- The fatigue life of metallic materials refers to the number of cycles a material can withstand before failure under repeated or fluctuating stress.
- Among the factors listed, surface roughness has the greatest impact on fatigue life. This is because surface irregularities such as scratches, notches, or machining marks act as stress concentrators, where cracks can easily initiate and propagate under cyclic loading.

- While grain size influences mechanical properties like strength and toughness, its effect on fatigue life is generally less pronounced than that of surface condition.
- Similarly, corrosion resistance affects fatigue performance primarily when the environment promotes corrosion fatigue, but in the absence of corrosive conditions, its impact is lower.
- Thermal conductivity relates to heat transfer in the material and does not directly affect fatigue behavior.

Therefore, improving surface finish by polishing or surface treatments effectively enhances fatigue resistance, making surface roughness the most critical factor.