  Starting from the classical definition of stress-life Wohler curve in the
form of Basquin's law, an analytical procedure for the calibration of the four
parameters Wohler curve (Weibull's law) for a plain specimen is proposed. The
obtained parameters are then adjusted by means of an additional slope factor
preserving the inflection point of the curve while changing its slope in order
to model the experimental observations in which an increase of the scatter in
life prediction is observed when reducing the stress amplitude. The same
approach has then been adopted to calibrate the Weibull's law parameters for a
notched specimen, and the fitting slope factor has been found to be a value
that changes with the material but remains constant with the stress
concentration factor. The findings have been validated with existing
experimental data on 2024-T3 aluminum alloy and normalized SAE 4130 steel.
