Fatigue Limit

What is Fatigue Limit

Fatigue limit, also known as endurance limit or fatigue strength, is the maximum stress a material can sustain without failing from repeated loading or stress. In other words, fatigue limit is the stress level below which a material can withstand an infinite number of loading cycles without undergoing fatigue failure.

Diagram showing the fatigue limit curve for a typical steel and aluminium materials — Fatigue limit curve for steel vs aluminium

The concept of fatigue limit is important in engineering and material science as many materials subjected to cyclic loading can fail due to fatigue, even if the maximum stress level is below the ultimate tensile strength of the material. Fatigue failure can occur due to the accumulation of small cracks over time, which can lead to sudden and catastrophic failure if not detected and treated.

One example of a fatigue limit can be seen in bridges. The bridge must be designed to have enough strength to safely support the weight of vehicles passing over it without collapsing. However, if the bridge is subjected to repetitive loads (such as wind or traffic), then its fatigue limit may be exceeded and cause structural failure.

Types of Fatigue Loading

There are several types of fatigue loading that a material can be subjected to. The type of loading can have a significant impact on the fatigue limit of the material. Some common types of fatigue loading include:

Axial loading: In axial loading, the stress is applied in the direction of the longitudinal axis of the material. This can occur in tension or compression and is commonly seen in applications such as machine components and structural members.
Bending loading: In bending loading, the stress is applied perpendicular to the longitudinal axis of the material, causing it to bend. This type of loading is commonly seen in beams, shafts, and other components that are subjected to bending moments.
Torsional loading: In torsional loading, the stress is applied in a twisting motion around the longitudinal axis of the material. This type of loading is commonly seen in components such as drive shafts and gears.
Combined loading: In combined loading, the material is subjected to more than one type of loading. For example, a component may be subjected to both axial and bending loading. This type of loading can be particularly damaging to a material and can significantly reduce its fatigue limit.

Factors Affecting Fatigue Limit

There are several factors that can affect the fatigue limit of a material, including:

Material properties: The chemical composition, microstructure, hardness, strength, ductility, and toughness of the material can all affect its fatigue limit. Materials with finer grains and higher purity tend to have higher fatigue limits. The presence of inclusions, impurities, or defects in the material can reduce the fatigue limit.
Loading conditions: The magnitude and frequency of the cyclic stress, the mean stress level, the stress ratio (ratio of minimum stress to maximum stress), and the mode of loading (tension, compression, bending, torsion, etc.) can all affect the fatigue limit. Higher stress levels, higher stress frequencies, and lower stress ratios tend to reduce the fatigue limit. Compressive stresses can increase the fatigue limit, while tensile stresses tend to decrease it.
Environmental factors: The temperature, humidity, corrosive environment, and presence of contaminants or lubricants can all affect the fatigue limit. High temperatures, high humidity, and corrosive environments can reduce the fatigue limit. The presence of contaminants or lubricants can either increase or decrease the fatigue limit, depending on the nature and amount of the contaminants or lubricants.

It is important to note that these factors can interact with each other in complex ways, and the effect of each factor can vary depending on the specific application and conditions. Therefore, accurate estimation of the fatigue limit of a material requires careful consideration of all the relevant factors and their interactions.

Applications of Fatigue Limit

The concept of fatigue limit is important in many fields of engineering and materials science and has several applications, including:

Design of mechanical components: Knowledge of the fatigue limit is critical for the design of mechanical components that will be subjected to cyclic loading, such as aircraft components, automotive parts, and machine components. Designing components with a sufficient fatigue limit ensures that they will have a longer service life and reduce the risk of sudden and catastrophic failure.
Infrastructure and construction industry: In the infrastructure and construction industry, fatigue limit is important for designing safe and reliable structures, such as bridges, buildings, and pipelines, which are subjected to cyclic loading over many years. Understanding the fatigue limit helps ensure that these structures are designed to withstand the required stress levels over their intended service life.
Manufacturing industry: Fatigue limit is important in the manufacturing industry for ensuring the quality and reliability of components produced using different manufacturing techniques, such as forging, casting, and machining. Understanding the fatigue limit helps ensure that the components meet the required standards for durability and reliability.
Material science research: Fatigue limit is an important area of research in material science, where researchers study the mechanisms of fatigue failure and develop new materials and manufacturing techniques that can improve the fatigue limit of materials and components. Understanding the fatigue limit is critical for advancing the field of material science and developing new materials and applications

Fatigue Limit Design Considerations

When designing any product or structure with components that experience repeated cycles of loading and unloading, engineers must consider several factors when determining the appropriate fatigue limit:

Material properties: Different materials have different fatigue limits based on their properties such as elasticity, ductility, hardness, etc., so engineers must select materials with appropriate properties for their designs.
Stress profile: The rate and magnitude of the load experienced by an object over time will affect its expected life span under those loads. Thus it’s important for engineers to accurately simulate real-world loading conditions when setting a fatigue limit.
Operating environment: Temperature and humidity can also affect an object’s expected life under load. Thus engineers must also account for environmental conditions when setting a fatigue limit.

How is fatigue limit determined?: Fatigue limit is typically determined by subjecting a material to cyclic loading at various stress amplitudes and counting the number of cycles required to cause failure. This process is repeated with different stress levels, until the stress amplitude is found at which the material can withstand an infinite number of cycles without failure.

What factors affect fatigue limit?: The fatigue limit of a material is affected by several factors, including the material’s microstructure, surface finish, temperature, loading frequency, and stress concentration. Other factors that can affect fatigue limit include the presence of notches or cracks, the loading history of the material, and the type of loading applied.

How does temperature affect the fatigue limit of a material?: Temperature can have a significant impact on the fatigue limit of a material. Generally, increasing the temperature can decrease the fatigue limit of a material due to the reduced strength and stiffness of the material at higher temperatures. This can cause a decrease in the resistance of the material to the formation and propagation of microcracks, leading to a lower fatigue limit.