Title:
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Body wave propagation in an analogue granular material in a cubical cell apparatus
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Shear and compression wave velocities are examined in assemblages of spherical glass beads
in isotropic, triaxial and multiaxial stress states in the Cubical Cell Apparatus (CCA). The
CCA enables application of non-rotational stress-states to a cube-shaped sample with
independent control of the three principal stresses. Waves are transmitted using
bender/extender elements (Lings & Greening 2001), the performance of which are assessed
by laser vibrometry. Uncertainties in the analysis of wave velocities are examined in relation
to the method of travel time assessment, extending to frequency and shape of the transmitted
pulse waveform. Results suggest that uncertainties over wave travel time result largely from
the mismatching of the transmitted waveform and the method by which its arrival is analysed.
Methods based on cross-correlation are the most universally applicable and can be semi automated
but still rely on the operator to correctly identify the wave arrival.
Results for velocities in isotropic stress states are curve-fitted to a power law; the coefficients
and exponents of these best fit curves appear to be related by an inverse power law, which is
backed up by published data. How this may relate to material properties remains unclear.
Additional laser vibrometry results from targets on the outside of a cubical sample enable
examination of the propagation of the wavefront through the sample. The received traces for
S-wave transmissions support the theory that the operation of the bender element produces
sideways propagating P-wave 'lobes' (Lee and Santamarina 2005). The trend of their arrivals
with distance between the bender and laser target supports the accepted use tip-to-tip travel
distance for S-wave transmissions but suggests it may be too short for P-waves; this latter
requires further experimental work.
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