Fatigue strength refers to the ability of a material to resist the initiation and propagation of cracks or fractures under cyclic loading conditions. It is an important property to consider in engineering design, as many components and structures are subjected to repeated loading over their lifetime.

Temperature: Least Affected
Temperature has a significant impact on the mechanical properties of materials, including fatigue strength. At elevated temperatures, the material's strength and hardness decrease, leading to reduced fatigue strength. Similarly, at low temperatures, the material becomes more brittle, making it more susceptible to fatigue failure. Therefore, fatigue strength is affected by temperature.

Stress Concentration: Least Affected
Stress concentration refers to the localized increase in stress around a geometric feature such as a notch, hole, or fillet. These stress concentrations can significantly reduce the fatigue strength of a material, as they create areas of high stress concentration where cracks are more likely to initiate and propagate. Therefore, stress concentration has a significant effect on fatigue strength.

Frequency: Least Affected
The frequency of loading refers to the number of cycles or repetitions of stress applied to a material within a given time period. It is commonly expressed as the number of cycles per second (Hz). The frequency of loading has a minimal effect on fatigue strength. Fatigue failure is primarily governed by the stress amplitude and number of cycles, rather than the frequency of loading. As long as the stress amplitude and number of cycles remain constant, the fatigue strength will not be significantly affected by the frequency of loading.

Conclusion
In summary, fatigue strength is least affected by frequency, as long as the stress amplitude and number of cycles remain constant. On the other hand, both temperature and stress concentration have a significant impact on fatigue strength. Elevated temperatures decrease the material's strength and hardness, while stress concentrations create areas of high stress concentration where cracks are more likely to initiate and propagate.