Biomedical Engineering Reference
In-Depth Information
material characterization in air have been around for almost a century. The technique is
described in basic acoustics text books such as Kinsler et al., (2000) and Temkin (1981).
Impedance tube methods based on standing waves and the transfer function method have
been accepted as standard methods by the American Society for Testing and Materials
(ASTM 1990 and 1995), thus they will not be discussed in this chapter. Instead the chapter
will focus on liquids, semiliquid and semisolid materials, many of them exhibiting
viscoelastic properties, i.e. those properties that are more representative of biomaterials
behavior.
2. Vibration fundamentals and analysis
The theoretical background that supports mechanical characterization of materials using
vibration/acoustic based methods is mainly based on the characterization of acoustic waves
propagating though the material. In that sense the analysis can be classified on the type of
material being tested, i.e. liquid, viscoelastic semifluid, and viscoelastic semisolid.
2.1 Liquid materials
The analysis of liquid samples can be further classified based on the type of container used
to confine the testing liquid. One of more important aspects to consider in this classification
is the rigidity of the container walls. Two cases are considered: containers with rigid walls
and containers with deformable/flexible walls.
2.1.1 Rigid wall containers
Since acoustic waves reveal useful information on the characteristic of the material through
which they travel, measurement of acoustical properties such as velocity, attenuation, and
phase changes resulting mainly from wave reflections in the transfer media are often used
as a tool for mechanical characterization of materials. Specifically, ultrasound has found a
wide range of applications in the measurement of the viscosity of liquids. Mason et al. (1949)
first introduced an ultrasonic technique to measure the viscosity of liquids. They used the
reflection of a shear wave in the interface between a quartz crystal and the sample liquid.
Since then many other ultrasonics related techniques have been developed to measure
viscosity of liquids [Roth and Rich (1953), Hertz et al. (1990), and Sheen et al., (1996)].
However, acoustical techniques that use sonic frequency have been rather limited. The main
reason for this has been the lack of practical approaches that can employ frequencies in the
sonic range to study the rheology of liquids. Tabakayashi and Raichel (1998) tried to use
sonic frequency waves for rheological characterization of liquids. They affixed a
hydrophone and a speaker to both ends of a cylindrical tube and analyzed the effect of the
liquid non-Newtonian behavior on the propagation of the sound waves. The approach used
by these authors is similar to the well known impedance tube method commonly applied to
gases contained in cylindrical tubes, known as waveguides, but their analysis did not
include the effect of the tube boundaries on sound propagation which can be of importance.
In that sense, the application of the impedance tube techniques to test liquids has been
limited because the loss of wall rigidity, i.e. the tube boundary, which normally it does not
occur in tubes filled with air or air waveguides. Thus, when the tube is filled with a liquid,
the tube may become an elastic waveguide and the rigid wall approximation loses its
validity. The key assumption of having rigid walls is related to the shape of the acoustic
