High-Cycle Fatigue vs. Low-Cycle Fatigue Testing 

Introduction to Fatigue Testing 

Fatigue refers to the gradual formation of microcracks and cracks in a material under the cyclic loading of stress. Those irregularities reduce the material’s capacity to withstand cyclic loads and ultimately lead to catastrophic failure. 

Fatigue testing provides the insight necessary to prevent failure by refining designs, predicting maintenance needs, and estimating materials’ lifespan. Without it, material failure can lead to potentially devastating consequences. For example, in early 2024, a Boeing 737 jet lost a plug from one of its doors mid-flight. The company announced that it discovered more loose parts just days later, making headlines nationwide. This frightening scenario underscores the critical nature of robust fatigue testing measures, which could have provided guidance into those parts’ lifespans and failure points.

Innovative Test Solutions (ITS) has offered expert fatigue testing and a full suite of testing since 2004. It operates an ISO/IEC 17025 accredited testing laboratory staffed by a team of trained engineers. ITS specializes in assessing the mechanical behavior of structural materials and structures. The team can perform rigorous standard tests following ASTM protocols. Additionally, ITS can customize testing protocols and design one-of-a-kind test rigs and procedures to ensure even non-standard samples can undergo critical testing. 

Definition of Fatigue Testing 

This mechanical testing method uses specialized equipment to apply cyclic loads to testing samples, allowing engineers to predict performance and reliability. By exposing the sample to repeated load applications, fatigue tests provide vital data revealing the sample’s ability to resist damage. It measures durability and longevity and provides critical insight into the failure point. The data collected during testing provides engineers and designers with the details necessary to make improvements and predict product performance and service life.

It helps identify weaknesses and potential flaws in materials, thereby enhancing engineers’ ability to optimize the materials they choose for a product. Fatigue testing helps identify materials’ endurance limits, which can maximize product lifespan and help end users implement predictive maintenance. The data collected from fatigue testing can also drive design improvements for an end product that is less likely to crack and fail due to fatigue. 

Types of Fatigue Testing 

Material failure can occur below its strength limits when it is exposed to a high number of cycles. It can also occur at a relatively low number of cycles with stressors that exceed the material’s yield strength. Fatigue accounts for an estimated 80% to 90% of instances of mechanical service failures, many of which can have costly consequences, both from a financial and safety standpoint. That is why fatigue testing is such a critical step in research and development, product safety, and material component verification.

As materials, components, and product design and manufacturing capabilities advance, so do fatigue testing techniques. This ongoing refinement and evolution are necessary to keep up with the demand for high quality products with longer service lives and improved performance. Although the testing is often performed on metals and alloys, it also provides valuable insight into the performance of other materials. A broad range of materials used in engineering, including composites, aggregates, and ceramics, can also experience fatigue.

Additionally, because applications and materials vary, multiple types of fatigue testing are necessary, including high-cycle fatigue and low-cycle fatigue testing. Each has unique characteristics, methodologies, and applications. A solid understanding of low and high-cycle fatigue testing and the use cases for each is essential. It can also aid designers, engineers, and manufacturers in the pursuit of optimizing product designs and material selections. 

High-Cycle Fatigue (HCF) Testing 

High-cycle fatigue often creates elastic deformation, marked by no significant or permanent deformation prior to material failure. This type of fatigue testing is designed to assess materials’ ability to endure many cycles under relatively low stress levels. The overarching goal is to determine the number of cycles the specimen can withstand before the initiation of a fracture and material failure. 

Definition and Characteristics 

HCF refers to one of two primary service-induced damages that materials and components may experience. In this case, the specimen’s typical operating environment produces a low stress amplitude, usually below that of the material’s yield strength. However, the conditions are also characterized by high frequency and a vast number of cycles — typically occurring after more than 10,000, up to 100,000 or more. Although the stress levels are within the material’s parameters, vibration and other forces present during normal operations can eventually cause wear and fractures. 

Testing Methodology 

HCF testing relies on state-of-the-art servo-hydraulic test frames outfitted with data acquisition systems. The sample is subjected to frequencies defined by the type of material and its typical environmental conditions, usually between 20 Hz and 100 Hz. Applying the specified stress and load control produces a Wöhler curve, more commonly known as an S-N (stress-life) curve. This provides a visual representation of the relationship between the stress and load level and the number of cycles it takes to achieve failure.

The horizontal axis represents the number of cycles it takes for the sample to fracture. The vertical one describes the stress/load range as it was applied. The test continues until a pre-set number of cycles has been reached or the sample fails. 

An HCF test is conducted according to industry standards and protocols, such as ASTM E466, which is among the most common. It provides a standard for testing axial unnotched and notched metallic specimens for strain defines the type of specimens that can undergo testing. Moreover, it establishes protocols for consistency in such testing to ensure reliable results. 

Applications 

High-cycle fatigue testing is critical across various industries, including aerospace. Imagine the repetitive stress that components and materials in wind and gas turbine blades undergo, for example, over the course of their lifespans. HCF testing is particularly valuable for components and materials that are subject to vibration and millions of stressors over the course of normal use.

Now consider the implications if those turbine blades fail. In gaining a deep understanding of how materials and components will withstand typical operating conditions, it enhances safety and provides insight into when replacements and repairs will likely be necessary.28 

In addition to the aerospace industry, other common industries that rely on HCF testing include: 

• Medical devices 
• Transportation 
• Oil and gas 
• Energy and power generation 

Low-Cycle Fatigue (LCF) Testing 

In contrast to HCF, low-cycle fatigue occurs when materials operate in a high stress environment, typically with fewer cycles. It causes what’s known as plastic deformation, or permanent changes to the material’s shape prior to fracture and failure. Low-cycle fatigue testing measures the specimen’s ability to withstand mechanical and/or thermal stresses. 

Definition and Characteristics 

Low-cycle fatigue refers to the second of the two primary service-induced damages that materials and components may experience. Unlike high-cycle fatigue, the specimen’s typical operating environment is one of a high stress amplitude, typically exceeding the material’s yield strength. Additionally, LCF provides insight into materials’ behavior in high stress, but relatively low cyclic loading.

On the other hand, HCF testing subjects specimens to a minimum of 10,000 cycles, while LCF is marked by its low number of cycles. It typically takes no more than 10,000, and often fewer. The data generated by a low-cycle fatigue test provides the information needed to assess materials and components’ behavior in intense settings. It can pinpoint plastic deformation, the initiation of cracks, and, ultimately, material failure.

Testing Methodology 

Like high-cycle fatigue testing, low-cycle fatigue tests are also conducted on servo-hydraulic test frames. Specimens are subject to frequencies ranging from 0.25 Hz to more than 5 Hz, with stress parameters set to replicate the sample’s real-world operating conditions. The test often includes attaching an extensometer to measure strain. 

The data generated an E-N or ε–N (total strain-life) curve. This provides a visual representation of the relationship between the amplitude of strain and the number of reversals it takes to achieve failure under the prescribed cyclic loading conditions. In this instance, the vertical axis represents half the range of strain in a loading/unloading cycle, or strain amplitude. The horizontal axis describes the number of reversals of applied strain the specimen withstands before failure.

Low-cycle fatigue tests are conducted according to industry standards, most commonly ASTM E606. This method outlines the requirements of the testing method and the scope of low-cycle fatigue testing. It provides a guide without any restrictions on temperature, pressure, or other environmental factors as long as they remain controlled during the test.

Applications 

Because fatigue manifests with few outward signs, determining how materials and components will hold up in high-stress environments is critical. Low-cycle fatigue testing is valuable for various components across multiple sectors. For example, in construction, a building in an earthquake-prone zone will require materials capable of enduring high stress loads. It’s frequently used for rotating components and materials with uses that expose them to high temperatures or mechanical stress. Common industries that use low-cycle fatigue testing include: 

• Aerospace 
• Architecture 
• Automotive 
• Manufacturing 
• Oil and gas 
• Power generation 

Comparison: High-Cycle Fatigue vs Low-Cycle Fatigue 

Low-cycle fatigue tests and high-cycle fatigue tests share a similar goal — to assess the behavior of materials and components in conditions simulating their operating environment. Both provide vital insight into specimens’ fatigue properties. The resulting data allows engineers to determine performance, suitability for various use cases, durability while in operation, and maintenance needs. Still, the two also differ in many ways.

Differences in Definitions and Characteristics 

For engineers and manufacturers, understanding the way materials behave under stress and strain is pivotal for choosing the optimal materials for various use cases. Both low-cycle fatigue testing and high-cycle fatigue testing provide critical data and insight, but with differing methodologies to test for each type of fatigue.

Low-cycle fatigue is characterized by high stress levels and low cycles, typically fewer than 10,000, although ASTM standards allow for up to 100,000. High-cycle fatigue is the opposite, occurring at relatively low stress levels with many cycles, typically 10,000 or more, up to millions.

Testing Methodologies 

Both tests are designed to mimic real-world operating conditions and typically continue until the specimen fails. LCF testing uses low frequencies and high force, while HCF uses high frequencies and relatively low force. 

Both tests occur using servo-hydraulic machines. High-cycle fatigue testing requires high-frequency cyclic loading at a constant amplitude. In contrast, low-cycle fatigue testing requires low frequencies and high stress, with varying amplitudes. Both testing methodologies may also rely on sensors or strain gauges attached to the sample for added monitoring of material response. 

Applications and Industry Relevance 

HCF testing is critical for materials and components used in high-vibrational, low-strain settings. For instance, in the aerospace industry, an airfoil is exposed to constant bending and vibration from compressor and turbine blade operations. LCF testing provides essential data for materials and components used in high-strain or extreme thermal environments.

A turbine blade in an engine, for example, can experience LCF damage as the engine’s rotational speeds increase. It undergoes mechanical stress from centrifugal force, even though the increase isn’t necessarily constant. Although industry relevance often overlaps, the common uses include the following: 

• High-cycle fatigue testing: Aerospace, automotive, defense, energy, medical devices, oil and gas, power generation 
• Low-cycle fatigue testing: Aerospace, architecture, automotive, construction, manufacturing, oil and gas, power generation 

Importance of Fatigue Testing

Fatigue testing offers value across many industries as it aids designers, engineers, and manufacturers in assessing and validating a component or material’s durability. By simulating real-world operating conditions, these tests provide data regarding structural damage. Ultimately, it provides the details necessary to refine designs or material selection, predict damage, and design maintenance and repair schedules accordingly. Fatigue testing provides data-driven insights into the engineering process that can save money and enhance safety. It is critical for products used in defense, transportation, infrastructure, and more. 

Impact on Safety and Reliability 

Several historic disasters can be attributed to mechanical fatigue. For example, the Versailles train crash in 1842, which killed an estimated 55 people, was caused by an axle that broke on the lead locomotive. At the time, mechanical fatigue and material degradation weren’t studied or understood in depth. August Wöhler (the Wöhler curve) and William Rankine are among the researchers who were inspired by the accident to take action. They spent decades studying and testing train axles after the accident, leading many historians to mark this as the beginning of mechanical fatigue testing and studies. 

Decades later, extensive investigations into the De Havilland Comet plane crashes in 1954 revealed that metal fatigue caused explosive decompression. An oil platform collapsed in 1980 in the North Sea, killing more than 120 workers, and was caused by fatigue cracking in a structural brace. Fatigue also caused another train incident in 1998. The Eschede disaster killed 101 people, all due to vibration-linked fatigue that led to the failure of a train wheel. That failure sparked a chain reaction that derailed more than a dozen train cars and caused a bridge collapse.

These examples represent just a small sample of the destructive effects of mechanical fatigue. With robust fatigue testing, engineers no longer blindly install components or design parts using materials that can’t withstand the operating environment’s stressors. The data allows for greater insight into reliability and performance, ensuring the safety and reliability of components and structures. 

Economic Benefits 

Mechanical fatigue failures cost an estimated 4% of the United States’ gross domestic product (GDP), which totaled more than $29 trillion in 2024. Given the enormous price of these types of failures, the savings associated with preventing them can be equally significant. With fatigue testing, engineers can identify failure mechanisms, which can otherwise go undetected before failure. This enables proactive mitigation to reduce failure risks and paves the way for the optimization of material selection and design. Associated benefits include a reduction of unplanned downtime and improved product performance and structural integrity. 

Regulatory Compliance 

Standards vary across different industries, each of which has strict safety standards. Fatigue testing is critical to ensure a product meets all requirements for the appropriate agencies to approve it for the marketplace. Common standards include, but are not limited to, ASTM 606, ASTM E466, ISO 12106, ISO 1099, BS ISO 114, BS 66072, EN 3874, and BS 7270. These standards vary from fatigue in metallic materials to constant amplitude fatigue testing for aerospace uses.

Conclusion 

Mechanical fatigue is the leading cause of mechanical failures, which can result in staggering costs and consequences. Low-cycle fatigue testing and high-cycle fatigue testing offer critical data to support product safety and reliability. Both are critical in many instances, particularly in sectors like aerospace, automotive, and power generation.

High-cycle fatigue testing uses relatively low stress levels with high cyclic loads to measure stress-life, using an S-N curve as the visual. This explores the relationship between the variables and elastic deformation in the materials. In contrast, low-cycle fatigue testing is characterized by high stress levels and relatively low cyclic loads. This type of fatigue testing measures strain-life, using an E-N curve as the visual representative of the relationship between strain and plastic deformation.

In both cases, the goal is to assess the number of cycles it takes for the material to fail. The resulting data provides engineers, designers, and manufacturers with the details necessary to optimize product design and material selection. Ultimately, it supports better, more durable, and reliable products capable of performing in rigorous environments. From aerospace and automotive to power generation and manufacturing, fatigue testing is a game changer for industries of all types.

Partnering with an expert testing laboratory ensures the highest quality data and results. Innovative Test Solutions has decades of experience and a team of expert engineers who can provide customized methodologies and testing strategies when necessary. The full-service laboratory is ISO/IEC 17025 accredited to perform both low and high-cycle fatigue testing to meet ASTM standards E466 and E606.

High-quality products require rigorous quality assurance testing. Fatigue testing is among the most critical tests to optimize the end product’s performance, safety, and reliability. Get in touch with ITS today to learn more about our testing services or to request a quote.