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The adjustment of the S-N curve through a general statistical model and thenormalization of the fatigue life of the material permit to establish equivalencies betweenstates of damage for different stress levels. On the other side, the analysis of the stiffnesscurves of composites under fatigue loading provides an important information about thestate of damage accumulation and the failure mechanisms inside of the material. Joiningboth procedures permits to present the stiffness evolution as a function of a normalizedvariable, instead of as a function of the number of cycles, supplying in this way thepossibility of a unified study for results belonging to various load levels and histories.