15 Mechanical Properties Of Engineering Material

Mechanical Properties

Under the  action of various kinds of forces,  the
 behavior of the  material is studied that measures
the   strength and   lasting characteristic of a
 material in  service. The  mechanical
properties of materials are  of great industrial
importance in the  design of tools,  machines
and structures. Theses properties are  structure
sensitive in  the  sense that they depend upon
 the crystal structure and  its  bonding forces,
 and  especially upon  the  nature and
 behavior of the imperfections which- exist within the
 crystal itself  or at the  grain boundaries.
The  mechanical properties of the  metals are
 those which  are  associated with the
 ability of the  material to resist mechanical forces
and  load. The main mechanical properties of the metal are
 strength, stiffness, elasticity, plasticity, ductility,
malleability, toughness, brittleness, hardness, formability,
castability and  weldability. These properties can
 be well  understood with help  of tensile test
and  stress strain diagram. The  few important and
 useful mechanical properties are  explained below.


1.  Elasticity

It is defined as  the  property of a material to
regain its  original shape after deformation when the
 external forces  are  removed. It can
 also  be  referred as  the  power
 of material to come  back  to  its
 original position after deformation when the  stress
or  load  is  removed. It is  also
 called as  the  tensile property of the

2.  Proportional limit

It is  defined as  the  maximum stress under
which  a  material will  maintain a
 perfectly uniform rate of strain to  stress. Though
its  value is  difficult to  measure, yet
 it can  be  used as  the  important
applications for  building precision instruments,
 springs, etc.

3.  Elastic limit

Many metals  can  be  put   under stress
slightly above   the   proportional limit without
taking a  permanent set.  The  greatest stress
that a  material can  endure without taking up some
 permanent set  is  called elastic limit. Beyond
this limit, the  metal does  not  regain its
original form  and  permanent set  will

4.  Yield point

At  a  specific   stress,  ductile metals
 particularly  ceases, offering resistance  to
 tensile forces.  This  means, the  metals
flow and  a relatively large permanent set  takes
place  without a noticeable increase in load.  This
 point is called yield  point. Certain metals such
 as mild  steel exhibit a definite yield  point,
in which  case  the  yield  stress is
simply the  stress at this point.

5.  Strength

Strength is defined as the  ability of a material to
resist the  externally applied forces  with breakdown
or yielding. The  internal resistance offered  by a
material to an  externally applied force  is
 called stress.  The  capacity of bearing load
 by  metal and   to  withstand destruction
under the   action of  external loads   is
 known as  strength. The   stronger the  
material the greater the  load  it can
 withstand. This  property of material therefore
determines the  ability to  withstand stress without
failure. Strength varies according to  the  type
  of loading. It is always possible to assess tensile,
compressive, shearing and  torsional strengths. The
maximum stress that any  material can  withstand
before  destruction is called its  ultimate strength.
The tenacity of the  material is  its  ultimate
strength in  tension.

6.  Stiffness

It is defined as the  ability of a material to resist
deformation under stress. The  resistance of a material to
elastic deformation or deflection is called stiffness or
rigidity. A material that suffers slight or  very
 less  deformation under load  has  a
 high  degree of stiffness or  rigidity. For
 instance suspended beams of steel and  aluminium may
 both  be  strong enough to  carry the
 required load  but  the  aluminium beam
will  “sag” or deflect further. That means, the
 steel beam is stiffer or more  rigid  than
aluminium beam. If the  material behaves elastically with
linear stress-strain relationship under Hooks   law,
 its  stiffness is  measured by  the  
Young’s modulus of elasticity (E). The  higher is
 the  value of the   Young’s modulus, the
 stiffer is  the material. In  tensile and
 compressive stress, it is  called modulus of
stiffness or  “modulus of elasticity”; in  shear, the
 modulus of rigidity, and  this is  usually 40%
 of the  value of  Young’s modulus for
 commonly used materials;  in  volumetric
distortion, the  bulk  modulus.

7.  Plasticity

Plasticity is defined the  mechanical property of a
material which  retains the  deformation produced
under  load  permanently. This   property of
 the   material is  required in  forging,
in stamping images on  coins  and  in
 ornamental work.  It is  the  ability or
 tendency of material to  undergo some  degree
of permanent deformation without its  rupture or  its
 failure. Plastic deformation takes  place  
only  after the   elastic range of  material has
  been   exceeded. Such property of material is
important in  forming, shaping, extruding and  many
other hot  or cold working processes. Materials such
 as  clay,  lead, etc. are  plastic at room
 temperature and  steel is plastic at forging
temperature. This  property generally increases with
increase in temperature of materials.

8.  Ductility

Ductility is termed as  the  property of a material
enabling it to be drawn into  wire  with the
 application of tensile load.  A ductile material
must be strong and  plastic. The  ductility is
usually measured by  the  terms, percentage
elongation and  percent reduction in  area which is
 often  used as  empirical measures of
ductility. The  materials those possess more  than 5%
elongation are  called as ductile materials. The
 ductile material  commonly used in engineering
practice in  order of diminishing ductility are  mild
 steel, copper, aluminium, nickel, zinc,  tin and

9.  Malleability

Malleability is the  ability of the  material to be
flattened into  thin sheets under applications of heavy
compressive forces  without cracking by  hot  or
 cold  working means. It is  a  special
case   of  ductility which   permits materials
to  be  rolled or  hammered into   thin
 sheets. A malleable material  should be
 plastic but  it is  not  essential to
 be  so  strong. The  malleable materials
 commonly used in  engineering practice in
 order of  diminishing malleability are lead, soft
 steel, wrought iron,  copper  and
 aluminium. Aluminium, copper, tin,  lead, steel,
etc. are  recognized as  highly malleable metals.

10.  Hardness

Hardness is  defined as  the  ability of a
 metal to  cut  another metal. A harder metal
can always cut  or  put   impression to
 the   softer metals by  virtue of  its
 hardness. It is  a  very important property of
the   metals and   has   a  wide
 variety of meanings. It embraces many different
properties such  as  resistance to wear, scratching,
deformation and  machinability etc.

11.  Brittleness

Brittleness is  the   property of  a
 material  opposite to  ductility. It is
 the   property of breaking of a  material with
little permanent distortion. The  materials having less
 than 5% elongation under  loading behavior are
  said   to  be  brittle materials. Brittle
materials when subjected to  tensile loads, snap off
without giving  any  sensible elongation. Glass, cast
 iron, brass and  ceramics are  considered as
 brittle material.

12. Creep

When a metal part when is subjected to a high  constant
stress at high  temperature for a longer period of time,
it will undergo a slow and  permanent deformation (in form
 of a crack which  may  further propagate
further towards creep  failure) called creep.

13.  Formability

It is  the  property of metals which  denotes
the  ease  in  its  forming in  to
 various shapes and  sizes. The  different
factors that affect  the  formability are
 crystal structure of metal, grain size  of metal hot
 and  cold working, alloying element present in the
 parent metal. Metals with smal1 grain size  are
 suitable for shallow forming while  metal with size
 are  suitable for heavy forming. Hot  working
increases  formability. Low  carbon steel possesses
good  formability.

14.  Castability

Castability is  defined as  the  property of
metal, which  indicates the  ease  with it can
 be casted into  different shapes and  sizes.
Cast iron,  aluminium and  brass are  possessing
good castability.

15.  Weldability

Weldability is  defined as  the   property of
 a  metal  which indicates the   two
 similar or dissimilar metals are  joined  by
 fusion with or  without the  application of
pressure and  with or  without the   use  
of  filler   metal  (welding) efficiently.
Metals  having  weldability in  the descending
order are  iron,  steel, cast  steels and
 stainless steels.