In general, boundary condition loads, at a level, do not cause any damage when applied once, but when repeatedly applied, damage occurs in the form of cracks or fractures. In other words, fatigue is the changes that occur in the internal structure of the material under variable stresses.
For more detail about fatigue: What is Fatigue
As it is known, fatigue is a structural damage of materials under cyclic loads. There are a few main types of fatigue.
- Mechanical Fatigue
- Creep Fatigue
- Thermal Fatigue
- Thermo-Mechanical Fatigue
- Corrosion Fatigue
- Fretting Fatigue
Mechanical fatigue, which is the topic of this article, could be described as damage and fracture caused of cyclic loading with fluctuating stresses and strains by boundary conditions.
Mechanical Fatigue Failure
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Fatigue life is an important characteristic of an engineering component and, it is measured by a number of cycles that can oppose before fatigue failure occurs. Mechanical fatigue can be divided into headings as follows:
- Monotonic Fracture Behaviour
- Extremely Low Cycle Fatigue / Ultra Low Cycle Fatigue
- Low Cycle Fatigue
- High Cycle Fatigue
- Very High Cycle Fatigue / Ultra High Cycle Fatigue
Monotonic Fracture Behaviour
The Monotonic Fracture behaviour means action under an applied stress that increases in one direction without cyclic loading. The difference between monotonic and cyclic load is, the monotonic load in only one direction, causing to failure to initiate on the load side. Monotonic Fracture behaviour is mainly in the early crack propagation stage and it is more likely to exhibit multiple crack nucleation sites.
Extremely Low Cycle Fatigue
In Extremely Low Cycle Fatigue, large plastic strains and plastic response of the material lead to ductile failure in a very low number of cycles, usually less than hundreds.
Extremely Low Cycle Fatigue can be found during earthquakes, for example, where buildings are subject to an intense repeated loading similar causing large plastic strain cycles.
Low Cycle Fatigue
Low Cycle Fatigue model is a type of fatigue that undergoes large plastic strains under a low number of load cycles before failure occurs.
Low Cycle Fatigue model is exposed at 104 cycles and below. In Low Cycle Fatigue life, maximum tensile stress and the strain amplitude are useful to predict the life of the material by plastic strain amplitude or stress amplitude. The stress level is usually into plastic range and, these large plastic strains cause a shorter life.
High Cycle Fatigue
High Cycle Fatigue model is a type of fatigue that is subject of elastic deformation under a high number of load cycles before failure occurs. High Cycle Fatigue deals with low strain and long-term cyclic loads. The material is exposed to elastic range of deformation by loads, which are mostly under yield stress.
High Cycle Fatigue requires a high number of loading cycles to reach fatigue failure mainly due to elastic deformation and the fatigue occurs above 104 load cycles. HCF has got lower stresses compare to LCF.
Very High Cycle Fatigue
Very High Cycle Fatigue failure life of structures such as wheel, gear, blade and so on is expected to be beyond 107 cycles (mostly between 108 and 1010 cycle). The Very High Cycle Fatigue life prediction and damage assessment methods in structural materials is a concern in the fatigue field.
The ELCF method is between the MF and LCF methods and controlled by the ductility and cyclic strain. The fatigue life of ELCF is under 100 cycles.
The LCF mode is strain controlled and related to large strain amplitudes or high stress levels that produce both plastic and elastic strain on components at each cycle, where fatigue life is ≤104 cycles. The HCF mode is stress controlled and associated with relatively low stress levels in elastic range. In this mode, the fatigue life is >104 cycles. In VHCF method, over 108 cyclic loads are observed for failure in material.
The real difference between the High Cycle Fatigue and Low Cycle Fatigue is the strain levels in the material. Low Cycle Fatigue is characterized by repeated plastic deformation, on the other hand High Cycle Fatigue is characterized by elastic deformation. If the Stress & Strain levels are well below the yield, the High Cycle Fatigue (stress based) could be used. Otherwise, the Low Cycle Fatigue (strain based) should be used.