What is fatigue?

Short Answer:

Fatigue is the progressive and permanent weakening or failure of a material when it is subjected to repeated or fluctuating stresses over time, even if these stresses are lower than the material’s ultimate strength. It is a common cause of failure in machine parts and structures under cyclic loading.

In simple words, fatigue means gradual damage due to repeated loading and unloading. Small cracks form in the material, which slowly grow and finally cause sudden fracture. Fatigue failure often occurs unexpectedly without much visible warning, making it dangerous in engineering applications like bridges, shafts, and aircraft components.

Detailed Explanation:

Fatigue

Definition and Meaning:
Fatigue is a time-dependent failure phenomenon that occurs when a material is subjected to repeated cyclic stresses. Unlike static loading, where a constant force causes deformation or fracture, fatigue failure results from alternating or fluctuating loads that cause microscopic cracks to form and grow over time.

Even though each individual load cycle is below the yield or ultimate strength of the material, the repeated application of stress leads to internal damage, crack propagation, and eventual fracture. This process makes fatigue one of the most common causes of mechanical failure in engineering components.

For example, an aircraft wing, rotating shaft, or bridge structure experiences continuous loading and unloading during operation. Over time, such cyclic stresses can lead to fatigue cracks and sudden failure.

Stages of Fatigue Failure

Fatigue failure generally occurs in three distinct stages, each representing a different phase in the life of the material:

  1. Crack Initiation Stage:
  • In this initial stage, microscopic cracks start to develop on the surface or near stress concentration points such as notches, sharp corners, holes, or scratches.
  • These cracks form due to localized plastic deformation caused by repeated stress reversals.
  • The duration of this stage depends on the material type, surface finish, and applied stress level.
  1. Crack Propagation Stage:
  • Once the crack is formed, it gradually extends or propagates with each stress cycle.
  • The crack grows perpendicular to the applied stress and advances through the material grain by grain.
  • This stage can be identified by the striations (fine lines) visible on the fracture surface, indicating the crack growth per cycle.
  • Most of the material’s fatigue life is consumed in this stage.
  1. Final Fracture Stage:
  • When the remaining cross-sectional area of the component becomes too small to carry the applied load, sudden fracture occurs.
  • The final fracture may happen at a stress level much lower than the material’s yield strength.
  • The fracture surface usually shows two regions: a smooth area (due to slow crack growth) and a rough area (due to sudden breakage).

Characteristics of Fatigue Failure

  • Fatigue fractures usually start at the surface where stress concentration is highest.
  • The fracture surface often has a beach mark or shell pattern appearance showing crack progression.
  • Failure occurs without noticeable plastic deformation, making it sudden and dangerous.
  • Fatigue life is defined by the number of cycles to failure at a given stress level.

Thus, fatigue is a silent type of failure that develops slowly but ends abruptly.

Factors Affecting Fatigue

  1. Stress Magnitude:
    • Higher alternating stress reduces fatigue life.
    • Even low stresses can cause fatigue if applied for a long time.
  2. Stress Concentration:
    • Notches, holes, sharp corners, and surface defects act as stress raisers and promote early crack initiation.
  3. Surface Finish:
    • Smooth and polished surfaces increase fatigue strength, while rough surfaces decrease it.
  4. Material Properties:
    • Ductile materials generally have better fatigue resistance than brittle ones.
    • Alloying and heat treatment can improve fatigue life.
  5. Temperature:
    • Elevated temperatures may soften materials and reduce fatigue strength, whereas very low temperatures can cause brittleness.
  6. Corrosive Environment:
    • Corrosion accelerates crack growth and reduces fatigue life (known as corrosion fatigue).
  7. Residual Stresses:
    • Compressive residual stresses improve fatigue resistance, while tensile residual stresses reduce it.
  8. Load Type:
    • Rotating bending and alternating loads are more damaging than steady or static loads.

Fatigue Testing

To study and measure fatigue behavior, a fatigue test is performed. In this test:

  • A specimen is subjected to repeated cyclic stresses (tension-compression, bending, or torsion).
  • The number of cycles to failure () is recorded for different stress amplitudes ().
  • The results are plotted on an S–N curve (Stress vs. Number of cycles), also called a Wöhler curve.

S–N Curve Interpretation:

  • The horizontal axis shows the number of cycles to failure (on a logarithmic scale).
  • The vertical axis shows the stress amplitude.
  • As the number of cycles increases, the allowable stress decreases.
  • Some materials (like steel) show a fatigue limit or endurance limit, below which fatigue failure does not occur even after infinite cycles.

For example, mild steel has a fatigue limit around half of its ultimate tensile strength, while non-ferrous metals (like aluminum or copper) do not have a clear fatigue limit.

Prevention of Fatigue Failure

  1. Avoid Stress Concentration:
    • Design smooth curves and fillets instead of sharp corners.
  2. Improve Surface Finish:
    • Polishing, shot peening, and case hardening reduce surface roughness and increase fatigue strength.
  3. Use Suitable Materials:
    • Use materials with high fatigue resistance or apply surface treatments like carburizing and nitriding.
  4. Control Operating Stresses:
    • Reduce load fluctuations and avoid overloading during service.
  5. Protect from Corrosion:
    • Use coatings, paints, or lubricants to prevent corrosion fatigue.
  6. Proper Maintenance:
    • Regular inspection helps detect small cracks before they lead to complete failure.

Examples of Fatigue Failure

  • Aircraft wings: Continuous cyclic loading during flight leads to fatigue damage.
  • Crankshafts and connecting rods: Repeated stresses during engine operation cause fatigue cracks.
  • Bridges and railway axles: Experience cyclic loads due to vehicle movement.
  • Springs and gears: Fail due to repetitive stress reversals.
  • Turbine blades: Undergo cyclic thermal and mechanical loading, causing fatigue over time.

Importance of Fatigue Study in Engineering

  • Helps predict the service life of components under cyclic loading.
  • Ensures safety and reliability of critical structures and machines.
  • Aids in material selection and design optimization to prevent premature failure.
  • Reduces maintenance costs by avoiding unexpected breakdowns.

Fatigue analysis is essential in industries like automotive, aerospace, railways, and power generation, where components experience millions of stress cycles during operation.

Example Calculation:

If a steel component is subjected to a cyclic stress of 250 MPa and fails after 10⁶ cycles, reducing the stress to 150 MPa might increase its life to over 10⁸ cycles, showing the sensitive relationship between stress and fatigue life.

Conclusion:

Fatigue is the progressive failure of materials under repeated cyclic loading. It begins with microscopic crack initiation, followed by crack propagation, and ends with sudden fracture. Fatigue strength depends on stress levels, material properties, surface finish, and environmental conditions. Since fatigue failure occurs without warning, engineers must carefully design, test, and maintain components to ensure safe operation. Understanding fatigue behavior is vital in designing durable, long-lasting mechanical and structural systems used in real-world applications.