This paper formulates a stochastic model of fatigue crack growth in ductile alloys under variable loading of the center
wing type. This center wing loading has three different load ratios to depict the most demanding operating conditions.
The cumulative distribution function of the crack length estimate is generated by numerically solving a stochastic
differential equation describing the physics of the crack growth. The model parameters are obtained by analyzing each
load span, and the variable model parameter is used in the corresponding load period. Simulations are used to show that
the analytical crack exceedance probability follows the experimental data fairly well.