PHYSICAL PRINCIPLES UNDERLYING ULTRASONIC NONDESTRUCTIVE EVALUATION OF ANISOTROPIC MEDIA (ULTRASOUND, COMPOSITES, GRAPHITE-EPOXY, BACKSCATTER, ATTENUATION)

The research described in this thesis is aimed toward extending the physics underlying the application of ultrasound to the investigation of materials. Some of the results presented deal with isotropic materials, both homogeneous and inhomogeneous. However, the primary focus is the physics of inhomogeneous, anisotropic materials, such as fiber-reinforced plastics.;In this thesis, we describe the physics underlying the use of ultrasound as a tool for the detection and characterization of damage in composite materials. The thesis is composed of two inter-related portions. The first portion serves as an introduction to wave propagation in isotropic and anisotropic media. These chapters provide a basis of insight and intuition for the non-destructive evaluation experiments described in the second portion.;The second portion deals with experimental application of ultrasonic techniques to several specific problems in nondestructive evaluation. The studies reported here examine parameters of ultrasonic propagation in materials with known flaws whose location and character have been verified by independent (non-ultrasonic) means. The ultrasonic parameters employed in these studies are backscatter, attenuation measured in transmission, and attenuation estimated from backscatter. Three chapters deal with the use of a backscatter-based technique known as polar backscatter. In this technique the effects of specular reflection from the water/sample interface in an immersion-type measurement are reduced by interrogating at nonperpendicular incidence. The anisotropic nature of composite laminates makes polar backscatter a very useful tool for investigation of impact damage and porosity. Two chapters discuss a theoretical approach and experimental results for estimating attenuation in inhomogeneous materials from backscattered ultrasound.;Fiber-reinforced plastics exhibit very favorable strength-to-weight ratios and can be fabricated to exhibit a wide range of structural properties. From a mechanical standpoint, two types of disadvantages of otherwise well-designed composite materials are fabrication defects (e.g., delamination, fiber-misalignment, and porosity) and wear damage (e.g., environmental degradation, fatigue stress, and impact damage).