FATIGUE LIFE METHODOLOGY FOR TAPERED COMPOSITE FLEXBEAM LAMINATES

The viability of a method for determining the fatigue life of composit e rotor hub flexbeam laminates using delamination fatigue characteriza tion data and a geometric non-linear finite element (FE) analysis was studied. Combined tension and bending loading was applied to non-linea r tapered flexbeam laminates with internal ply drops. These laminates, consisting of coupon specimens cut from a full-size S2/E7T1 glass-epo xy flexbeam were tested in a hydraulic load frame under combined axial -tension and transverse cyclic bending loads. The magnitude of the axi al load remained constant and the direction of the load rotated with t he specimen as the cyclic bending load was applied. The first delamina tion damage observed in the specimens occurred at the area around the tip of the outermost ply-drop group. Subsequently, unstable delaminati on occurred by complete delamination along the length of the specimen. Continued cycling resulted in multiple delaminations. A 2D finite ele ment model of the flexbeam was developed and a geometrically non-linea r analysis was performed. The global responses of the model and test s pecimens agreed very well in terms of the transverse flexbeam tip-disp lacement and flapping angle. The FE model was used to calculate strain energy release rates (G) for delaminations initiating at the tip of t he outer ply-drop area and growing toward the thick or thin regions of the flexbeam, as was observed in the specimens. The delamination grow th toward the thick region was primarily mode II (shear), whereas dela mination growth toward the thin region was almost completely mode I (o pening). Material characterization data from cyclic double-cantilevere d beam tests was used with the peak calculated G values to generate a curve predicting fatigue failure by unstable delamination as a functio n of the number of loading cycles. The calculated fatigue lives compar ed well with the test data.