Which statement best defines the fatigue limit?

• A. The maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.
• B. The stress level at which failure will occur for some specified number of cycles.
• C. The minimum stress amplitude that causes fatigue cracks to initiate.
• D. The average of the yield strength and the ultimate tensile strength.

Answer: (A)

The fatigue limit is the stress amplitude threshold below which a material can endure essentially infinite cycles without failing. In practice, this means that if you cyclically stress the material at or below this level, cracks do not propagate to cause failure, even as the number of cycles becomes very large. This creates a distinct region on the S-N curve where life is effectively infinite for that amplitude, which is why this statement best defines fatigue limit.

If you stress above this limit, the material will eventually fail after a finite number of cycles, and the higher the amplitude, the fewer cycles to failure. Some materials don’t have a true fatigue limit and don’t exhibit a horizontal plateau on the S-N curve, but the given definition captures the standard concept for materials that do have a fatigue limit. The other statements describe failure at a specified finite life, initiation thresholds, or unrelated material properties, which do not define the fatigue limit.