Fatigue is progressive and localised structural damage caused by cyclic loading, where the maximum stress is below the ultimate tensile strength. The relationship between stress level, cyclic frequency, and operating life depends on whether the fatigue is high-cycle/low-stress or low-cycle/high-stress.

High-cycle fatigue (low stress-high cycle):

• This occurs at lower stress levels over a high number of cycles, resulting in elastic deformation. The component can withstand more cycles at these lower stress levels, and its life expectancy is determined by the S-N curve, which predicts the number of cycles before failure at a given stress level. For example, fatigue in turbocharger blowers often results from prolonged vibration over numerous cycles.

Low-cycle fatigue (high stress-low cycle):

• This occurs at high-stress levels over fewer cycles, causing plastic deformation in the material. This type of fatigue is typically assessed by a strain curve. If the stress level increases, the component's operating life decreases, as higher stress accelerates the onset of failure. For example, air receivers filling automatically face high stress and experience fewer cycles before failure.

If stress levels or the number of cycles increase beyond the material’s capacity, failure will occur sooner. It is important to keep stress levels within allowable limits for extended component life.

• Surface roughness, porosity, inclusions, and abrupt section changes all create stress concentrations, lowering fatigue strength.
• Coarse grain size, specific chemical compositions, and cold working introduce residual stresses that reduce fatigue resistance.
• Corrosion, erosion, and decarbonisation weaken the material and accelerate fatigue crack initiation and propagation.
• Faulty workmanship during assembly or processing introduces defects that may significantly shorten the component's life.