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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 ofthe 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

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 making an artificialcut (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