机械设计制造及其自动化英文

2021年1月25日发(作者：爱词霸词典)

英文原文
:

Mechanical properties of materials


The material properties can be classified into three major headings: (1) physical,
(2) chemical, (3) mechanical
Physical properties



Density
or
specific
gravity,
moisture
content,
etc.,
can
be
classified
under
this
category.


Chemical properties
Many
chemical
properties
come
under
this
category.
These
include
acidity
or
alkalinity, react6ivity and corrosion. The most important of these is corrosion which
can be explained in layman
’
s terms as the resistance of the material to decay while in
continuous use in a particular atmosphere.



Mechanical properties



Mechanical properties include in the strength properties like tensile, compression,
shear,
torsion,
impact,
fatigue
and
creep.
The
tensile
strength
of
a
material
is
obtained by dividing the maximum load, which the specimen bears by the area of
cross-section of the specimen.



This
is
a
curve
plotted
between
the
stress
along
the


This
is
a
curve
plotted
between
the
stress
along
the
Y-axis(ordinate)
and
the
strain
along
the
X-axis
(abscissa)
in
a
tensile
test.
A
material
tends
to
change
or
changes
its
dimensions
when
it
is
loaded,
depending
upon
the
magnitude
of
the
load.
When
the
load
is
removed
it
can
be
seen
that
the
deformation
disappears.
For
many
materials
this
occurs op to a certain value of the stress called the elastic limit Ap. This is depicted
by the straight line relationship and a small deviation thereafter, in the stress-strain
curve (fig.3.1)
. Within the elastic range, the limiting value of the stress up to which the stress and
strain are proportional, is called the limit of proportionality Ap. In this region, the
metal obeys hookes
’
s law, which states that the stress is proportional to strain in the
elastic
range
of
loading,
(the
material
completely
regains
its
original
dimensions
after
the
load
is
removed).
In
the
actual
plotting
of
the
curve,
the
proportionality
limit is obtained at a slightly lower value of the load than the


elastic limit. This may be attributed to the time-lagin the regaining of the original
dimensions
of
the
material.
This
effect
is
very
frequently
noticed
in
some
non-ferrous metals.


Which iron and nickel exhibit clear ranges of elasticity, copper, zinc, tin, are
found
to
be
imperfectly
elastic
even
at
relatively
low
values
low
values
of
stresses.
Actually
the
elastic
limit
is
distinguishable
from
the
proportionality
limit more clearly depending upon the sensitivity of the measuring instrument.


When the load is increased beyond the elastic limit, plastic deformation starts.
Simultaneously the specimen gets work-hardened. A point is reached when the
deformation starts to occur more rapidly than the increasing load. This point is
called they yield point Q. the metal which was resisting the load till then, starts
to deform somewhat rapidly, i. e., yield. The yield stress is called yield limit Ay.


The
elongation
of
the
specimen
continues
from
Q
to
S
and
then
to
T.
The



stress-strain relation in this plastic flow period is indicated by the portion QRST
of the curve. At the specimen breaks, and this load is called the breaking load.
The value of the maximum load S divided by the original cross- sectional area of
the specimen is referred to as the ultimate tensile strength of the metal or simply
the tensile strength Au.


Logically speaking, once the elastic limit is exceeded, the metal should start to
yield, and finally break, without any increase in the value of stress. But the curve
records an increased stress even after the elastic limit is exceeded. Two reasons
can be given for this behavior:


①
The strain hardening of the material;


②
The diminishing cross-sectional area of the specimen, suffered on account
of the plastic deformation.


The more plastic deformation the metal undergoes, the harder it becomes, due
to work- hardening. The more the metal gets elongated the more its diameter (and
hence,
cross- sectional
area)
is
decreased.
This
continues
until
the
point
S
is
reached.


After S, the rate at which the reduction in area takes place, exceeds the rate at
which
the
stress
increases.
Strain
becomes
so
high
that
the
reduction
in
area
begins to produce a localized effect at some point. This is called necking.


Reduction in cross-sectional area takes place very rapidly; so rapidly that the
load value actually drops. This is indicated by ST. failure occurs at this point T.


Then percentage elongation A and reduction in reduction in area W indicate
the ductility or plasticity of the material:


A=(L-L0)/L0*100%


W=(A0-A)/A0*100%


Where L0 and L are the original and the final length of the specimen; A0 and
A are the original and the final cross-section area.


The Two Types Of Power Transmission


In hydraulic power transmission the apparatus (pump) used for conversion of the
mechanical
(or
electrical,thermal)
energy
to
hydraulic
energy
is
arranged
on
the
input of the kinematic chain ,and the apparatus (motor) used for conversion of the
hydraulic energy to mechanical energy is arranged on the output (fig.2-1)


The theoretical design of the energy converters depends on the component of the
bernouilli equation to be used for hydraulic power transmission.


In
systerms
where,
mainly,
hydrostatic
pressure
is
utilized,
displacement
(hydrostatic)
pumps
and
motors
are
used,
while
in
those
where
the
hydrodynamic
pressure
is
utilized
is
utilized
gor
power
transmission
hydrodynamic
energy
converters (e.g. centrifugal pumps) are used.


The
specific
characteristic
of
the
energy
converters
is
the
weight
required
for
transmission of unit power. It can be demonstrated that the use of hydrostatic energy
converters for the low and medium powers, and of hydrodynamic energy converters
of high power are more favorite (fig.2-2). This is the main reason why hydrostatic
energy
converters are used in
industrial apparatus. transformation of the
energy in
hydraulic transmission.

1.

2.

3.

4.

5.

6.

7.

driving motor (electric, diesel engine);
mechanical energy;
pump;

hydraulic energy;


hydraulic motor;

mechanical energy;

load variation of the mass per unit power in hydrostatic and hydrodynamic energy
converters




1
、
hydrostatic; ynamic
Only
displacement
energy
converters
are
dealt
with
in
the
following.
The
elements
performing
converters
provide
one
or
several
size.
Expansion
of
the
working chambers in a pump is produced by the external energy admitted, and in the
motor by the hydraulic
energy.
Inflow of the
fluid occurs
during
expansion of the
working
chamber,
while the outflow
(displacement) is
realized during
contraction.
Such devices are usually called displacement energy converters.


The Hydrostatic Power


In order to have a fluid of volume V1 flowing in a vessel at pressure work spent
on compression W1 and transfer of the process, let us imagine a piston mechanism
(fig.2-3(a)) which may be connected with the aid of valves Z0 and Z1 to the external