Which statement correctly defines fatigue strength?

• 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 energy required to propagate a crack.
• D. The average of the yield strength and the ultimate tensile strength.

Answer: (B)

Fatigue strength is the stress level that will cause failure after a specified number of cycles under cyclic loading. In practice, you determine it from an S-N curve: for a given life target, the fatigue strength is the stress at which the material will fail after that many cycles. This makes the concept precise for most materials, since many do not have a true infinite-life limit. The idea of an infinite life at low stress is a special case for some materials, but it isn’t universal, so the general definition uses a defined number of cycles. The energy required to propagate a crack relates to fracture mechanics, not fatigue strength, and the average of yield and ultimate tensile strength has no direct bearing on fatigue behavior.