amplitude and strain, with a linear decrease then increase
in amplitude for a minimum amplitude at 0.27 strain.
For all tests conducted for a constant ultrasonic displacement
amplitude of the lower platen, the mean flow stress
dropped to about 40% of the static flow stress. The mean
stress path returns elastically to the static stress path when
the ultrasonic excitation is discontinued.
In order to assess any response contributions due to
machine dynamics, throughout all the tests the displacement
amplitude response of the lower and upper platens
was measured using a laser vibrometer, on the top surface
of the block horn and on the upper platen, Fig. 4. For the
compression tests reported here, the block horn was driven
at a nominal constant peak amplitude of 8 lm, with the
measured response being between 8 and 9 lm during ultrasonic
compression. The displacement amplitude response
of the upper platen increased linearly with specimen height
reduction, but showed only a small transmission of ultrasonic
vibration with the amplitude increasing from 0.5 to
1.5 lm during compression.
The test results reported here were repeated for many
specimens and also repeated for different cross-head speeds