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Document #1
MECHANICAL PROPERTIES OF MATERIALS
§1 The quality that is required from any material depends on its use: it's
obvious that wires and springs can't be manufactured out of the same metal as
the former must possess ductility and the latter elasticity. Aluminium will
certainly not do for a drill but is advisable for electricity. Thus a designer
must estimate the loads that a piece will carry and determine which material,
shape and size is best suited.
§2 External forces can effect solid materials in different ways, but basically
their effects can be divided into three categories: compression, tension and
shear. A material is said to be stressed when an external force tends to cause
it to change shape. Sometimes the force applied is so strong that the shape is
actually permanently altered: the piece is said to be strained i.e. distorted.
§3 Tests performed on specimens enable to determine their mechanical properties.
Hardness, for instance, is measured by applying a constant vertical load onto a
test area (Brinell test). Tensile tests are carried out by exerting a
progressively increased increased load to a specimen, until elongation and
finally fracture happens. It's also possible to determine how tough a material
is by means of an Izod test : a pendulum hits the specimen with a given kinetic
energy at a given speed, and the energy absorbed in bending the specimen
indicates its toughness.
§4 The natural properties of materials can sometimes be widely altered by
various treatments. Thus corrosion, which attacks metals in particular, can be
reduced by applying paint or by electrical treatment such as electro-plating.
Metallic elements can also be added so as to improve the original properties:
nickel added to steel increases toughness and ductility, whereas brass (a copper
and zinc alloy) is much stronger than copper though less ductile.
§5 The properties of ferrous metals depend on the amount of carbon that they
contain. Cast iron contains approximately 3% carbon whereas in steel the
proportion ranges from 0.1 to 1.5%. Depending on the exact percentage of carbon
and of various other metals which have been added, alloy steel can exhibit such
different properties as ductility, hardness, elasticity or plasticity.
§6 Moreover, heating metals, either during or after shaping, provides them with
new physical or mechanical properties: atoms are arranged as 3-D shapes ( called
crystals) which determine the properties. When heated, the structure of some
crystals (especially iron) is changed, thus resulting in different properties.
The basic operation consists in heating steel (for instance) up to a determined
temperature and cooling it down more or less slowly depending on the result to
be achieved.
COMPREHENSION : answer the questions
1 - When choosing a material, what must a designer always think about ?
-According to the text, which properties should wires possess ? What about
springs ?
-What can aluminium be used for ?
-As a result, how does a designer choose a material ?
2 - What are the effects of external forces on materials ?
-What is the definition of 'stress' ?
-What has caused a piece to be strained ?
3 - List the tests mentioned in the paragraph.
-What are these tests for ?
4 and 5 - What can you say about the following summary ?
Various techniques enable us to alter a material properties: painting or plating
develop corrosion resistance, as well as alloying which also increases toughness
and ductility. Ferrous metals may contain various proportions of carbon, which
gives them varying qualities.
6 - How can metals be supplied with new properties ?
-Which basic operations must then be carried out ?
-Explain briefly the relationship between the properties and the chemical
structure of a metal.
LANGUAGE STUDY
1 - Definitions
*Toughness can be defined as the ability to resist / withstand repeated bendings
(adjective : tough).
*A material which can be easily drawn into wires exhibits / possesses the
property of ductility (adjective : ductile).
*A material which easily breaks when subjected to impacts or elongation is said
to be brittle (noun : brittleness).
Using the same structures, make definitions and give the corresponding noun or
adjective.
-malleability - can be easily given a new shape by hammering
-hardness - resistance to scratching or indentation
-elasticity - ability to return to its original shape
-plasticity - does not return to its original shape
-stiff (adjective) - resistance to deflection, in the case of springs
-density - ratio between mass and volume
-fusiblility - the material can easily melt
2 - Look at the following table.
Temperature Cooling New properties
annealing 800 C very slow ductile +
soft +
tempering 250 C quenching
= immediate
cooling in oil
or water tough +
ductile+
hard -
hardening 800 C hard +
normalizing 800 C slow hard +
ductile -
a/ When /if normalized, steel acquires / gains increased hardness and decreased
ductility.
Make similar sentences with the other three operations.
b/ annealing :
* after the metal is heated (= after heating the metal) up to 800 C, it is very
slowly cooled down.
* before the metal is very slowly cooled down (= before very slowly cooling the
metal down), it is heated up to 800 C.
Make similar sets of sentences for each of the other three operations.
3 - Transform as in the example :
If we add chromium to steel, corrosion-resisting properties are improved. -->
Adding chromium to steel results in / brings about improved corrosion-resisting
properties.
-If we add nickel to steel, ductility and toughness are increased.
-If we quench a piece, hardness and strength are improved.
-If we anneal steal, ductility is incresaed.
-When we use tin to coat another metal, corrosion-resisting properties are
enhanced (=improved).
-If we vary the proportion of carbon , steel will possess quite different
properties.
4 - Transform as in the example :
If you add chromium TO steel, it'll resist corrosion. -->Steel TO WHICH chromium
is added will resist corrosion.
-If you incorporate chromium INTO steel, it'll withstand corrosion. --> Steel...
-If you apply a high load TO a piece, it'll break. --> a piece...
-If a rivet is submitted TO a shear force, the latter can break the rivet. --> a
shear force ...
-If you exert a tensile force ON an elastic material, it'll return to its
original shape. --> an elastic material ...
5 - Fill in the blanks with a preposition (or [Ø])
Brass is made ... zinc and copper. Painting iron prevents it ... rusting. The
properties of steel depend ... the amount of carbon it contains . Quenching
consists ... cooling metals in water . Cast iron is not capable ... resisting
impacts. Copper can be drawn ... wires . When heated, steel is provided ... new
properties. Painting steel helps it to resist ... corrosion. When a material is
subjected ... a tensile load, it tends to extend. Adding certain properties ...
steel or removing some ... it may be done by heating it.
6 - Turn into the passive - See Figure 1
(Note: don't forget to download figure 1. See below.)
If two forces act on body, each force exerts an influence on it. Each separate
effect produces a total effect which we can represent as a vector: we call this
vector the'resultant'. Thus we can draw a parallelogram, in which the diagonal
represents the resultant of the two forces. If we add E to the system, it
produces equilibrium, so that we say E is the equilibrant.
7 -a/ Look at Fig. 2 and write a description of the steel tensile test. You may
use the following expressions:
as the applied force increases - a rise in the extension - proportional to -
return to its original shape - if the load is removed - up to point M - general
extension continues until - phenomenon called 'waisting' ( cross-sectional area
becomes narrower) - notice that the load - elongation continues until - break at
a load of x KN.
b/ write a similar descrption of the copper tensile test.
(Don't forget to download figures 1 and 2 (just below). Click on the right
mouse-button and choose 'save' in the menu.)
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