Pulse propagation techniques

Recourse to pulse propagation measmements is often prompted by their apparent simplicity, involving measurements of the time-of-flight of a disturbance propagating through a visco-elastic material [Joseph et al, 1986]. 

In plane-harmonic shear-wave propagation in a linear medium of density p, G and G are given by, 

Such simple measmements belie the complicating effects of visco-elastic wave dispersion, which may render their analysis unreliable. The tendency of pulse frequency components to travel at different velocities in dispersive media distorts the pulse, thereby influencing measurements of damping to a degree dependent on the mediiun and the spectral content of the pulse. The latter, in turn, depend on pulse shape. This visco-elastic wave dispersion, associated with dissipative stresses, can severely restrict the application of pulse propagation techniques in which the measmed velocity may correspond to a group velocity, U, not the requisite phase velocity, v. As U and v may differ 

In some instruments the phase velocity, v, is measured directly (using continuous shear waves), as in the virtual gap rheometer, VGR, a multiple path shear wave interferometer which operates in the frequency range 100 Hz to ca. 2 kHz [Williams and Wilhams, 1992], 

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Inevitably, the conjunction of frequency-dependent visco-elastic properties and wave propagation leads to consideration of visco-elastic wave dispersion and its influence on conventional wave-based measurements, such as those involving resonance phenomena and pulse propagation techniques. 

One example of a relatively new technique for the non-invasive, non-destructive characterization of network structures involves pulse-propagation measurements [288,289]. In this technique, the delay Af in a pulse passing through the network is used to obtain information on the network structure, for example, the chain length between cross-links or between entanglements. The technique is illustrated schematically in Figure 12 [282]. 

M. Sinha, Ph. D. Physics Probing Polymer Networks Using Pulse Propagation and Brillouin Light Scattering Techniques. University of Cincinnati, 2000. 

In this section, a problem of a nonlinear waveguide excitation by a non-stationary light beam has been investigated. Optical pulse propagation through the junctions of nonlinear waveguides was simulated by the numerical technique described in Section 2. 

A method for preparing a-methylstyrene to investigate its radiation-induced polymerization yields samples which exhibit reproducible kinetics. The kinetic results are interpreted as indicating that free radicals, carbonium ions, and carbanions can all propagate simultaneously, the relative importance of each species depending upon the dryness of the monomer and all associated glassware. This viewpoint is further supported by data from a preliminary investigation of the transients formed in a-methylstyrene, as studied by the pulse radiolysis technique. 

One method used for studying the orientation of polymers which does not make use of electromagnetic radiation is that of sonic velocity or pulse propagation. This technique is particularly suited to the characterisation of orientation in fibres (or samples having rodlike geometry) and, although the technique suffers somewhat from not having a sound theoretical basis, it is of particular usefulness in instances where an orientation index or parameter is desirable for relative comparisons. 

Another example of a relatively new technique for the noninvasive, nondestructive characterization of network structures involves ultrasound pulse-propagation measurements. The goal here is the rapid determi-... 

The distributed signals can be measured and spatially resolved by an optical time-domain reflectometry technique. An optical time domain reflectometer is based on the measurement of backscattered light attained from a light pulse propagating through an optical fiber. Light is backscattered because of inhomogeneities and impurities... 

Instead of temperature fields, one can use pressure discontinuities in order to probe charge or polarization distributions in dielectrics. According to the pressure excitation method, one can distinguish three basic configurations (a) the LIPP (laser-induced pressure pulse. Fig. 21) (Sessler et al. 1981) or PWP (pressure wave propagation) technique (Alquie et al. 1981), (b) the piezoelectrically generated pressure step technique (PPS, cf. Fig. 22) (Eisenmenger and Haardt 1982), and (c) the pulsed electroacoustic method (PEA, cf Fig. 23) (Takada et al. 1987). 

Newer techniques for characterizing elastomers include Brillouin scattering [43], and pulse propagation measurements [44]. 

The common civil engineering seismic testing techniques work on the principles of ultrasonic through transmission (UPV), transient stress wave propagation and reflection (Impact Echo), Ultrasonic Pulse Echo (UPE) and Spectral Analysis of Surface Waves (SASW). 

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