• Visual inspection: The crack surface will have a smooth, polished finish, while the remaining material will show a granular texture.
• Crack pattern: The fatigue crack surface will display a series of curved visible lines, which are a result of the cyclical loading and stress.
• Non-Destructive Testing (NDT): Techniques such as Dye-Penetrant Testing or Magnetic Particle Testing are commonly used to identify cracks in the material.

Stage I: Initial Crack Initiation:

• The first crack forms at a point of high stress, usually around sharp corners, notches, or surface defects. This is the stage where microscopic cracks begin to form due to repeated loading.

Stage II: Progressive Crack Growth:

• The initial crack propagates slowly under cyclic loading. This stage is characterized by relatively slow, stable crack growth. The crack propagates most rapidly in a direction perpendicular to the main tensile stress.

Stage III: Final Fracture:

• Once the crack has grown to a certain size, the remaining material can no longer withstand the applied stress. The crack grows rapidly, leading to a catastrophic failure of the component. This is the final stage of fatigue failure, often happening suddenly.

• The crankshaft should be made from a material with high fatigue strength, as opposed to high ultimate tensile strength (UTS). Materials with higher fatigue strength are better able to resist the initiation of cracks.
• Forging the crankpin and webs from a single piece of material ensures a continuous grain flow, enhancing strength and reducing stress concentrations. The forging process itself also helps to consolidate material, reducing the number of internal defects.
• Cold rolling fillets (radii) at stress concentration points reduces stress concentration by removing sharp corners and inducing compressive residual stresses. This smoothing improves the fatigue resistance.
• Shot Peening/Laser Peening treatments introduce compressive residual stresses near the surface, thereby offsetting the tensile stresses during operation and making crack initiation more difficult. Laser peening imparts a deeper compressive layer compared to shot peening.
• Increased web thickness improves the component's ability to accommodate tensile stresses, reducing the likelihood of fatigue crack initiation.
• The High-Frequency Mechanical Impact Treatment (HFMIT) method is particularly effective for welded surfaces, improving their fatigue resistance.