Explanation:
This is a somewhat arbitrary number, but some justification for the names and demarcation can be obtained by looking briefly at the figure in the reading for sub-subunit 3.3.6 Cyclic Fatigue. Note that the plot of stress versus lifetime has an inflection point near N = 1000; that is, the shape of the curve changes from concave-up to concave-down near this point. This point may be different for different materials and conditions.
