Fatigue is a type of failure that occurs in materials when they are subjected to repeated or cyclic loading. It is a common type of failure in engineering structures and components, such as bridges, aircraft, and machinery, that are subjected to repeated loads over their lifetime.
When a material is subjected to a load, it will undergo some level of deformation, which is called strain. When the load is removed, the material will return to its original shape, which is called recovery. If a material is subjected to a load that is below its yield strength (the point at which it begins to deform permanently), it will recover completely when the load is removed. However, if the material is subjected to a load that is above its yield strength, it will undergo some level of permanent deformation, which is called plastic deformation.
When a material is subjected to repeated or cyclic loading, it will undergo alternating periods of strain and recovery. If the cyclic load is high enough, the material may undergo plastic deformation with each cycle, which can lead to the accumulation of strain over time. As the strain accumulates, it can cause the material to become weaker and more susceptible to failure. This process is called fatigue.
The fatigue limit of a material is the maximum stress that it can withstand under cyclic loading without failing due to fatigue. The fatigue limit is usually lower than the ultimate tensile strength of the material, which is the maximum stress that it can withstand before breaking under a single, static load.
In engineering design, it is important to consider the fatigue properties of materials and to design structures and components to avoid fatigue failure. This may involve selecting materials with high fatigue limits, designing structures to distribute loads evenly, or using fatigue-resistant design techniques.
Frequently Asked Questions
1. What is fatigue in materials?
Fatigue in materials is the phenomenon of damage and failure that occurs when a material is subjected to cyclic loading, leading to the initiation and propagation of cracks over time.
2. How does fatigue differ from other forms of material failure?
Fatigue is distinct from sudden and catastrophic failures. Unlike static loading, where a material withstands a constant load, fatigue failure occurs due to repeated loading and unloading cycles below the material’s ultimate strength.
3. What types of materials are susceptible to fatigue?
Almost all materials, including metals, polymers, and composites, can experience fatigue. However, it is more commonly observed in metals due to their crystalline structure and susceptibility to crack initiation and propagation.
4. What are the primary factors influencing fatigue life?
Factors affecting fatigue life include stress amplitude, mean stress, material properties, surface conditions, temperature, and the presence of notches or defects.
5. How is fatigue strength determined?
Fatigue strength is typically determined through fatigue testing, where a specimen undergoes repeated loading and unloading cycles until failure. The results are used to create an S-N (stress versus number of cycles to failure) curve.
6. What is the endurance limit?
The endurance limit, also known as the fatigue limit, is the maximum stress level below which a material can endure an infinite number of cycles without fatigue failure. Not all materials have a distinct endurance limit.
7. Can materials exhibit fatigue even under relatively low loads?
Yes, materials can experience fatigue failure even under loads much lower than their ultimate strength. Fatigue is particularly insidious as it often occurs at stress levels well below the static strength of the material.
8. How can fatigue life be extended?
Improving fatigue life involves factors such as design modifications to eliminate stress concentrations, surface treatments to reduce crack initiation sites, and selecting materials with better fatigue resistance.
9. Are there industry standards for fatigue testing?
Yes, there are standardized procedures for fatigue testing, such as ASTM (American Society for Testing and Materials) standards, which provide guidelines for conducting fatigue tests on various materials.
10. How is fatigue addressed in engineering design?
Engineers address fatigue in design by incorporating factors of safety, considering materials with high fatigue resistance, and employing techniques such as stress relief and surface treatments to enhance the fatigue life of components.
