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Prof. Arthur Every

The Impact of Elastic Anisotropy on Ultrasonic Inspection

and Materials Characterization

 

A. G. Every

University of the Witwatersrand, Johannesburg, South Africa 

Abstract

In this talk I will address the question “Of what importance is elastic anisotropy in ultrasonic inspection  and materials characterization? Is anisotropy something NDT professionals should be aware of, and are they likely to encounter it in their activities?” 

An elastically anisotropic medium is one in which properties such as elastic moduli and wave speed depend on direction. I will describe the principal types of anisotropic materials:

  • Crystals: the semiconductors Si, Ge, GaAs, etc., transducer materials such as quartz, LiNbO3, etc., Ni-based super-alloys used for gas turbine blades, etc.,
  • Polycrystalline textured materials: thin protective coatings in which the crystallites are preferentially aligned, austenitic welds, etc.,
  • Fiber composites: carbon fiber epoxy, glass fiber, etc.,
  • Fibrous organic materials: wood, bone, etc.,
  • Meta-materials: superlattices, phononic crystals, oriented microfracture,
  • Stressed solids: the elasto-acoustic effect,
  • Geological structures: stratification, tectonic effects.

Still in a purely descriptive mode, I will expand on single crystal turbine blades, fiber composites and austenitic welds, areas where anisotropy has made major inroads into research in NDE. 

The second part of my talk will be to sketch some of the basic ideas that come up in the treatment of elastic wave propagation in anisotropic solids: the wave equation, the elastic stiffness matrix and how it simplifies depending on the degree of symmetry of the medium (e.g. cubic, transverse isotropy, isotropy), the distinction between phase and group velocity, the slowness and wave surfaces and energy focusing. 

The third part of the talk will deal with a number of applications that I have worked on, namely:

Ballistic phonon transport and phonon imaging in crystals (with J.P. Wolfe et al.),

Phonon imaging at ultrasonic frequencies with transmission acoustic microscopy (with W. Grill et al.),

Pulsed laser scan imaging on silicon (with W. Sachse et al.),

Acoustic microscopy wall thickness measurement of single crystal superalloy gas turbine blades (with G. Amulele),

Surface Brillouin scattering on bulk solids (superalloys) and thin coatings (TiN on steel)  (with J.D. Comins et al.),

Laser transient grating measurements on periodically patterned thin film structures encountered in the semiconductor industry (with A.A. Maznev).