Short Answer:

Fatigue is the progressive and localized structural damage that occurs in a material when it is subjected to repeated or cyclic loading. Even if the applied stress is below the material’s yield strength, continuous loading and unloading can eventually cause cracks and lead to failure.

In simple words, fatigue means the weakening of a material due to repeated stress or strain over time. It usually starts with small cracks that grow gradually and finally cause sudden fracture. Fatigue failure is common in machine parts like shafts, springs, connecting rods, and aircraft components that experience fluctuating or reversing loads.

Detailed Explanation :

Fatigue

In mechanical engineering, fatigue refers to a phenomenon where a material fails after being subjected to repeated or fluctuating stresses over an extended period. Unlike static loading, where the material fails when the stress exceeds the ultimate strength, fatigue failure can occur even when the stress levels are much lower.

Fatigue is one of the most common causes of failure in mechanical components and structures that are continuously subjected to cyclic or alternating forces. Examples include rotating shafts, bridges under traffic loads, airplane wings, and railway axles. These components experience stress cycles due to varying loads during operation, which eventually leads to fatigue failure.

Definition of Fatigue

Fatigue can be defined as:

“The progressive and permanent structural damage in a material caused by repeated or cyclic stresses that are less than the material’s ultimate tensile strength.”

It is a time-dependent failure phenomenon caused by fluctuating or alternating loads that lead to the initiation and growth of cracks, ultimately resulting in fracture.

Fatigue failure usually occurs suddenly and without warning, even though the stresses involved are lower than the yield limit.

Stages of Fatigue Failure

The process of fatigue failure generally occurs in three stages:

Crack Initiation:
- This is the first stage of fatigue failure.
- Small cracks form at stress concentration points such as surface scratches, notches, sharp corners, holes, or inclusions.
- These cracks usually originate on the material surface where the stress is highest.
Crack Propagation:
- Once a crack starts, it grows progressively with each load cycle.
- The crack propagates slowly at first but accelerates as the cross-section weakens.
- Under a microscope, the crack surface shows “beach marks” or “striations,” which represent the progress of the crack over time.
Final Fracture:
- When the remaining cross-sectional area becomes too small to withstand the load, the material fractures suddenly.
- This stage occurs quickly and results in complete separation of the part.

The fracture surface often shows two distinct regions — a smooth area (slow crack growth) and a rough area (final rapid fracture).

Types of Fatigue Loading

Fatigue loading can occur in several forms, depending on how the stress varies with time:

Completely Reversed Stress:
- Stress changes from equal tension to equal compression (e.g., rotating shafts).
Fluctuating or Alternating Stress:
- Stress alternates between maximum and minimum values, but not symmetrically about zero.
Repeated Stress:
- Stress varies from zero to a maximum value repeatedly (e.g., springs or beams under periodic loading).

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

The relationship between the applied stress and the number of cycles to failure is represented by an S–N curve (also called a Wöhler curve).

S represents the stress amplitude, and N represents the number of cycles to failure.
The curve shows that as the applied stress decreases, the material can withstand more load cycles before failing.
For materials like steel, the curve becomes nearly horizontal after a certain stress level — this stress is called the endurance limit or fatigue limit.

Endurance Limit:
It is the maximum stress that a material can withstand for an infinite number of cycles without failure.

Factors Affecting Fatigue

Magnitude of Stress:
Higher stress levels lead to fewer cycles before failure.
Stress Concentration:
Sharp corners, notches, and holes increase stress concentration, accelerating crack initiation.
Surface Finish:
Smooth surfaces resist fatigue better, while rough surfaces promote crack formation.
Material Type:
Ductile materials (like steel) have higher fatigue resistance than brittle materials (like cast iron).
Temperature:
High temperatures reduce fatigue life, while low temperatures may improve it.
Residual Stresses:
Compressive residual stresses on the surface can improve fatigue life by preventing crack initiation.
Environment:
Corrosive environments (like humid or salty air) accelerate fatigue failure through corrosion fatigue.

Methods to Improve Fatigue Strength

To reduce fatigue failure and increase the life of mechanical components, the following methods are used:

Polishing and Surface Finishing:
Smooth surfaces help reduce crack initiation.
Shot Peening:
Induces compressive residual stresses on the surface, increasing fatigue strength.
Proper Material Selection:
Use high-strength alloys or heat-treated materials for better fatigue performance.
Avoid Sharp Corners:
Use fillets and smooth transitions to reduce stress concentration.
Surface Hardening:
Processes like carburizing and nitriding improve surface resistance.
Regular Inspection and Maintenance:
Helps in early detection of cracks before complete failure.

Examples of Fatigue Failure

Aircraft wings and propellers – Repeated aerodynamic loads.
Bridge components – Vibration and traffic loading.
Crankshafts and connecting rods – Repeated stress due to piston movement.
Railway axles – Alternating bending stresses during rotation.
Springs – Continuous compression and relaxation cycles.

Importance of Fatigue Study

Understanding fatigue is vital in mechanical design because:

It helps in predicting the service life of components.
Prevents sudden and catastrophic failures.
Aids in selecting suitable materials and shapes for cyclic load conditions.
Ensures safety, reliability, and durability of machines and structures.

Conclusion

The fatigue of materials is a gradual process of failure due to repeated or cyclic loading, even at stresses below the yield limit. It starts with crack initiation, followed by crack growth, and ends with sudden fracture. Fatigue is influenced by stress level, material properties, surface finish, and environmental conditions. By improving design, surface treatment, and material quality, engineers can significantly enhance fatigue life and ensure safe, long-lasting performance of components subjected to cyclic stresses.