ANS:B - Grain coarsening

Grain coarsening cannot increase the fatigue strength of a material. Fatigue strength is the ability of a material to withstand repeated loading and unloading cycles without failure. Grain coarsening refers to the phenomenon where the grain size of a material increases, typically due to high-temperature exposure or mechanical deformation. Coarse grains are generally associated with lower strength and toughness compared to fine grains. Therefore, grain coarsening can decrease the fatigue strength of a material by reducing its resistance to fatigue crack initiation and propagation. The other options listed—grain refining, surface hardening, and shot peening—can increase the fatigue strength of a material through various mechanisms such as refining the microstructure, inducing compressive stresses on the surface, and introducing residual compressive stresses, respectively.