Wave Propagation in Anisotropic Media

Abstract

Modern turbine blades are manufactured from single-crystal nickel-based superalloys for the excellent mechanical properties these materials exhibit at elevated temperatures. However, single-crystal materials are highly elastically anisotropic, meaning the elastic proprieties vary with direction. Wave propagation in anisotropic materials is significantly more complex than in isotropic materials. This severely complicates the inspection of these materials using ultrasound NDE methods. To enable the reliable inspection of single-crystal turbine blades with ultrasound, the propagation of ultrasonic waves in anisotropic material must be fully understood. In this chapter, the theory of bulk wave propagation in both isotropic and anisotropic solid materials is presented. Initially, a brief background of the research in wave propagation in solids is laid out to provide a historical context for this thesis. A short theory of bulk wave propagation is given and the important concepts of group and phase velocities are explained. Analytical models are then developed to describe wave propagation in anisotropic materials. Specifically, the developed analytical models simulate the variation in velocity with direction and the variation in beam amplitude from a point-force acting on the surface of an anisotropic material. These models are utilised in subsequent chapters to correct ultrasonic array imaging algorithms for the inspection of anisotropic materials. Finally, the analytical models are validated against numerical finite-element models.