Fatigue

Definition

Fatigue refers to the weakening of materials due to repeated loading and unloading over time, often leading to eventual failure. In structural engineering, fatigue is a critical consideration because materials are subjected to cyclic stresses, which can cause cracks and lead to catastrophic failure if not properly accounted for in design and analysis.

5 Must Know Facts For Your Next Test

1. Fatigue failure typically occurs at stress levels lower than the material's ultimate tensile strength, highlighting the importance of considering fatigue in structural design.
2. The fatigue process generally involves three stages: crack initiation, crack propagation, and final fracture.
3. Materials with poor ductility are often more susceptible to fatigue failure, as they may not exhibit visible signs before breaking.
4. Environmental factors such as temperature, corrosion, and the presence of notches can significantly influence a material's fatigue life.
5. Design codes often include specific guidelines and safety factors to account for potential fatigue in structures subjected to cyclic loads.

Review Questions

• How does cyclic loading contribute to the phenomenon of fatigue in materials used in structures?
  • Cyclic loading contributes to fatigue by subjecting materials to repeated stress cycles that can lead to gradual weakening over time. Each cycle creates small changes within the material's structure, which can initiate micro-cracks. As these micro-cracks grow with each subsequent load cycle, they can eventually lead to significant structural failures if not properly addressed during design. Understanding how cyclic loading affects materials is crucial for engineers in order to prevent unexpected failures.
• What are the stages of fatigue failure in structural materials, and how can engineers mitigate these risks?
  • Fatigue failure consists of three main stages: crack initiation, crack propagation, and final fracture. Engineers can mitigate these risks by designing structures with materials that have a high endurance limit and implementing surface treatments to improve resistance to crack initiation. Additionally, using proper design techniques like avoiding sharp corners or notches can help distribute stresses more evenly and reduce the likelihood of crack formation during cyclic loading.
• Evaluate the impact of environmental factors on the fatigue life of structural materials, particularly under real-world conditions.
  • Environmental factors such as temperature fluctuations, humidity, and corrosive environments can significantly reduce the fatigue life of structural materials. For instance, elevated temperatures may accelerate crack propagation while corrosion can weaken material integrity and promote premature failure. Analyzing how these factors interact with cyclic loading is essential for accurate predictions of a structure's lifespan. Therefore, engineers must consider both material properties and environmental conditions when designing structures that will face real-world challenges.

Related terms

Cyclic Loading: The application of load that varies over time, causing materials to experience repetitive stress cycles.

Endurance Limit: The maximum stress level a material can withstand for an infinite number of loading cycles without failing due to fatigue.

Crack Propagation: The process by which a small crack in a material grows over time due to repeated stress, eventually leading to failure.