Fatigue Life is defined as?

• A. the rate of crack growth per cycle
• B. the time to corrosion under cyclic loading
• C. the maximum load during cyclic loading
• D. the number of load cycles required to initiate and propagate to a critical size

Answer: (D)

Fatigue life is the total number of load cycles a component can endure before it fails, accounting for both the initiation of a crack and its growth until the crack reaches a critical size that leads to fracture. In practice, after repeated loading a small flaw forms and then grows with each cycle; the sum of cycles needed for that crack to reach a size that can cause sudden failure defines the component’s fatigue life. This is different from the rate of crack growth per cycle, which describes how fast an existing crack widens (not the total cycles to failure). It’s also not simply the maximum load in a cycle or the time to corrosion under cyclic loading. Understanding fatigue life helps engineers use S-N curves to estimate how many cycles a part can survive under a given stress range and environment.