Define fatigue.

Short Answer:

Fatigue is the progressive and localized structural damage that occurs in a material when it is subjected to repeated or fluctuating stresses over time. These stresses are usually below the material’s ultimate tensile strength and can lead to failure after many cycles of loading and unloading.

In simple terms, fatigue is the weakening or cracking of a material due to cyclic loading. It often starts as small cracks on the surface, which grow gradually until the material finally breaks. Fatigue failure is sudden, without significant warning, and is one of the main causes of failure in mechanical components like shafts, springs, gears, and aircraft structures.

Detailed Explanation:

Fatigue

Fatigue is one of the most important failure mechanisms in mechanical engineering. It refers to the failure of a material under repeated or cyclic stresses, even when those stresses are much lower than the material’s yield or ultimate strength. Unlike static failure, which occurs under a single large load, fatigue failure happens due to repeated loading and unloading over a long period of time.

During fatigue, small cracks form at points of high stress concentration such as notches, holes, or sharp corners. As the loading continues, these cracks grow slowly and eventually lead to complete fracture of the material. The fracture usually occurs suddenly, often without noticeable deformation, making fatigue failure dangerous and unexpected.

Fatigue is a time-dependent and stress-cycle-dependent phenomenon that plays a critical role in the design of components that experience fluctuating loads, such as automobile parts, bridges, turbines, and airplane wings.

Stages of Fatigue Failure

Fatigue failure occurs in three distinct stages, each involving different mechanisms of material behavior:

  1. Crack Initiation:
    • In this stage, tiny microscopic cracks form on the surface of the material due to cyclic stress.
    • These cracks generally start at points where stress concentration is high, such as sharp edges, surface scratches, corrosion pits, or defects in the material.
    • The number of cycles needed to initiate the crack depends on the stress level, material properties, and surface condition.
  2. Crack Propagation (Growth):
    • Once the crack is initiated, it grows gradually with each loading cycle.
    • The crack propagates perpendicular to the direction of the applied stress.
    • This stage covers most of the fatigue life of a component, and the rate of crack growth increases with higher stress levels.
    • The surface often shows distinct striations (marks) that represent the crack movement with each cycle.
  3. Final Fracture:
    • The final stage occurs when the crack grows large enough that the remaining material cross-section cannot support the applied load.
    • This results in a sudden and complete fracture of the component.
    • The fracture surface typically shows two regions: a smooth region (slow crack growth) and a rough region (sudden final fracture).

Characteristics of Fatigue Failure

  • Fatigue failure occurs gradually but ends with sudden fracture.
  • The fracture surface is usually smooth and flat, showing beach marks or striations.
  • It occurs under repeated or cyclic loading rather than static load.
  • The failure often originates from surface defects or stress concentration points.
  • Fatigue cracks propagate in a direction perpendicular to the maximum tensile stress.

Factors Affecting Fatigue

  1. Magnitude of Stress:
    • Higher alternating or fluctuating stress levels shorten the fatigue life of a material.
  2. Mean Stress:
    • The average value of stress during the cycle affects fatigue strength; tensile mean stress reduces fatigue life, while compressive mean stress increases it.
  3. Surface Finish:
    • Rough or scratched surfaces initiate cracks more easily; polished surfaces improve fatigue strength.
  4. Temperature:
    • High temperatures can reduce fatigue resistance by softening the material.
  5. Environment:
    • Corrosive environments accelerate crack initiation and propagation (known as corrosion fatigue).
  6. Material Properties:
    • Ductile materials generally resist fatigue better than brittle materials.
  7. Stress Concentration:
    • Holes, notches, keyways, and sharp corners act as stress raisers and reduce fatigue life.

Fatigue Testing

To study fatigue behavior, materials are tested under repeated loading conditions using machines such as a rotating bending machine or axial fatigue testing machine.

The results are represented graphically by an S–N Curve (Wöhler Curve), where:

  • S = Stress amplitude (applied cyclic stress)
  • N = Number of cycles to failure

This curve shows the relationship between the stress applied and the number of cycles a material can endure before failure.

For many ferrous materials like steel, there exists a fatigue limit or endurance limit — a stress level below which the material can theoretically endure an infinite number of cycles without failure. However, non-ferrous materials like aluminum do not have a distinct fatigue limit; they eventually fail after sufficient cycles.

Improving Fatigue Strength

To increase the fatigue life of components, several design and manufacturing techniques can be used:

  1. Polishing and Surface Finishing:
    Removes surface irregularities that cause stress concentration.
  2. Shot Peening:
    Introduces compressive residual stresses on the surface, preventing crack initiation.
  3. Case Hardening:
    Hardens the surface layer, improving resistance to fatigue.
  4. Avoid Sharp Corners:
    Using fillets or rounded corners helps distribute stress uniformly.
  5. Reduce Mean Stress:
    By proper design, the mean stress can be reduced to improve fatigue life.
  6. Protective Coatings:
    Prevent corrosion and surface oxidation that accelerate fatigue failure.

Applications of Fatigue Concept

Fatigue analysis is essential in designing components that undergo repeated loading cycles, such as:

  • Shafts, gears, springs, connecting rods, and crankshafts in engines.
  • Aircraft wings, fuselage, and landing gears.
  • Bridges and rotating machine parts.
  • Turbine blades and railway axles.

Understanding fatigue ensures these components remain reliable and safe throughout their service life.

Conclusion

Fatigue is a type of material failure that occurs due to repeated cyclic stresses over time, even when the stresses are below the yield strength of the material. It involves three stages — crack initiation, crack propagation, and final fracture. Fatigue is dangerous because it occurs without visible warning and can cause sudden failure. By improving surface quality, using stronger materials, and applying compressive surface stresses (like shot peening), fatigue life can be significantly increased. Hence, understanding fatigue is crucial for designing safe and long-lasting mechanical components.