Broadband viscoelastic spectroscopy (BVS) is a technique for studying
viscoelastic solids in both bending and torsion. It provides the ability to measure viscoelastic behavior over eleven
decades (orders of magnitude) of
time and
frequency: from 10
−6 to 10
5 Hz.
BVS is typically either used to investigate viscoelastic properties
isothermally
In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and ...
over a large frequency range or as a function of temperature at a single frequency.
It is capable of measuring mechanical properties directly over these frequency and temperature ranges; as such, it does not require
time-temperature superposition or the assumption that material properties obey an
Arrhenius-type temperature dependence.
As a result, it can be used for
heterogeneous
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
and
anisotropic
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
specimens for which these assumptions do not apply.
BVS is often used for the determination of
attenuation coefficients,
,
and especially
damping ratio
Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples inc ...
s.
BVS was developed primarily to overcome shortcomings in the functional ranges of other viscoelastic characterization techniques. For example,
resonant ultrasound spectroscopy Resonant ultrasound spectroscopy (RUS) is a laboratory technique used in geology and material science to measure fundamental material properties involving elasticity. This technique relies on the fact that solid objects have natural frequencies at ...
(RUS), another popular technique for studying viscoelastic solids, experiences difficulty in determining a material's parameters below its
resonant frequency
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
.
Furthermore, BVS is less sensitive to sample preparation than RUS.
History
BVS was first developed by C. P. Chen and R. S. Lakes in 1989 in order to address the shortcomings of existing laboratory techniques for studying viscoelastic materials.
It was later refined by M. Brodt et al. to improve the rigidity and resolution of the apparatus, which were sources of error in the original design.
First used to study
poly(methyl methacrylate)
Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylite, ...
(PMMA),
it has since seen applications in determining the properties of
bone,
s,
high damping metals,
and other such viscoelastic materials.
Design
The BVS apparatus consists of a specimen surrounded by
Helmholtz coils and isolated from external vibrations by a framework constructed from insulating foam and either lead or brass.
The specimen is affixed with both a permanent magnet and a mirror. The orientation of the coils with respect to the magnet when a current is driven through them determines whether the specimen undergoes bending or torsion. Angular displacement of the specimen is measured by an
interferometer
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
that detects the spatial movement of a reflected laser. This spatial waveform is converted to an electrical one by a light detector and read out on an
oscilloscope
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
. This oscilloscope also displays the
torque or
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
waveform from the capacitor driving the current in the Helmholtz coils.
Phase delay is determined by comparing these waveforms.
Resonance is minimized through the use of short specimens—which have higher resonant frequencies—and by reducing the inertia (
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
and
mass moments) of the magnet. Cubic
samarium-cobalt magnets are ideal for high frequency studies.
Due to the sample geometry being a short rectangular bar or cylinder, the equation governing the resonance of the BVS specimen geometry has an exact analytic solution, which allows the technique to yield results even for high loss materials.
This exact solution provides a relationship between dynamic moduli, angular displacement, and geometric parameters.
The inherent lack of drift and friction in the apparatus is responsible for its large range of operating frequencies.
References
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Spectroscopy