-
英文原文：
1. Introduction
to Mechanics of Materials
Mechanics
of
materials
is
a
branch
of
applied
mechanics
that
deals
with
the
behavior of solid bodies
subjected to
various types of loading.
It
is
a field
of study
that is known by a
variety of names, including “strength of
materials” and “mechanics
of
deformable
bodies.”
The
solid
bodies
considered
in
this
book
include
axially
-
loaded
bars,
shafts,
beams,
and
columns,
as
well
as
structures
that
are
assemblies
of
these
components.
Usually
the
objective
of
our
analysis
will
be
the
determination of the
stresses, strains, and deformations produced by
the loads; if these
quantities can be
found for all values of load up to the failure
load, then we will have
obtained a
complete picture of the mechanical behavior of the
body.
Theoretical analyses
and experimental results have equally important
roles in the
study of mechanics of
materials. On many occasions we will make logical
derivations
to obtain formulas and
equations for predicting mechanical behavior, but
at the same
time we must recognize that
these formulas cannot be used in a realistic way
unless
certain properties of the
material are known. These properties are available
to us only
after suitable experiments
have been made in the laboratory. Also, many
problems of
importance
in
engineering
cannot
be
handled
efficiently
by
theoretical
means,
and
experimental
measurements become a practical necessity. The
historical development
of mechanics of
materials is a fascinating blend of both theory
and experiment, with
experiments
pointing
the
way
to
useful
results
in
some
instances
and
with
theory
doing so in others. Such famous men as
Leonardo da Vinci(1452
-
1519)
and Galileo
Galilei(1564
-
1642)
made
experiments
to
determine
the
strength
of
wires,
bars,
and
beams, although they did not develop
any adequate theories (by today’s standards) to
their test results. By contrast, the
famous mathematician Leonhard
Euler(1707
-
1783)
developed
the
mathematical
theory
of
columns
and
calculated
the
critical
load
of
a
column
in
1744,
long
before
any
experimental
evidence
existed
to
show
the
significance of his results. Thus,
Euler’s theoretical results remained unused for
many
years, although today they form
the basis of column theory.
The
importance
of
combining
theoretical
derivations
with
experimentally
determined
properties of materials will be evident as we
proceed with our study of the
subject.
In this section we will begin by discussing some
fundamental concepts, such
as
stress
and
strain,
and
then
we
will
investigate
the
behavior
of
simple
structural
elements subjected to tension,
compression, and shear.
2.
Stress
The
concepts
of
stress
and
strain
can
be
illustrated
in
an
elementary
way
by
considering
the
extension
of
prismatic
bar.A
prismatic
bar
is
one
that
has
constant
cross
section
throughout
its
length
and
a
straight
axis.
In
this
illustration
the
bar
is
assumed to
be loaded at its ends by axial forces P that
produce a uniform stretching,
or
tension, of the bar. By making an artificial cut
(section mm) though the bar at right
angels to its axis, we can isolate part
of the bar as a free body. At the
right
-
hand end
the
tensile
force
P
is
applied,
and
at
the
other
end
there
are
forces
representing
the
removed
portion
of
the
bar
upon
the
part
that
remains.
These
forces
will
be
continuously
distributed
over
the
cross
section,
analogous
to
the
continuous