(a) Fatigue in Crankshafts

Fatigue failure occurs when a crankshaft section is subjected to cyclic stress reversals. Over time, the material's properties deteriorate, weakening its ability to withstand tensile stress, leading to crack initiation. With continued operation and stress reversals, this crack progressively grows until complete failure occurs.

• Smaller crankshafts, such as those in 4-stroke medium-speed engines, are typically solid forged, with a continuous grain line structure. This enhances fatigue resistance and overall strength.
• Larger crankshafts, as used in 2-stroke slow-speed crosshead engines, are often semi-built or fully built. These are fabricated from separate forged crank throws, with main journals shrink-fitted into machined holes in the webs. While the crank throw may have continuous grain flow, the shrink-fit areas are potential weak spots.
• The webs are in tension, and crankpins are subjected to bending, especially when bearing heights vary. Although designed to operate within the fatigue limit, fatigue cracks can initiate from flaws such as slag inclusions, which are more likely in larger forgings due to the volume of material involved.

1. Eliminating or reducing stress raisers:
   • Avoid sharp corners and abrupt changes in cross-section, which concentrate stress.
2. Preventing surface damage:
   • Avoid surface tears from punching, stamping, or improper machining.
   • Ensure smooth surface finishes to prevent irregularities that could initiate cracks.
3. Maintaining correct shrink-fit allowances:
   • Ensures proper stress distribution and prevents relative movement that may lead to fatigue.
4. Heat treatment:
   • Used to eliminate tensile residual stresses induced during manufacturing, reducing the risk of crack initiation.

1. Initial Stage – Crack Initiation:
   • Difficult to detect visually.
   • Cracks often begin at pin-to-web transitions or around shrink-fit areas.
   • Crack detection techniques (such as dye penetrant, magnetic particle, or ultrasonic testing) are required.
2. Intermediate Stage – Progressive Crack Growth:
   • The crack continues to grow with each stress cycle.
   • Some surface features may become visible during close inspection.
   • The crack path is typically along stress concentration zones.
3. Final Stage – Sudden Fracture:
   • Rapid failure of the remaining cross-section.
   • The fracture surface displays two distinct regions:
     • A smooth, polished area with curved beach marks, showing the progressive crack growth.
     • A rough, grainy region indicating the final brittle fracture, usually at an angle to the original surface.