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Ultrasonic vibrations, commonly called acoustic emissions (AE), are observed to be
generated during the course of metal cutting operations. AE has two major advantages as
a monitoring tool: it is considered to be related to the micro-mechanical activities of
the cutting process, and, as a high frequency signal, unwanted low frequency noise
signals can be filtered out. Although still not successfully implemented in industry,
quite a lot of research effort has been put into the use of AE to monitor the state of
cutting processes. However, the mechanism by which AE is generated is still not fully
understood.

The objective of the present thesis is the investigation of the generation mechanisms
of AE from metal cutting. As continuous-type AE is generated due to plastic deformation
from metal cutting with continuous chips, and as dislocation motion is the main
mechanism of plastic deformation of metals, a relationship between AE and dislocation
motion is investigated 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; 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 when 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. As a result,
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, four sets of original
metal cutting experiments are accomplished for four different work materials, by
varying cutting conditions and workpiece temperature. Both energy and frequency
information are computed from the experimental data using the most appropriate data
processing technics. In addition, a semi-empirical model is utilized to predict basic
metal cutting parameters. As the experimental results are in close agreement with the
predictions provided by the model, a very significant contribution to the knowledge of
the mechanisms of AE generation from metal cutting is undoubtedly achieved.
