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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.

CHART MECHANICAL Properties


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  material.

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  occurs.

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  lead.

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.

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