(a) Fatigue is the process leading to localized structural damage in materials subjected to cyclic loading.

• Fatigue failure can occur even when stress levels are below the ultimate tensile strength of the material.
• It primarily results from repetitive cyclic loads, causing microscopic crack formation and growth over time until the component fails.

Contributing factors include:

• High cyclic loading
• Vibrations
• Material or manufacturing defects
• Corrosion
• Improper handling and machinery operation

(b) Difference between high stress/low cycle and low stress/high cycle fatigue

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.

• Defects in metal concentrate stress locally, raising stress levels in those areas and leading to early failure.

Types of Defects That Reduce Fatigue Strength:

• Surface roughness, porosity, inclusions, abrupt section changes, and manufacturing flaws act as stress risers.
• Residual stress due to coarse grain structure, improper chemical composition, and cold working.
• Environmental factors like corrosion and erosion.
• Faulty assembly or workmanship can introduce additional stress, decreasing component life.

Fuel Timing Faults:

• Pre-ignition creates excessive peak pressures, potentially causing cracks in bed plates, especially around bearing pockets.
• Delayed injection can lead to after-burning and scavenging fires, loosening tie bolts and causing fatigue cracks.

Cylinder Power Balance:

• Proper fuel injection timing and balanced cylinder power minimize vibration.
• Any imbalance in timing or power can increase vibration, transmitting stress to bed plates and causing fatigue cracks.