\chapter*{\center{Abstract}}
\addcontentsline{toc}{chapter}{Abstract}

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 since it is 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.

It is the general objective of the current thesis to investigate the generation
mechanism of AE during the course of metal cutting. In order to fulfill the objectives,
a comprehensive review on the most relevant theories and techniques attained in the
fields of metal cutting, AE, and AE from metal cutting is undertaken. The novelty of
the present work arises from the fact that a set of metal cutting experiments (turning
with continuous-type chip formation) is performed for a wide range of cutting
conditions and work materials, whose data results are analysed with the most advanced
data processing techniques: the energetic component of AE is obtained with a technique
that removes the influence of unwanted outlying values (AE mode); the frequency
component of AE is obtained from a technique that divides the AE spectrum into two
parts of equal energy (mean frequency). In order to obtain important metal cutting
parameters that characterize the physics of the cutting process, such as the geometry,
stresses, strains, strain rates, and temperatures, a well documented semi-empirical
theory is employed, so that a relationship between AE and the plastic deformation
accompanying metal cutting can be derived. And finally, since dislocation motion is
considered to be the main mechanism responsible for plastic deformation, a theory
relating AE and dislocation motion is investigated. A novel dislocation model is
developed, whose predictions are in agreement with the experimental results.

From the experimental results, it is readily concluded that the power of AE
($I_\mr{power}$) is related to the cutting speed ($V$) and feed rate ($t_1$)
according to a relationship of the type $I_\mr{power} \propto V^{n_1}t_1^{n_2}$,
where $n_1$ and $n_2$ are material dependent, and $n_1 > 1$ and $n_2 < 0$.  Moreover,
a relationship of the type $I_\mr{power} \propto w^{n}$, where $w$ is the width of
cut, and $0 < n < 1$, is observed. The mean frequency ($f_\mr{mean}$) is observed to
remain unchanged with $t_1$  and $w$ but to decrease with increasing $V$. Good
agreement is also found between the experimental result from the present thesis and
published experimental results. It was also observed that AE is affected by chatter
(high amplitude vibrations).

Since most published models are based on the relationship $I_\mr{power} \propto
\dot{U}$, where $\dot{U}$ is the cutting power, this relationship is investigated,
and it is concluded that only if $t_1$ and $w$ are kept constant, the proposed
relationship is partially verified, otherwise, the proposed relationship can not be
sustained.

A very simple model is derived in order to show that when $w$ is increased, the
volume of AE activity is also increased, but AE events originating mostly further
apart reach a remote location out of phase. Consequently, although an increase in $w$
results in a proportional increase of AE activity at the generation source, at a
remote location, this increase in AE is not proportional due to the effect of
destructive phase interference; this explains the relationship between $I_\mr{power}$
and $w$.

By using the above-mentioned semi-empirical theory, a relationship of the type
$I_\mr{power} \propto \gamma^{n_1}\dot{\gamma}^{n_2}$ was derived, where $\gamma$
represents the total shear strain, $\dot{\gamma}$ represents the shear strain rate,
$n_1$ and $n_2
> 1$, and $n_1$ and $n_2$ are material independent, although the relationship could
only be validated for carbon steel materials. Moreover, $I_\mr{power}$ is observed to
decrease with cutting temperature if the workpiece temperature is externally raised
before cutting.

Finally, it is concluded that at the high strain rates and temperature encountered in
metal cutting, plastic deformation is governed by the motion of dislocations, which are
opposed by damping forces (proportional to dislocation speed) and also opposed by
forces due to dislocation obstacles. However, although theses two forces are
responsible for the dynamics of the motion of dislocations, it is concluded that only
the force opposed by the obstacles could provide the high frequency AE\@. According to
the proposed model, the following trends can be derived: $I_\mr{power}$ increases with
strain and strain rate, but it decreases with temperature; $f_\mr{mean}$ increases with
strain rate, decreases with temperature, and remains unchanged with strain. These
trends are confirmed by experimental results and also predictions from the
semi-empirical metal cutting theory.
