Short Answer:

Fatigue is the failure of a material that occurs due to repeated or fluctuating loads over time, even when the applied stress is below the material’s ultimate strength. It results in the gradual growth of cracks that finally lead to fracture. Fatigue failure often happens suddenly without any visible warning.

In simple words, fatigue occurs when a material is subjected to continuous cyclic loading, such as in rotating shafts, bridges, or aircraft wings. Over many cycles, the material weakens and fails due to the accumulation of microscopic cracks caused by repeated stress variations.

Detailed Explanation :

Fatigue

Fatigue is one of the most common and dangerous forms of material failure in mechanical engineering. It refers to the progressive and localized structural damage that occurs when a material is subjected to cyclic or fluctuating stresses over a long period. Even if these stresses are much lower than the material’s yield strength, they can cause the material to fail after many cycles of loading and unloading. This kind of failure is particularly critical in machine parts that experience repeated motion, vibration, or load reversals.

The phenomenon of fatigue is important because it leads to failure without prior warning. The material initially shows no visible sign of damage, but microscopic cracks start forming internally at points of stress concentration such as sharp corners, holes, or surface defects. With time, these cracks grow and finally lead to complete fracture.

Stages of Fatigue Failure

Fatigue failure generally occurs in three main stages:

1. Crack Initiation:
   In the first stage, small microscopic cracks start to form on the surface of the material. These cracks usually develop at points of high stress concentration like notches, welds, or scratches.
2. Crack Propagation:
   Once a small crack forms, it begins to grow gradually with each loading cycle. The crack moves deeper into the material as stress continues to act repeatedly.
3. Final Fracture:
   In the last stage, the crack becomes large enough to cause sudden and complete failure of the material. The fracture often occurs suddenly and without much deformation, making fatigue failures dangerous and difficult to detect in advance.

Characteristics of Fatigue Failure

• Fatigue failures occur under repeated or cyclic loading rather than a single static load.
• Failure happens at stress levels much lower than the tensile or yield strength of the material.
• The fracture surface is usually smooth and shiny due to slow crack growth, followed by a rough surface at the final fracture zone.
• The process is time-dependent, meaning the longer the material is exposed to fluctuating stress, the higher the chance of fatigue failure.

Factors Affecting Fatigue

1. Magnitude of Stress:
   Higher alternating stress causes faster crack growth and earlier failure.
2. Surface Finish:
   A rough surface has more irregularities that act as stress concentrators, leading to earlier crack initiation. A smooth surface increases fatigue life.
3. Temperature:
   High temperatures can accelerate fatigue by softening the material, while extremely low temperatures can make it brittle.
4. Environment:
   Corrosive environments (like moisture or chemicals) promote crack initiation and reduce fatigue strength, leading to corrosion fatigue.
5. Material Properties:
   Ductile materials generally have better fatigue resistance than brittle ones because they can absorb energy and deform plastically before failure.

S–N Curve (Stress–Number of Cycles Curve)

Fatigue behavior of materials is often represented using an S–N curve, where S represents the stress amplitude and N represents the number of cycles to failure.

• As the stress amplitude decreases, the number of cycles before failure increases.
• Some materials like steel have a fatigue limit, which means they can withstand stress below a certain level indefinitely without failure.
• Other materials, like aluminum, have no true fatigue limit and will eventually fail regardless of how low the stress is if enough cycles occur.

Methods to Improve Fatigue Strength

1. Polishing or Surface Finishing:
   Reducing surface roughness minimizes stress concentrations and delays crack initiation.
2. Shot Peening:
   This process introduces compressive surface stresses that counteract tensile stresses, improving fatigue life.
3. Case Hardening:
   Increasing surface hardness through carburizing or nitriding helps resist crack formation.
4. Proper Design:
   Avoiding sharp corners, sudden changes in cross-section, and stress concentration points helps prevent early fatigue.
5. Material Selection:
   Choosing materials with high ductility and good fatigue resistance, such as alloy steels, can significantly improve performance.

Examples of Fatigue in Real Life

• Aircraft wings and helicopter blades experience continuous cyclic stress due to air pressure changes.
• Bridges undergo repeated stress due to moving vehicles.
• Crankshafts, connecting rods, and springs in engines and machines are common examples of fatigue-prone parts.

Such components are designed with fatigue analysis to ensure safety and reliability during operation.

Conclusion

In conclusion, fatigue is the failure of a material caused by repeated or fluctuating loads over time, even at stresses below the yield strength. It occurs gradually through crack initiation, crack propagation, and final fracture. Fatigue is influenced by factors like stress magnitude, surface condition, environment, and temperature. Understanding fatigue and improving fatigue strength are essential in mechanical design to prevent sudden and catastrophic failures in machines and structures.