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Thursday, 22 October 2015

PROPERTIES OF MATERIALS

                     Properties of materials include various types of properties like mechanical properties (such as strength, hardness, and toughness), thermal properties (conductivity), optical properties (refractive index), and electrical properties (resistance) etc. Here, however, we shall concentrate only on mechanical properties which are most important in manufacturing process and also in everyday life and we use these terms quite often. To understand the mechanical properties, it is useful to first understand the behavior of the material when subjected to a force which causes deformation.

The mechanical properties of metals are:
  1. Strength
  2. Elasticity
  3. Plasticity
  4. Ductility
  5. Malleability
  6. Toughness (or tenacity)
  7. Brittleness
  8. Hardness
  9. Fatigue
  10. Creep
  11. Stiffness and resilience
  12. Endurance

(1)STRENGTH:

                     The strength of metal is its ability to withstand various forces to which it is subjected during a test or in service. It is usually defined as tensile strength, compressive strength, proof stress; shear strength etc. strength of material is a general expression for the measure of capacity of resistance processes by solid masses or pieces of various kinds to any cause tending to produce in them a permanent and disabling change of form or positive fracture.
Accordingly, one may broadly classify strength into following two categories:

        a) depending upon the value of stress, the strengths of a metal may be Elastic or Plastic.
       b) Depending upon the nature of stress, the strength of a metal may be tensile, compressive, shear, bending and torsional. Now, we study all these types of strengths.

>> Read more about strength

(2)ELASITICITY:

                     A material is said to be perfectly elastic if the whole of the stress produced by a load disappears completely on the removal of load. However, in nature no material is perfectly elastic, i.e., a certain limit exists for every material beyond which it will not be able to regain its original shape and size. This limit is termed as elastic limit. The modulus of elasticity or young's modulus (E) is the proportionality constant between stress and strain for elastic materials. Young's modulus is the property called stiffness; small values of E indicates flexible materials and large value of E reflect stiffness and rigidity.

The property of spring back is a function of modulus of elasticity.

TABLE-- MODULUS OF ELASTICITY OF SOME IMPORTANT METALS           


(3)PLASTICITY:

              Plasticity is the property that enables the formation of permanent deformation in a material. It is reverse of elasticity; a plastic material will retain exactly the shape it takes under load, even after the load is removed. It is of importance in deciding manufacturing process like forming, shaping, extruding operations etc. Metals possess more plasticity at high temperatures. Usually, plasticity of a material increases  with increase in temperature and this is important in deciding the further operations.

                   Gold and lead are the highly plastic materials. Plasticity is used in stamping images on coins and ornamental work.

(4)DUCTILITY:

                    This property relate to the plasticity of materials. Ductility refers to plastic deformation under tensile load; it is the ability of a metal to withstand elongation or bending. due to this property, wires are made by drawing out through a hole. The material shows a considerable amount of plasticity during the ductility extension. This is a valuable property in chains, ropes etc.

            A measure of ductility is "percentage elongation". Before the tensile test begins two punch marks are made on the stem of the tensile test piece. Distance between these marks is noted and is known as gauge length (l0). After the tensile test piece fractures in two pieces, the two pieces are retrieved and placed together as close to each other as possible. Now the distance between the two punch marks is measured and noted again. Let this distance be l1. the % elongation is calculated as *100.

                    High values of percentage elongation indicate that material is very ductile. Low values indicate that material is brittle and has low ductility. For mild steel, the percentage elongation usually is 20% or more. 

(5)MALLEABILITY:

                This Property relate to the plasticity of the material. Malleability refers to the ability of plastic deformation under compressive loads. This is the property by virtue of which a material may be hammered or rolled into thin sheets without rupture. This property generally increases with the increase of temperature. The metals in order of their ductility and malleability (at room temperature) are given below. 

TABLE-- COMMON METALS IN ORDER OF THEIR DUCTILITY AND MALLEABILITY


(6)TOUGHNESS:

                   Toughness ( or tenacity ) is the strength with which the materials opposes rupture. It is due to the attraction which the molecules have for each other; giving them power to resist tearing apart.
It is expresses as work units/unit volume, i.e., kg fm/m3

(7)BRITTLENESS:

           Lack of ductility is called brittleness. When a body breaks easily when subjected to shocks it is said to be brittle.

              It is a property which is possessed in great measure by glass and other ceramics. A piece of glass if dropped on a hard surface shatters and is broken in many pieces. The real cause of brittleness is inability of the material to withstand shock loads. Of course, glass is an extreme case of brittle material.

(8)HARDNESS:

                    Hardness is generally defined as resistance of material to penetration. Hard materials resist scratches or bending worn out by friction with another body.

                          Hardness is primarily a function of the elastic limit (i.e., yield strength) of the material and to a lesser extent a function also exerts a slight effect on hardness. Lack of hardness is called softness.

(9)FATIGUE:

                    When subjected to fluctuating or repeating loads (or stress), materials tend to develop a characteristic behavior which is different from that (or materials) under steady loads. Fatigue is the phenomenon that leads to fracture under such conditions. Fracture takes place under repeated or fluctuating stresses whose maximum value is less than tensile strength of the materials (under steady loads).

(10)CREEP:

                         Creep is the slow plastic deformation of metals under constant stress or under prolonged loading usually at high temperature. It can take place and lead to fracture at static stress much smaller than those which will break the specimen by loading it quickly. Creep is specially taken care of while designing I.C E ngines, boilers and turbines.

(11) STIFFNESS AND RESILIENCE

                        A material with high value of modulus of elasticity is said to be stiff and a material with low value of modulus of elasticity is said to be resilient. Consider a material undergoing tensile stress within the elastic range. If the material possesses a high value of Young's modulus (which is the modulus of elasticity corresponding to tensile stress), the material will not stretch much. It will behave as a "stiff" material. In this case, the slope of the line will be more. Resilience is a property which is totally opposite to stiffness. A beam made of stiff material will deflect to a lesser extent as compared to another made of resilient material under identical loading condition.

(12)ENDURANCE

                  This is defined as the property of a metal by virtue of which it can withstand varying stresses (same or opposite nature). The maximum value of stress, which can be applied for an indefinite times without causing its failure, is termed as its endurance limit. For ordinarily steel, the endurance limit is about half the tensile strength.

                  This property of a metal is of great importance in the design and production of parts in a reciprocating machines and components subjected to vibrations. It is always desirable to keep the working stress of material well within the elastic limit. 

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