Which fatigue concept is associated with infinite fatigue life?

• A. Fatigue II
• B. Fatigue limit state
• C. Fatigue I
• D. Fracture limit state

Answer: (C)

The key idea is that some materials can endure an unlimited number of loading cycles if the stress range is kept at or below a certain threshold. This threshold, known as the endurance limit, marks a regime where damage per cycle is effectively zero, so the life can be infinite. The concept that best captures this situation is the one described as Fatigue I, which embodies the condition of infinite fatigue life under sub-threshold stress. In other words, when you stay within that limit, the material does not accumulate enough damage to fail no matter how many cycles occur.

The other notions you see in fatigue analysis refer to different ideas, such as states or boundaries related to failure or finite life, but they don’t describe the indefinite life scenario unless the stress is kept below the endurance limit. So, recognizing Fatigue I as the concept tied to infinite fatigue life helps you connect the idea of an endurance limit to the behavior of materials under cyclic loading.