Machinery components such as crankshafts, propeller shafts, connecting rods, gears, and other rotating parts are subjected to repeated or cyclic loading during normal operation. Even when the applied stress is below the material's yield strength, repeated loading can cause fatigue failure. Fatigue failure occurs due to the initiation and gradual propagation of small cracks until the component fractures suddenly.

1. Crack Initiation

Small imperfections are always present in engineering materials. These may include:

• Machining marks
• Corrosion pits
• Material inclusions
• Weak welds
• Manufacturing defects
• Sharp changes in section or poor design features

These imperfections act as stress concentration points. During repeated cyclic loading, the local stress at these points becomes much higher than the average stress, resulting in the formation of a small fatigue crack.

2. Crack Propagation

Once initiated, the crack opens and closes during every loading cycle.

• The crack tip experiences very high stress concentration.
• Each stress cycle extends the crack by a very small amount.
• Initially, crack growth is slow.
• As the crack becomes longer, the stress intensity at the crack tip increases, causing the crack to grow more rapidly.
• Corrosion can further accelerate crack growth by making the material brittle and more susceptible to fatigue.

3. Reduction of Load-Carrying Area

As the crack propagates, the effective cross-sectional area of the component decreases.

• The remaining uncracked section has to carry the entire operating load.
• This increases the stress on the remaining material.
• The higher stress further accelerates crack growth, creating a continuous cycle of weakening.

4. Critical Crack Size and Final Failure

Eventually, the crack reaches a critical size, where the remaining cross-section is no longer able to withstand the applied load.

At this stage:

• Rapid crack propagation occurs.
• The remaining section fractures suddenly without warning.
• The component fails completely, often causing serious machinery damage.

Factors Causing Fatigue Cracking

Fatigue cracking may be initiated or accelerated by:

• Mechanical fatigue due to repeated cyclic loading.
• Poor material selection.
• Manufacturing defects.
• Weak welds.
• Design flaws causing stress concentration.
• Corrosion and oxidation, which make the material brittle and increase susceptibility to fatigue cracking.