  This work employs the Birnbaum--Saunders distribution to model the fatigue
life of metallic materials under cyclic loading and compares it with the normal
distribution. Fatigue-limit models are fitted to three datasets of unnotched
specimens of 75S-T6 aluminum alloys and carbon laminate with different loading
types. A new equivalent stress definition that accounts for the effect of the
experiment type is proposed. The results show that the Birnbaum--Saunders
distribution consistently outperforms the normal distribution in fitting the
fatigue data and provides more accurate predictions of fatigue life and
survival probability.
