How do materials behave under fatigue?

 Materials behave under fatigue through a progressive, localized degradation that occurs when they are subjected to repeated or cyclic loading, often at stress levels below their yield strength. The fatigue process is typically divided into three stages:​

Stages of Fatigue

1. Crack Initiation: Fatigue cracks begin at stress concentrations, such as surface defects, sharp corners, or microscopic discontinuities. Even in ductile materials, these cracks can form due to localized plastic deformation, work hardening, or persistent slip bands.​

2. Crack Propagation: Once initiated, the crack grows incrementally with each loading cycle, often leaving visible striations on the fracture surface. This stage is characterized by the gradual accumulation of damage, which may not be visible from the outside​

3. Final Fracture: When the crack reaches a critical size, the remaining cross-section can no longer support the applied load, leading to rapid, often catastrophic failure. This final stage usually resembles a brittle fracture, even in normally ductile materials.​

Key Observations

• Irreversibility: Fatigue damage is permanent; materials do not recover strength even after rest.​

• Stress Levels: Failures can occur at stress levels much lower than the material’s ultimate or yield strength, making fatigue a critical consideration in engineering design.​

• Environmental Effects: Factors like temperature, corrosion, and surface finish can accelerate fatigue and reduce the material’s life.​

• Microstructural Influence: The material’s microstructure, grain size, and metallurgical properties play a significant role in how fatigue develops and propagates.​

Fatigue Testing and Analysis

Engineers use S-N (stress vs. number of cycles) curves to predict a material’s fatigue life. These curves plot the relationship between cyclic stress amplitudes and the number of cycles before failure, helping in selecting materials for applications involving repeated loading.​

In summary, fatigue causes materials to weaken and fail under cyclic stresses, even at levels below their maximum strength, making it a fundamental concern in structural integrity and material design