Fatigue limit

S-N curves

The fatigue limit, also known as the endurance limit, is a critical concept in the field of materials science and mechanical engineering. It refers to the maximum stress level a material can withstand for an infinite number of loading cycles without experiencing fatigue failure. This property is particularly important in the design and analysis of components subjected to cyclic loading, such as aircraft wings, automobile components, and bridge structures.

Definition

The fatigue limit is typically defined as the stress level below which a material can endure an infinite number of cycles without failing. For many materials, especially ferrous alloys like steel, there is a distinct fatigue limit. However, for non-ferrous alloys such as aluminum and copper, a true fatigue limit may not exist, and the material may eventually fail regardless of the stress level if the number of cycles is sufficiently high.

Factors Affecting Fatigue Limit

Several factors can influence the fatigue limit of a material, including:

• **Material Composition**: Different materials have different fatigue limits. For example, high-strength steel typically has a higher fatigue limit compared to aluminum.
• **Surface Finish**: A smoother surface finish can improve the fatigue limit by reducing the number of stress concentrators.
• **Temperature**: Elevated temperatures can reduce the fatigue limit of a material.
• **Residual Stresses**: Compressive residual stresses can improve the fatigue limit, while tensile residual stresses can reduce it.
• **Environmental Conditions**: Corrosive environments can significantly lower the fatigue limit due to the combined effects of corrosion and cyclic loading.

Measurement

The fatigue limit is usually determined through fatigue testing, where a sample is subjected to cyclic loading at different stress levels until failure occurs. The results are plotted on an S-N curve (stress-number of cycles curve), which shows the relationship between the stress level and the number of cycles to failure. The fatigue limit is identified as the stress level below which the curve becomes horizontal, indicating that the material can withstand an infinite number of cycles.

Applications

Understanding the fatigue limit is crucial in various engineering applications to ensure the longevity and safety of components. It is particularly important in:

• Aerospace engineering: Designing components that can withstand the cyclic stresses experienced during flight.
• Automotive engineering: Ensuring that parts such as crankshafts and suspension systems do not fail under repeated loading.
• Civil engineering: Designing structures like bridges and buildings to endure the stresses caused by traffic and environmental factors.

Related Pages

• Fatigue (material)
• S-N curve
• Stress concentration
• Cyclic loading
• Materials science
• Mechanical engineering