Volume 1, Issue 8 (September 2004)
Fatigue Life Variability Prediction Based on Crack Forming Inclusions in a High Strength Alloy Steel
Fatigue cracks in metals have been observed to form at nonmetallic inclusions in the matrix of the material, with the size of such features influencing fatigue life. A probabilistic method was used to predict the variability of fatigue life in quenched and tempered AISI 4340 steel. Crack growth analyses, presented as cumulative distribution functions of probability of failure, were made at two stress levels by growing inclusion size fracture mechanics flaws using an existing linear elastic fracture mechanics crack growth model. Initial flaw sizes were obtained from two separate distributions: one representing the overall inclusion population, and the other a subdistribution representing crack forming inclusions. In general, analyses correlated well with experimental lives, typically being within 25 %. The two initial flaw size distributions produced similar results; however, predictions based on the total inclusion distribution were heavily dependent on the threshold value. With the appropriate value of crack growth threshold, good correlation with variability can be made without requiring fatigue testing, particularly for the more critical shorter lives.