Nondestructive Characterization/ Evaluation of Functional Sandwich and Hybrid Carbon-Carbon Composites

Abstract

Carbon/carbon (C/C) composites were developed on a conceptual basis with a goal of developing low cost functional composites. The current approach in chooing a precursor material system deviates from conventional C/C systems in that nonwoven carbon mat reinforcement is used in conjunction with continuous fabric. C/C composite systems considered in this study are classified into two categories; hybrid and sandwich. These composites were fabricated from continuous woven carbon fabric in plain, eight harness satin (8HS) and stretch broken weave architectures along with nonwoven carbon mat reinforcement. In sandwich composites, the nonwoven mat is sandwiched between continuous fabric reinforcement; while in hybrid composites, the nonwoven mat is alternated with continuous 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 densification. Ultrasonic velocity measurements in the in-plane and through-the-thickness directions were found to be influenced by the evolving material microstructure and fabric architecture. Vibration damping of sandwich and hybrid composites was found to be an order of magnitude higher than conventional woven C/C composites. Acoustic emission (AE) 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 processing, the incorporation of nonwoven carbon mat in conjunction with continuous reinforcement enabled rapid infiltration during densification stage of sandwich and hybrid C/C composites.