Nondestructive evaluation of graphite fiber based composites using acoustic and vibration techniques

The thesis entitled "Nondestructive Evaluation (NDE) of Graphite Fiber Based Composites using Acoustic and Vibration Techniques" contains information pertaining to experimental work on nondestructive testing of Graphite/Epoxy (Gr/Ep) and Carbon-Carbon (C/C) composites. The NDE techniques adopted in this study were: Acoustic Emission (AE), Vibration-Based NDE (VNDE), and Acousto-Ultrasonics (AU). Defects in the form of overlapped layers, precut fibers, misaligned layers and semi-cured material state in Gr/Ep composites have been identified and characterized through AE.;For C/C composites, emphasis was placed on nondestructive evaluation of the material at various processing stages. The as-cured material that was comprised of Graphite/Phenolic (Gr/Ph) was repeatedly subjected to processes of carbonization and densification. At each stage of processing, nondestructive evaluation methods including AE, AU and VNDE, were applied. Microstructural studies were also performed to correlate changes in microstructure with a nondestructive test parameter from each NDE technique.;The failure modes of C/C were characterized ex-situ, at each processing stage by AE. The parameters considered in data interpretation included; amplitude, event duration, and energy content of the AE signals. A novel scheme for the representation of AE parameters in three-dimensions using Statistical Analysis System (SAS) was devised. Such a representation gave excellent visual discrimination of fiber breaks, matrix crack development, and the onset and propagation of delamination related modes of failure. Pattern recognition through cluster analysis of the AE data was also performed to investigate failure modes.;Resonance frequency and material damping were found to be two useful vibration parameters to monitor global material condition in the case of Gr/Ep and C/C composites. In the case of Gr/Ep composites, manufacturing and service-induced damage conditions were characterized by using VNDE. For C/C composites, changes in material condition at each stage of processing were studied in relation to VNDE parameters. These parameters were correlated qualitatively with microstructural changes of the composite.;In the case of AU testing, time and frequency domain signal analysis was found to provide indications regarding manufacturing induced defects in Gr/Ep composites. In C/C composites, material changes at each stage of processing were characterized by changes in 'Stress Wave Factor' defined in terms of energy and/or counts. The peaks in the frequency spectrum were found to be indicative of the changes occurring in the material. Primarily, the development of a network of transverse cracks, interfacial debonds, and large amounts of distributed porosity was found to have a considerable influence on the AU signature. Microstructural studies were also conducted by considering the crimp angle variations and stacking sequences in plain and satin weave composites.