dc.description.abstract
Ultrasonics is a well-established method for the determination of elastic properties of solids. Depending on the application, it may have advantages over classical quasi-static tests, in terms of its non-destructive nature, of simpler specimen preparation, of easy and fast experimental realization, and of high precision measurements of normal and shear ('diagonal') elasticity tensor components. These advantages, however, rely on the appropriate application of the method within specific ranges, the elucidation of which is the primary focus of this thesis. The application ranges concern specimen geometry and microstructure (Publication 1), Poisson's ratios and off-diagonal elasticity tensor components (Publication 2), and different dense and porous glass-ceramic, biological, and metal-based materials from the engineering and biomedical fields (Publication 1 - 5).<br />Publication 1 covers the influence of specimen shape and porosity on elastic wave velocity and stiffness determination through ultrasonic contact pulse transmission. It turns out that bar-shaped specimens with a slenderness ratio larger than ten, excited by low-frequency signals, transmit (1D) extensional or bar waves, whereby the specimen needs to be the more slender the higher the signal frequency to be transmitted as extensional wave.<br />Beyond a quite narrow extensional-bulk-wave transition regime, less slender bar-type specimens excited by higher frequency signals transmit (3D) bulk waves, whereby specimens need to be the less slender the lower the frequency to be transmitted as bulk waves. As for porous non-slender specimens, the wave propagation type depends on the 'pore-diameter-over-wavelength' ratio and on the porosity. Cube-shaped porous specimens excited by low frequency signals transmit bulk waves relating to the effective porous medium (long-wavelength-limit), whereby the specimen needs to be the more porous, the higher the frequency to be transmitted as effective wave 'feeling' the porous medium. Beyond a long-to-short wavelength transition period, which is increasing with increasing porosity and with decreasing direction-dependent wave propagation velocity, cube-shaped porous specimens excited by higher frequencies transmit bulk waves relating to the solid matrix (short-wavelength-limit). Thereby, specimens need to be the less porous, the lower the frequencies to be transmitted as waves 'feeling' the solid matrix. Publication 2 deals with the determination of Poisson's ratios in isotropic, transversely isotropic, and orthotropic materials by means of combined ultrasonic-mechanical testing of normal stiffnesses. Poisson's ratios of isotropic, transversely isotropic, and orthotropic non-axially auxetic materials are expressed as functions of normal elastic stiffnesses, considering the positive definiteness of the stiffness and compliance tensors. The relevance of our method is shown by comparing Poisson's ratios computed from normal elastic stiffnesses given in the literature, to experimentally given Poisson's ratios, for a range of materials including (isotropic) aluminum, (transversely isotropic) aluminum matrix-fiber composite and (orthotropic) stainless-steel weld metal. Finally, the method is applied to (orthotropic) wood (namely spruce), by measuring four normal stiffnesses, and relying on a spruce-specific universal constant involving longitudinal Poisson's ratios and on reasonable estimates for the radial Young's modulus. Resulting ranges of Poisson's ratios agree well with ranges of Poisson's ratios obtained from direct mechanical measurements on spruce.<br />In porous materials, especially such with very high porosity, the determination of material stiffness may be strongly biased by inelastic deformations occurring in the material specimens. In contrast, ultrasonic waves propagating through a material generate very small stresses and strains (and also strain rates lying in the quasi-static regime). Therefore, elastic properties of such materials can be reliably accessed through ultrasonics, which we used for multiple-scale elastic characterization of porous biomaterials (Publication 3) and of titanium scaffolds for biomedical applications (Publication 4).<br />Ultrasonically determined elastic properties finally helped us to understand the micromechanics of bioresorbable porous CEL2 glass ceramic scaffolds for bone tissue engineering (Publication 5). Interesting details on various aspects of ultrasonic testing are collected in Appendices A - H.
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