Which statement best defines material fatigue and how an S–N curve assists in designing for life expectancy?

Which statement best defines material fatigue and how an S–N curve assists in designing for life expectancy?

• A. Fatigue is failure under repeated cyclic loading at stresses below the static strength; an S‑N curve plots stress amplitude against the number of cycles to failure.
• B. Fatigue is failure under a single overload.
• C. Fatigue cannot be predicted; S‑N curves are irrelevant.
• D. Fatigue is corrosion under load.

Answer: (A)

Fatigue is failure that happens after many cycles of loading, even when the peak stresses are below the material’s static strength. An S–N curve, also called a Wöhler curve, shows how the stress amplitude relates to the number of cycles to failure for a given material. This curve lets designers predict how long a component will last under a given cyclic loading by mapping the expected stress range to the cycles you can expect before failure. It also guides decisions about safe operating limits, required safety factors, and maintenance intervals, since lower stress or more cycles generally extends life and some materials show a fatigue limit where infinite life is possible at low enough stresses. Fatigue is not a single overload failure, and it can be predicted using S–N curves; fatigue is distinct from corrosion, though environmental factors can influence fatigue behavior.

Fatigue is failure that happens after many cycles of loading, even when the peak stresses are below the material’s static strength. An S–N curve, also called a Wöhler curve, shows how the stress amplitude relates to the number of cycles to failure for a given material. This curve lets designers predict how long a component will last under a given cyclic loading by mapping the expected stress range to the cycles you can expect before failure. It also guides decisions about safe operating limits, required safety factors, and maintenance intervals, since lower stress or more cycles generally extends life and some materials show a fatigue limit where infinite life is possible at low enough stresses. Fatigue is not a single overload failure, and it can be predicted using S–N curves; fatigue is distinct from corrosion, though environmental factors can influence fatigue behavior.