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The objective of the present thesis is the investigation of  the generation mechanism
of the ultrasonic vibrations, commonly called acoustic emissions (AE), detected during
the course of metal cutting, since, although quite a lot of research effort has been
put into the use of AE to monitor metal cutting condition, the mechanism by which AE is
generated is still not fully understood.

If chip generation is continuous, without built-up edge, and a sharp tool is used,
continuous-type AE is normally assumed. Most published models relate the energy of AE
to the total cutting power, but this can be shown to be rather incorrect. Consequently,
as continuous-type AE is mostly generated due to plastic deformation, and as
dislocation motion is the main mechanism of plastic deformation of metals, a
relationship between AE and dislocation motion is developed for the typical plastic
deformation regimes encountered in metal cutting (due to the high temperatures, flow
stress decreases with temperature in the so-called diffusion controlled regime, and due
to the high strain rates, opposing viscous damping becomes the dominant mechanism
governing dislocation movement). Although viscous damping governs the mechanics of
deformation in metal cutting, it is proposed that AE is generated due to the
interaction between dislocations and obstacles, since as a dislocation approaches an
obstacle, strain energy is stored, which is rapidly released as soon as the dislocation
surmounts the obstacle, resulting in the emission of an AE event. The detected AE is a
result of many consequential likewise events. Consequently, a qualitative original
model of AE generation is developed, in which the energetic level of AE is predicted to
increase with strain and strain rate, but decrease with temperature, and the frequency
content of AE is predicted to increase with strain rate, decrease with temperature, and
remain unchanged with strain.

In order to access the validity of the above-mentioned model, two sets of metal cutting
experiments were accomplished for four different work materials, in which the cutting
conditions were varied over a wide range, and the workpiece temperature was
artificially modified. Both energy and frequency information were computed from the
experimental data using the most appropriate data processing technics, i.e.\ AE mode
and mean frequency, respectively. In addition, a semi-empirical metal cutting theory
was utilized to predict basic metal cutting parameters. As the experimental results are
in close agreement with the predictions provided by the qualitative model, it is
concluded that the main source of AE in metal cutting comes from the interaction of
moving dislocations with obstacles, whose dynamics is, however, dictated by viscous
damping.
