NONDESTRUCTIVE CHARACTERIZATION EVALUATION OF FUNCTIONAL SANDWICH ANDHYBRID CARBON-CARBON COMPOSITES/

Carbon/carbon (C/C) composites were developed on a conceptual basis wi th a goal of developing low cost functional composites. The current ap proach in choosing a precursor material system deviates from conventio nal C/C systems in that nonwoven carbon mat reinforcement is used in c onjunction with continuous fabric. C/C composite systems considered in this study are classified into two categories; hybrid and sandwich. T hese composites were fabricated from continuous woven carbon fabric in plain, eight harness satin (8HS) and stretch broken weave architectur es along with nonwoven carbon mat reinforcement. In sandwich composite s, the nonwoven mat is sandwiched between continuous fabric reinforcem ent; while in hybrid composites, the nonwoven mat is alternated with c ontinuous reinforcement. Ultrasonic and vibration based nondestructive evaluation/characterization (NDE/C) was conducted at three processing stages, namely; as-cured, after first carbonization and after first d ensification. Ultrasonic velocity measurements in the in-plane and thr ough-the-thickness directions were found to be influenced by the evolv ing material microstructure and fabric architecture. Vibration damping of sandwich and hybrid composites was found to be an order of magnitu de higher than conventional woven C/C composites. Acoustic emission (A E) signatures from the two composite systems were correlated to matrix cracking, fiber filament and bundle fracture, onset and occurrence of delamination and failure of the nonwoven layer(s). With regard to pro cessing, the incorporation of nonwoven carbon mat in conjunction with continuous reinforcement enabled rapid infiltration during densificati on stage of sandwich and hybrid C/C composites.