Stress–strain curves derived from the measured load–
displacement and load–time data are plotted in Fig. 2,
which compares compression test data from a conventional
static test with data from a test where ultrasonic excitation
of the lower platen was applied during plastic deformation.
For clarity, during applied ultrasonic excitation the paths
of the maximum, mean and minimum oscillating stress
are included in the figure, rather than the oscillations.
The figure shows that the mean flow stress is significantly
reduced at the onset of ultrasonic excitation and follows
a path parallel to the static stress path. The paths of the
maximum and minimum oscillating stress initially show a
gradual linear decrease in the oscillating stress amplitude,
with an amplitude minimum at a strain of 0.27. Subsequently
the stress amplitude increases gradually and linearly
with strain.
Fig. 3 shows the stress–strain curves for static compression
and for static-ultrasonic compression for two intervals
of ultrasonic excitation of the lower platen during plastic
deformation. In this test, ultrasonic excitation was applied
for 6 s then discontinued and, after 6 s of continued static
compression, the ultrasonic excitation was applied again
for 12 s. Comparing Fig. 3 with Fig. 2, the two figures
can be overlaid, exhibiting exact correspondence between
the paths of the maximum, mean and minimum oscillating
stress during ultrasonic excitation intervals. The figures
show the same relationship between oscillating stress