Strain-Life and Stress-Life Fatigue Testing

From strain-controlled (E-N) or load-controlled (S-N) fatigue tests to data analysis required to calculate fatigue parameters, HBK performs the testing and characterisation necessary for obtaining accurate fatigue life estimates from FEA and fatigue analysis.

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Stress-Life Fatigue Testing

Stress-life fatigue testing, also known as S-N testing (Stress vs. Number of cycles), is a fundamental method used to evaluate the fatigue behaviour of materials under cyclic loading. This type of testing is essential for understanding how materials respond to repeated stress applications, especially when the stress levels are below the material’s yield strength.

The primary goal of stress-life testing is to generate an S-N curve, which represents the relationship between the cyclic stress range and the number of cycles to failure. By subjecting specimens to a constant amplitude cyclic load until failure, engineers can determine the material’s fatigue life. This data is critical for the safe and reliable design of components in industries such as aerospace, automotive, and structural engineering, where materials are routinely exposed to fluctuating stresses.

Stress-life testing is most applicable to materials/components that operate in the high-cycle fatigue regime, where material exhibit fully elastic behaviour. It is used to determine the fatigue limit—a stress level below which the material can theoretically endure infinite cycles without failure. The results provide valuable insight into the durability and long-term performance of materials under realistic service conditions.

Strain-Life Fatigue Testing

Strain-life fatigue testing, often referred to as E-N testing (strain vs. number of cycles), is a fatigue analysis method used to characterise the behaviour of materials under high strain or low-cycle fatigue conditions. Unlike stress-life (S-N) testing, which is more applicable to high-cycle fatigue with lower stress levels, strain-life testing focuses on situations where materials experience plastic deformation during each load cycle.

In this method, a specimen is subjected to controlled, repeated strain cycles, and the number of cycles to failure is recorded. The resulting data is used to develop an ε-N curve, which describes how a material’s fatigue life relates to the imposed strain amplitude. This approach accounts for both the elastic and plastic components of strain, making it particularly valuable for analysing components that undergo large stress reversals, such as in turbine engines, automotive suspensions, and pressure vessels.

Strain-life testing is crucial for accurate fatigue life predictions in applications where materials are pushed near or beyond their yield strength. It provides insights into crack initiation behaviour—a key phase in fatigue failure—making it a vital tool for engineers designing components for demanding mechanical environments.

Key Benefits of Strain-Life Testing

Works for both low-cycle and high-cycle fatigue regimes
Provides more useful material properties compared to stress-life testing:

o Strain-life curve & stress-life curve

o Cyclic stress-strain curve

o Cyclic strength coefficient and hardening exponent (K' & n')

o Elastic modulus and yield strength (E & σ_γ)

o Fatigue strength coefficient and exponent (σ'_f & b)

o Fatigue ductility coefficient & exponent (ε'_f & c)

o Endurance limit

Improve product durability and reliability

Testing Capabilities

Load capacity: 1 - 250 kN
Specimen geometry: Flat sheet / Cylindrical / Bespoke specimens
Range of Materials: Metals / Polymers / Composites / Welds / Adhesives
Temperature: room temperature to 900°C
Frequency: up to 50 Hz

*All capabilities are subjected to specimen geometry and material

Test to standards

Tests are performed to international, internal or bespoke standards.

Stress-life testing

ASTM E466-21
Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials
BS ISO 1099:2017
Metallic materials — Fatigue testing — Axial force-controlled method

Strain-life testing

ASTM E606-21
Standard Test Method for Strain-Controlled Fatigue Testing
BS 7270:2006
Metallic materials — Constant amplitude strain controlled axial fatigue — Method of test
ISO 12106:2017
Metallic materials — Fatigue testing — Axial-strain-controlled method

Results for CAE

Reassured with analysed data - We cover comprehensive data analysis of the testing results.
Results for CAE - We provide parameters that are ready-to-use in CAE & simulation. Data can be easily imported into nCode DesignLife.
Confidence & Reliability - We provide design curves for product optimisation.

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Accredited

HBK’s AMCT is an established fatigue testing facility with over 25 years of operational experience and ISO 9001:2015 certification.

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Strain-Life and Stress-Life Fatigue Testing

Stress-Life Fatigue Testing

Strain-Life Fatigue Testing

Key Benefits of Strain-Life Testing

Testing Capabilities

Test to standards

Results for CAE

Accredited

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