Fatigue failure

Definition

Fatigue failure refers to the progressive structural damage that occurs when a material is subjected to cyclic loading over time, eventually leading to fracture. This type of failure is particularly significant because it can occur at stress levels much lower than the material's ultimate tensile strength, and is influenced by factors like the number of load cycles, load magnitude, and the environment in which the material operates.

5 Must Know Facts For Your Next Test

1. Fatigue failure typically begins with the initiation of small cracks that grow over time under repeated stress.
2. Factors such as surface roughness, material imperfections, and environmental conditions can significantly impact the fatigue life of a material.
3. Different materials have different fatigue characteristics; for example, metals may exhibit distinct fatigue limits while polymers might not have a well-defined endurance limit.
4. The phenomenon of cumulative damage can occur where materials fail due to the combined effect of multiple loading types, including static and dynamic loads.
5. Predicting fatigue life requires understanding the material properties and applying appropriate design considerations to minimize stress concentrations.

Review Questions

• How does the S-N curve help engineers predict fatigue failure in materials?
  • The S-N curve provides engineers with critical information about how materials respond to cyclic loading. By plotting stress levels against the number of cycles to failure, it allows engineers to determine a material's fatigue limit and make informed decisions on safe loading conditions. Understanding the S-N curve enables engineers to design components that avoid operating in regions that could lead to premature fatigue failure.
• What role does crack propagation play in the process of fatigue failure, and how can this knowledge inform design practices?
  • Crack propagation is essential to understanding fatigue failure as it illustrates how small defects can lead to catastrophic failure over time. By recognizing that cracks can grow unnoticed under cyclic loads, engineers can implement design strategies that include regular inspections or incorporating features that reduce stress concentrations. This proactive approach helps in prolonging the lifespan of components subject to repeated loading.
• Evaluate how environmental factors might influence fatigue failure in materials under cyclic loading, and what measures could be taken to mitigate these effects.
  • Environmental factors like temperature, humidity, and corrosive agents can significantly affect a material's susceptibility to fatigue failure by altering its mechanical properties or promoting crack initiation. For example, corrosion can reduce a material's strength and accelerate crack growth. To mitigate these effects, engineers might select materials with higher corrosion resistance, apply protective coatings, or use design modifications that reduce stress concentrations. Additionally, performing tests under varying environmental conditions can provide insights into potential fatigue performance.

Related terms

S-N curve: A graphical representation that shows the relationship between the cyclic stress (S) applied to a material and the number of cycles to failure (N), used in fatigue analysis.

Endurance limit: The maximum stress level below which a material can endure an infinite number of loading cycles without failing, typically determined through fatigue testing.

Crack propagation: The process by which a small crack grows in size due to repeated loading and unloading cycles, often leading to fatigue failure.