material fatigue

Fatigue is a material damage caused by repeated loading for a long time. If a metal is subjected to repeated loads (stress or strain), the metal will fracture. Damage due to repeated loads is called fatigue failures, generally this occurs after the use of the material for a long time. The damage occurs without warning, suddenly, and completely. More than 90% of the causes of mechanical failure are caused by fatigue fracture.

Phases in Fatigue Fracture:

  1. Initiation crack
  2. Crack propagation
  3. Fracture failure
Fatigue fracture phases. Source: PT. Hesa Laras Brilliant

In general, the process of crack initiation occurs on the surface of the weak material or areas where there is a concentration of stress on the surface, such as scratches, notches, holes, etc., due to repeated loading. Furthermore, the beginning of these cracks develops into microcracks, the propagation or combination of these microcracks then forms macrocracks which will lead to failure. After that, the material will experience a final fracture, because the material has undergone a stress and strain cycle that results in permanent damage.

Basically, fatigue failure begins with the occurrence of cracks on the surface of the material. It proves that fatigue properties are very sensitive to surface conditions, which are influenced by several factors, including surface roughness, changes in surface properties, and surface residual stress. Therefore, the endurance limit is highly depends on the quality of the surface finish. Surface treatment may change the surface condition and residual stress on the surface. Surface treatment that produces compressive residual stress will result in increased fatigue resistance, while surface treatment that produces tensile residual stress will decrease its fatigue resistance.

At the surface of the material the highest concentration of compressive or tensile stress occurs. If the surface conditions are receiving tensile stresses, the residual compressive stress on the surface will result in a greater resultant compressive stress. The compressive stress will inhibit the initiation of crack, so that the fatigue resistance will increase, and the opposite will happen if there is residual tensile stress on the surface. The initial location of cracks in components or metals that are subjected to dynamic or cyclic loading is at the point of the region that has the minimum strength and or the point of the region experiencing the maximum stress.

Failure of components or structures can be divided into two main categories. First, quasi-static failure (failure that does not depend on time, and resistance to failure is expressed by strength). Second, time-dependent failure (resistance to failure is expressed by age or life time). Metal fatigue (fatigue fracture) is included in the time-dependent failure.

Factors Affecting Metal Fatigue

  1. Loading
    • Load type: uniaxial, bending, torsion
    • Load pattern: periodic, random
    • Load amount
    • Load cycle frequency
  2. Material condition (grain size, strength, solid solution reinforcement, second phase reinforcement, strain reinforcement, microstructure, surface finish), component size).
  3. Working process (casting process, forming process, welding process, machining process, heat treatment process)
  4. Operating temperature
  5. Environmental conditions
Fatigue cracked surface. Source: Ewing & Humfrey (1903)

Contributor: Feri Wijarnako

By Caesar Wiratama

aeroengineering services is a service under CV. Markom with solutions especially CFD/FEA.

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