Which of the following methods can improve the fatigue strength of a circular mild steel(MS) shaft?

The correct methods that can improve the fatigue strength of a circular mild steel (MS) shaft are:
1. Enhancing surface finish
2. Shot peening of the shaft
3. Reducing relative humidity

Let us break down the engineering and physical principles behind why each of these methods improves the fatigue strength of a mild steel shaft:

1. Enhancing Surface Finish:
Fatigue failures almost always initiate from surface defects, micro-cracks, or irregularities where localized stress concentrations occur. A rough surface contains microscopic notches that act as stress raisers under cyclic loading. By enhancing the surface finish (e.g., through polishing, grinding, or superfinishing), these micro-notches are minimized or removed. This reduces the stress concentration factor at the surface, thereby delaying crack initiation and significantly increasing the fatigue strength of the shaft.

2. Shot Peening of the Shaft:
Shot peening is a cold working process where the surface of the shaft is bombarded with small spherical media (shot). Every impact creates a tiny indentation, causing the surface layer of the metal to yield plastically in tension. As the surrounding elastic material tries to restore the surface to its original shape, a high-magnitude compressive residual stress layer is induced at the surface. Since fatigue cracks propagate under tensile stresses, this protective compressive stress layer must first be overcome by external cyclic tensile loads, thereby increasing the resistance of the shaft to both crack initiation and propagation.

3. Reducing Relative Humidity:
Fatigue strength is greatly reduced in corrosive environments, a phenomenon known as corrosion fatigue. High relative humidity supplies water vapor and moisture that react with the mild steel surface, promoting oxidation and rust formation. These corrosive pits act as severe stress concentrators that accelerate crack nucleation. By reducing the relative humidity of the surrounding environment, the rate of atmospheric corrosion is minimized, which effectively preserves the fatigue strength of the material compared to a humid, corrosive setting.