In fatigue evaluation, which statement best describes the effect of higher stress range?

• A. Accelerates crack initiation only
• B. Accelerates both crack initiation and crack growth
• C. Reduces crack growth
• D. Has no effect

Answer: (B)

Higher stress range increases the driving force for fatigue damage in two stages: initiation and propagation. When the cyclic stress amplitude is larger, microcracks at defects or stress concentrations reach a critical size more quickly, shortening the initiation life. Once a crack exists, its growth rate depends on the stress intensity factor range (ΔK); for most materials, the crack growth per cycle, da/dN, increases with (ΔK)^m, so a higher stress range makes the crack advance faster each cycle. So, increasing the stress range speeds up both the formation of a crack and its subsequent growth, making the correct statement that higher stress range accelerates both initiation and crack growth. The other options don’t fit because higher stress range does not slow growth, and it does not leave initiation or growth unaffected.