Fatigue behavior of composites under different loading modes

The fatigue characteristics of S-glass/epoxy (Scoth-ply SP-250-SF1) and graphite/epoxy (Rigidite 5209-T300) laminates were studied for tension and shear loading. Emphasis was placed on fiber orientations where the matrix contribution to fatigue strength is significant. A general regression curve was used for data analysis, and a two-parameter Weibull model was used for reliability studies. An existing plane-stress macroscopic failure theory for static strength was extended to predict fatigue strength. It was found that the fatigue failure theory provides a conservative approach to first-ply failure which should be useful for preliminary design of composite structures.