Geology Reference
In-Depth Information
Chapter 4
Mechanical Fundamentals
4.1 Introduction
There are some very broad distinctions that can usefully be made in classifying
types of mechanical behavior and the approaches to their study. The first of these
distinctions is between brittle and ductile behavior. We can define brittleness as
the liability to gross fracturing without substantial permanent change of shape in
response to loading beyond the elastic range. Conversely, ductility is the capacity
for substantial permanent change of shape without gross fracturing. In this context,
''gross'' means on the scale of the whole body or region under consideration and
the use of the terms brittle and ductile is only meaningful with proper reference to
scale. For the study of brittle behavior, see Jaeger (
1969
), Paterson and Wong
(
2005
), and Jaeger et al. (
2007
). In this chapter, we are mainly concerned with
ductile behavior or plastic deformation.
A second distinction is that between athermal and thermal regimes of behavior.
These terms refer to the degree of sensitivity of the flow behavior to change in
temperature and are to be regarded as relative only. Thus, in an athermal regime,
the behavior is relatively insensitive to change in temperature and can often be
treated as being independent of temperature, although temperature effects may not
be absent altogether. Also, athermal behavior cannot be associated uniquely with
relatively low temperatures and thermal behavior with relatively high tempera-
tures. Rather, there is a tendency for there to be three regimes of behavior:
1. Low-temperature thermal
2. Athermal
3. High-temperature thermal
These regimes are depicted schematically in Fig.
4.1
in a plot of the flow stress
against temperature (the flow stress is the stress needed to bring about plastic flow).
The scheme of Fig.
4.1
derives originally from the observed behavior of body-
centered cubic metals, but it seems to have a wider applicability (Di Persio and
