Ultrasonic characterization of elastic constants and defects in composite materials
This thesis consists of two parts. The first part deals with the determination of anisotropic elastic constants of silicon carbide particulate (SiC[subscript] p) reinforced Al matrix composites using ultrasonic velocity measurements. The composite materials, fabricated in the form of plates by a powder metallurgy extrusion process, included 2124, 6061 and 7091 Al alloys reinforced by 10-30% by volume of [alpha]-SiC[subscript]p. All of the reinforced composite samples exhibited orthotropic behavior, with the maximum elastic anisotropy existing between the extrusion direction and the out-of-plan direction. Microstructure was examined and revealed that the observed elastic anisotropy could be attributed to the preferred orientation distribution of SiC[subscript]p. A theoretical model based on the Eshelby's method and Mori-Tanaka's theory was used to predict the elastic constants of the Al/SiC[subscript]p composites. A 6 x 6 matrix form of effective stiffness expression was developed to investigate the effect of particle characteristics on the anisotropic properties. Ultrasonically measured constants were compared with both the tensile test data and the model prediction; reasonably good agreement was obtained. The second part of the thesis is concerned with the theoretical and experimental studies of the ultrasonic velocity for the nondestructive evaluation of porosity in carbon fiber reinforced plastics (CFRP). A fiber reinforced composite containing voids was used to study the effect of void characteristics and fiber properties on the ultrasonic velocity propagating normal to the fiber. The results showed that the velocity decreased with increasing void content. The void shape was found to have a significant effect on the rate of velocity decrease. However, the volume fraction of transversely isotropic fibers had a negligible effect on the ultrasonic velocity of composites containing voids. Ultrasonic spectroscopy was employed to measure the phase velocity and the attenuation in a through transmission, immersion testing mode. The composite materials studied included carbon (graphite) fiber reinforced epoxy and polyimide laminates containing different level of porosity. Experimental results showed that void content had a correlation with ultrasonic phase velocity. With increasing void content, the velocity decreased substantially. In addition, the velocity of composites containing voids was found to be more dispersive than that of void-free composites. Finally, the relationship between ultrasonic attenuation and velocity dispersion was tested using the local approximation of the exact Kramers-Kronig relation.