MATERIALS-notes2

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29 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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MATERIALS


by

Ikmalzatul Abdullah

THERMAL MOVEMENT


Thermal properties


connection with heat transfer
and heat loss.


Thermal movement is caused by the expansion or
contraction of materials.


Materials generally expand when heated and
contract when cooled.


Problems arise related to the thermal movement:


Large walls and glass in windows may crack
due to this process if they are restrained.


The actual amount of thermal movement depends
on the type of materials used.

MOISTURE CONTENT & WATER
ABSORPTION


Many building materials contain water or
moisture to a greater or lesser extent.


The moisture may be the remainder after
the material was prepared and used or it
can occur within the substance naturally.


Inappropriate water content causes many
problems. (e.g: timber)

Cont’d


Some materials do not absorb or give
out any moisture (e.g: bituminous
materials, metals and most plastics)


These materials frequently used for
water proofing, as vapor barriers to stop
the penetration of water vapor and for
damp
-
proof courses respectively.


Cont’d


The actual level of moisture within a given
material depends also on the temperature
and relative humidity of the surrounding
air.


In damper environments moisture levels
in materials are higher than in drier
environments.


The moisture content can also be greatly
modified depending on what the material
is in contact with.


POROSITY


As the void content of a material increases, so
does the ability to absorb water by capillary
action.


The ability to absorb moisture depends on the
size of the voids and how accessible the voids
are to the water at the surface.


Generally, there is no direct correlation
between porosity and other properties such as
durability.


Porosity = volume of pores / bulk volume x
100


Porosity = (solid density


bulk density) /
solid density x 100

ELASTICITY


Elasticity is the forces cause the material in a
building to squash, stretch and bend.


The forces frequently caused by gravity.


Squashing forces = compressive


Stretching forces = tensile


The columns / walls / foundations holding a
building up are in compression while the wires
holding up a suspended walkway are in
tension.


Beams have bending forces acting on them and
are partly in compression and partly in tension.


Cont’d


Some materials behave well in tension while
others perform better in compression.


Most materials have elastic behavior up to a
certain level of applied force.


Glass, steel, iron, timber are examples of
materials which exhibits elastic behavior to a
greater or lesser extent.


Fresh putty and sealants do not have any
elastic properties.


Elastic behavior is important because it is
predictable.


STRAIN


The strain measures the change in length of a
material compared with the original length.


A measure of the deformation of a material.


A material in tension has tensile strain and in
compression it has compressive strain.

FORCE


Measured in Newton (N).


Can be applied in any direction.


Gravity causes vertical forces to act.


The force of gravity acts on all objects.


Force = mass x gravity



STRESS


A measure of the force causing deformation.


Stress in the ‘concentration’ of the applied
force.


Stress is very important concept.


Suppose a given load bearing column is at the
maximum allowed stress.


For a given stress the load is directly
proportional to its cross
-
sectional area.


Stress = force / cross sectional area

YOUNG MODULUS


If a force is applied to a given sample
material, the strain can be measure as the
stress is increased.


Young modulus = stress / strain


Also known as the elastic modulus.


A measure of the stiffness of the material or it
resistance to squashing or stretching.


Unit = N/mm2

STRENGTH OF MATERIALS


Yield stress is the stress at which the material
starts to change from elastic to plastic behavior.


Yield stress = force at yield point / original
area



Breaking stress = the stress at the fracture, that
is at the point the material breaks.


Breaking stress = force at fracture / original
area


Cont’d


Ultimate stress = the greatest value of stress
the material can withstand before failure.


Ultimate stress = maximum force / original
area


Yield stress, breaking stress and ultimate
stress are all measured N/mm2 or N/m2.


These are very important quantities for
building engineers.


These are all measures of the tensile strength
of materials.


Compressive strength = force applied at
failure / area of specimen




OTHER RELEVANT MECHANICAL PROPERTIES


Materials which have substantial plastic
phase when tensile forces are applied are
said to be ductile.


Ductility = also the ability of a material to
be drawn into wires.


For engineering purposes, ductility is an
important consideration.


The property of ductility allows loaded
materials to redistribute localized stresses
which may build up at the position of
notches.

Cont’d


Localized stresses may build up as
there is no yielding and continue to do
so until failure suddenly occurs.


Hardness is the resistance of a material
to becoming indented.


Hardness is an important factor to
consider for the construction of floor and
walls.



Cont’d


For metals, hardness is determined by
measuring the resistance offered by the
material to the penetration of either a hardened
steel ball or a diamond into its surface under
standard loading conditions.


Tough materials can easily absorb energy
impact.


For example, ceramic materials tend to be
very strong in compression but they are not
tough.


They are easy to break by sudden blows.


ELECTRICAL INSULATION


Materials which are good electrical
conductors can pass large electric current.


A very good electrical conductor is copper.


Copper wires have very low electrical
resistance and are frequently used
power cables, telephone wires, etc.


Aluminum too, has long been in use for
electrical conductors.


Cont’d


At the other extreme, plastics are very poor
conductors


to be electrical insulators and
have very high resistances.


Plastics are used to electrically insulate
conducting wires and electrical fittings
inside buildings.


The ability of a given type of material to
oppose the flow of an electric current is called
the electrical resistivity.



Cont’d


Refer to the table given.


From the table, metals have very low
resisitvity.


Most non
-
metals have very high resistivities.


Metals are very good conductors while non
-
metals are very good insulators of electricity.


Electrical resisitivity = (area of cross section x
resistance) / length

CHEMICAL PROPERTIES


Besides the physical properties, chemical
properties are also one of the important
properties to be discussed.


In a chemical reaction, the atoms or molecules
of two or more substances combine irreversibly
to form a new material.


Metals suffer from various chemical reactions
which cause corrosion.


These reactions result in degradation of the
material.

OXIDATION


A common chemical reaction between oxygen
and many metals.


This reaction often causes damage to the
building material.


Examples of the oxidation process:


Unprotected iron and steel will react with
oxygen in air only in the presence of
water:


Iron + oxygen = iron oxide (rust)


The water is essential in this reaction to
take place.


The reaction is speeded up when salts are
present.

Cont’d


The rust formed flakes away from the
remaining metal and not only spoils the
appearance but more importantly reduces
the strength of the structure.


Where moisture is present, it is important
to protect the steel from rusting.


Unlike rust, aluminum oxide sticks to the
surface of the metal and protects the
material from further oxidation.

Cont’d


Nevertheless, aluminum is usually
anodized.


In this process, the natural oxide film
is thickened. This increases the
resistance to degradation.


The process of anodizing also
improves the appearance of the
surface.


ELECTROLYTIC CORROSION


If two different metals are joined together, for
example in plumbing, electrolytic corrosion may
occur if precautions are not taken.


In this electrical and chemical process, one of
the two metals is gradually ‘eaten away’ by
corrosion.


For any two metals joined together, it is easy to
find out which of them will corrode.


When two different metals are connected
together, the one having the more negative
electrode potential has an increased willingness
to supply electrons.

CORROSION PROCESS


The corrosion process can be accelerated
further:


As the salt or acid concentration in the
solution is increased


If the area of the anode is small
compared with the cathode


If there is an increase in temperature of
the solution