Fatigue is one of the main causes of crankshaft failure.

[Sketch notes: The most likely location of a fatigue crack in a crankshaft is at the fillet (the radius) between the crank web and the journal (the crankpin or the main journal), where the stress concentration is highest. The crack typically starts at the fillet and propagates into the web.]

The fatigue crack most commonly initiates at the fillet radius between the crank web and the journal (crankpin or main journal), where the bending and torsional stresses are concentrated. It propagates from the fillet into the web, eventually causing failure.

A fatigue failure is identified by:

1. The characteristic appearance of the fracture surface: a fatigue crack has a smooth, polished "beach mark" (clam shell) region where the crack has propagated slowly, and a rough, crystalline "final fracture" region where the remaining material failed suddenly.
2. The location of the crack (at the fillet, where stress concentration is highest).
3. The absence of gross plastic deformation (fatigue is a brittle-type failure).
4. The history: the crankshaft has been subject to cyclic loading (bending and torsion) over a long period.
5. Detection by non-destructive testing (e.g. magnetic particle inspection, ultrasonic testing) which reveals the crack before it causes failure.

A fatigue crack is initiated by:

1. A stress concentration: a notch, a sharp fillet, a machining mark, a keyway, or a corrosion pit at the surface concentrates the stress.
2. Cyclic loading: the crankshaft is subject to repeated bending and torsional stresses (from the gas pressure and the inertia forces), which cause the material at the stress concentration to yield locally and form a micro-crack.
3. The micro-crack propagates with each cycle (fatigue crack growth) until it reaches a critical size and the remaining material fails suddenly.
4. Corrosion or fretting can accelerate the initiation by creating pits or surface damage.

[Sketch notes: The methods include: (1) a generous fillet radius at the web-journal junction to reduce the stress concentration; (2) surface hardening (induction hardening, shot peening) of the fillet to create compressive residual stress; (3) polishing the fillet to remove machining marks; (4) avoiding sharp notches/keyways.]

The methods used to inhibit fatigue cracks are:

1. A generous fillet radius at the web-journal junction to reduce the stress concentration.
2. Surface hardening of the fillet (induction hardening, nitriding, or shot peening) to create a compressive residual stress in the surface, which resists the tensile stress that drives the crack.
3. Polishing the fillet to remove machining marks and notches.
4. Avoiding sharp notches, keyways, and abrupt changes of section.
5. Correct design (adequate section size) and correct alignment to avoid excessive bending stress.
6. Regular inspection (NDT) to detect any crack early.

These measures prevent the initiation and propagation of fatigue cracks, extending the crankshaft life.