Accelerated but realistic for dynamic stress testing under abrasive conditions.
Simple setup, repeatable and versatile (flat/notched samples, various media).
It provides clear quantitative indicators (e.g., ductility ratios) and fractographic evidence.

However, this method has some limitations:

The tension rate must be carefully chosen: too slow or too fast can compromise precision.
The implications are aggressive: Materials may fail testing but perform adequately in service (requiring correlation with other tests such as field data or fracture mechanics).
Interpretation can be complex: Reduced ductility may result from crack formation, propagation, or both.
It is not ideal for all materials: Some criticisms suggest that it may exaggerate the risks compared to static loading.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide low strain rate testing (SSRT) services to businesses.