Fatigue

Definition

Fatigue is a phenomenon that occurs in materials when they are subjected to repeated loading and unloading cycles, leading to the formation of cracks and eventual failure. This process can happen at stress levels lower than the material's yield strength, making it a critical consideration in engineering design and materials selection. Understanding fatigue is essential for ensuring the longevity and reliability of structures and components that experience fluctuating loads over time.

5 Must Know Facts For Your Next Test

1. Fatigue failure often starts with microscopic cracks that gradually grow over time due to repetitive stress.
2. Factors such as material properties, loading conditions, temperature, and environmental influences can significantly affect fatigue behavior.
3. Different materials exhibit different fatigue resistance; for instance, metals typically have a well-defined fatigue limit, while polymers may not.
4. The S-N curve (stress vs. number of cycles) is a graphical representation used to illustrate the relationship between cyclic stress and the number of cycles to failure for a given material.
5. Preventing fatigue failure often involves design strategies like using fillets to reduce stress concentrations, selecting materials with high fatigue resistance, and implementing regular inspections.

Review Questions

• How does fatigue differ from other types of material failure, and why is it important to consider in engineering applications?
  • Fatigue differs from other types of material failure because it can occur at stress levels lower than the yield strength of a material and develops through repeated loading cycles over time. This gradual process often results in unexpected failures after many cycles rather than sudden catastrophic failure. It's important to consider fatigue in engineering applications because many components experience cyclic loads during their service life, and understanding this behavior helps ensure safety and reliability.
• Discuss the factors influencing fatigue behavior in materials and how engineers can mitigate the risks associated with fatigue failure.
  • Several factors influence fatigue behavior in materials, including material composition, surface finish, temperature, and environmental conditions. Engineers can mitigate risks associated with fatigue failure by selecting materials with appropriate fatigue properties, employing effective surface treatments to improve durability, designing components to minimize stress concentrations, and incorporating regular inspection protocols to detect early signs of crack propagation. Additionally, understanding the loading conditions a component will face helps inform better design practices.
• Evaluate the significance of the S-N curve in understanding material fatigue and its implications for engineering design decisions.
  • The S-N curve plays a crucial role in understanding material fatigue by illustrating the relationship between applied stress levels and the number of cycles until failure. This information is vital for engineers when designing components subjected to cyclic loads since it helps them determine safe operating limits and select appropriate materials based on expected service conditions. By evaluating data from S-N curves, engineers can make informed decisions that enhance safety, performance, and longevity of structures or components while minimizing the risk of fatigue-related failures.

Related terms

Fatigue Limit: The maximum stress level below which a material can endure an infinite number of load cycles without failing.

Crack Propagation: The process through which a crack grows in a material as a result of cyclic loading, eventually leading to fracture.

Endurance Limit: The highest stress level that a material can withstand for an indefinite number of cycles without experiencing fatigue failure.