What is the primary controlling factor of fatigue life?

What is the primary controlling factor of fatigue life?

• A. Displacement amplitude
• B. Material yield strength
• C. Stress (range)
• D. Ambient temperature

Answer: (C)

The most important factor for fatigue life is the stress range—the amplitude of the cyclic stress experienced by the member. under repeated loading, the number of cycles a component can endure before crack initiation and eventual failure scales with how large the alternating stress is from cycle to cycle. This is why S-N curves show that increasing the stress range dramatically reduces life, while lower ranges extend life. Mean stress, temperature, and static yield strength can modify the curve or influence crack growth rates, but they do not control fatigue life as directly as the stress range does. In practice, designers and inspectors focus on areas where high fluctuating stresses occur, because those are the conditions that primarily govern how long a structure will last under cyclic loading.

The most important factor for fatigue life is the stress range—the amplitude of the cyclic stress experienced by the member. under repeated loading, the number of cycles a component can endure before crack initiation and eventual failure scales with how large the alternating stress is from cycle to cycle. This is why S-N curves show that increasing the stress range dramatically reduces life, while lower ranges extend life. Mean stress, temperature, and static yield strength can modify the curve or influence crack growth rates, but they do not control fatigue life as directly as the stress range does. In practice, designers and inspectors focus on areas where high fluctuating stresses occur, because those are the conditions that primarily govern how long a structure will last under cyclic loading.