In Situ (Autoclave) Cure Monitoring of Composites with IR Transmitting Optical Fibers

Abstract

The genesis of this work owes itself to the efforts of Young and Chang at NASA Langley [5]. They have reported on chemical characterization of various thermosetting resins used in composites using FTIR spectroscopic techniques. Their work, conducted over the past several years, demonstrates that FTIR is an excellent and powerful tool for the characterization of advanced materials of interest to the military community. Their laboratory analysis revealed that cure cycles of these resins can be closely monitored using FTIR techniques. Of late, their work has largely concentrated on graphite fiber composites and prepregs. The characterization of cured composites containing thermosetting resins presents a problem for infrared techniques because the material is opaque and the matrix resin is insoluble. Young solved this problem by using a diffuse re- flectance technique (DRIFT). As illustrated in Fig. 1, diffuse reflectance arises from radiation penetrating into the interior of an opaque sample and re-emerg- ing after being scattered numerous times. Since selected wavelengths are ab- sorbed during this process, the diffuse component contains valuable optical information about the sample. Off-axis ellipsoidal mirrors collimate the re- flected radiation and return it to the FTIR detector.