Stiffness degradation model for biaxial braided composites under fatigue loading

Biaxial braided composites are gaining popularity, in particular for the small business jets, where FAA requires take off weights of 5670 kgf (12,500 lb). In the present research carbon/epoxy biaxial braided composites were manufactured using low cost vacuum assisted resin transfer molding (VARTM). Extensive tension-tension fatigue tests were performed on biaxial braided carbon/epoxy composites for various braid angles. Experimental data clearly indicated that S-N diagram could be represented by Sigmoidal function. This paper specifically addresses stiffness degradation of braided composites with braid angle of 25°. In addition to Sigmoidal function, it was observed that the three stages of stiffness degradation curve can be represented by Bradley function. Sigmoidal function in conjunction with Bradley function is explored to predict the fatigue life and the residual fatigue modulus.