Estimation of Crack Opening Stresses from Strain-Life Fatigue Data

摘要

The fatigue life of components subjected to variable amplitude service loading is dominated by crack growth. It is generally assumed that only the portion of the stress-strain cycle during which the crack is open is effective in increasing crack growth. In order to calculate the effective stress or strain range the stress or strain at which the crack starts to open must be determined. An empirical formula suggested by DuQuesnay et al. that correlates the steady state crack opening stress with the minimum and maximum applied stresses has two experimentally determined constants a and β. This paper correlates these constants to the material hardness using data generated from six materials. Each material was heat-treated to a different hardness level to give eleven different data sets. The six materials are a Microalloyed SAE10V40 steel, a SAE 4140 steel, a SAE 4340 steel, a 3OOM tool steel, a SAE 1045 steel, and a low carbon CC007A steel. Since only the range of strain for which a fatigue crack is open is operative in propagating the crack, then for a given fatigue life, Nf, the difference in strain range between a closure-free fatigue-life curve which was obtained using periodic overloads and the constant amplitude curve is the range of strain for which the constant amplitude cycle is closed. Adding the range of stress for which the crack is closed and the minimum stress gives the crack opening stress for given maximum and minimum stress values in the constant amplitude tests. The constants of the crack opening stress are from a best fit of Duquesnay's formula to this data. The correlation of the two constants of the steady state crack opening stress formula with material hardness is good.