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.

• Cylinder overload due to excess power puts excessive stress on the girders.
• Incorrect crankshaft alignment induces uneven loading and stress concentrations.
• Material defects, high residual stresses in welds, heat-affected zone hardening, and the presence of dissolved oxygen all reduce fatigue resistance.
• Tank top deformation from pressurisation or overheating adds stress to the bedplate.

To minimise the risk of fatigue cracking:

(i) Constructional strength:

• Bed plates are made up of M.S. plates with four steel casting, which are assembled and welded together so that the bed plate is strong longitudinally & transversely with good resistance to twisting along its length.
• Longitudinal strength is obtained by fabricating each side of the bed plate in the form of a box girder.
• The cast steel cross girder in which the main bearing is placed contributes to the bed plate's transverse strength and resistance against twisting along its length.
• Resin cast chocks are used between the bedplate and the double bottom tank top to absorb the shocks & stress.

(ii) Maintenance:

• Monthly checks on the bolt tension.
• Monthly checks on engine load using power cards & measuring cylinder peak pressure.
• Regular checking of tension for main bearing jack bolts as recommended by engine manufacturers.
• Regular checks on crankshaft alignment by taking deflection & compare with recommended value.
• By maintaining engine operations at specified load, temperature, pressure, speed, etc.