Shafts are fundamental components in rotating and reciprocating machinery, transmitting torque, supporting loads, and ensuring precise alignment. Fatigue failure is one of the most common causes of shaft breakdown, often initiating at the surface due to repeated stress cycles. Surface treatments, such as nitriding and carburizing, are widely applied to enhance fatigue life by improving hardness, wear resistance, and residual stress profiles. 1. Understanding Shaft Fatigue Fatigue in shafts is caused by cyclic stresses that lead to the initiation and propagation of cracks, often at stress concentrators such as keyways, fillets, or surface defects. Surface properties significantly influence the fatigue limit because the first cracks usually appear at the outermost