MECHANICAL PROPERTIES OF MATERIALS

MECHANICAL PROPERTIES OF MATERIALS

Some important mechanical properties are:

(i)      Strength (in tension, compression, shear, bending and torsion)

(ii)     Elasticity                     (iii)     Plasticity

(iv)     Ductility                     (v)      Brittleness 

(vi)     Malleability               (vii)    Impact strength

(viii)   Fatigue                        (ix)     Creep

(x)     Hardness.                     (xi) Stiffness

(i) Strength:- The strength of a  material is its ability to sustain loads without undue material should have adequate strength when subjected to tension, compression, shear, bending or torsion as per the intended use. For example, a beam of a building should have a proper bending strength, a column should have adequate compressive strength, a shaft of an automobile should have proper torsional strength. The maximum stress that any material will withstand is called ultimate strength or tenacity.

(ii) Elasticity:- It is the property of a material by virtue of which it regains its original size and shape after deformation when the loads causing deformation are removed.

(iii) Plasticity:- Lack of elasticity is called plasticity. The plasticity of a material is the ability to change its shape without destruction under the action of external loads and to regain the shape given to it when the forces are removed.

(iv) Ductility :- t is the property of a material to undergo a considerable deformation under tension before rupture A body possessing ductility can be reduced from large sections to thinner and thinner sections i.e. it can be drawn wires. This is a tensile quality of a material.

The usual measures of ductility are the percentage elongation in gauge length of the test piece upto fracture percentage reduction in cross-sectional area upto fracture in the tension test.

In a laboratory, in addition to tension test, cold bend test is also performed to measure the ductility of metals. In this test, ductility is judged by non-cracking of metals under the conditions of the test. Generally, mild steel and tor steel bars of different diameters are tested for ductility. If the cracks are observed on the bend portion, the bar is said to have lost its ductility.

(v) Brittleness:- Lack of ductility is called brittleness. The brittleness ofa material is the property of breaking, fracturing9 or shattering without prior warning or without much permanent distortion under load. There are many materials which shatter before much deformation takes place. e.g. cast iron, glass, concrete, stone, etc. These materials are suitable for resisting compressive loads but usually less suitable for resisting tensile and impact loads. The compression test is generally performed for testing the brittleness of a material. Brittleness is a compressive quality of a material.

(vi) Malleability:- It is the property of a material by virtue of which it gets permanently deformed by compression without rupture. It is the ability of a material to be rolled or beaten up into thin sheets without cracking by rolling and hammèring. This is also a compressive quality of a material. Gold is the most malleable of all metals. Silver, aluminium, copper, tin are also malleable materials.

(vii) Impact Strength:- The amount of shock energy absorbed by a specimen before it fractures is called its impact strength or toughness. Izod and Charpy impact tests are generally conducted to measure the toughness of a material. The energy required to break the given specimen is measured in joules (N - m). (vii) Hardness: The ability of a material to resist wear, abrasion, scratching,or indentation (penetration) by harder bodies is called hardness. Tests such as Brinell, Rockwell, Vickers are generally performed to measure the hardness of a material.

(ix) Fatigue :- Some of the machine parts such as axles, shafts, springs, connecting rod, pinion teeth, etc. vibrate as they run and this is known as repetition of loading. A material may fail under fluctuating or repeated loads (or stresses) even though. the maximum applied stress is considerably less than the tensile strength of the material under steady loads. This phenomenon of failure of a material under fluctuating or repeated loading is called fatigue or endurance. Fatigue fracture is progressive, and starts as minute cracks at the centres of stress concentration within the material or on the surface. These cracks go on extending more and more under the action of the fluctuating stresses causing the failure.

The maximum stress that a material can sustain without failure for a specific large number of cycles of stresses is known as its fatigue value or endurance limit.

(x) Creep :- Many structural members and machine parts sustain steady loads for long periods of time. For example, beams in a R.C.C. building, plastic mountings for the parts of electronic devices, blades of turbine rotor, etc. Under such conditions the material may continue to deform and will ultimately break. Creep continues as long as the load is applied. Therefore, it is a time dependent phenomenon. The greater the time, the more will be the creep. The continuous deformation with time which the material undergoes due to application of external steady loads is called creep or time-yield or plastic flow.

(xi) Stiffness:- The ability of a material to resist elastic deformation is called stiffness. Mathematicaly, it is the load required to produce unit deformation. A material which deforms by a lesser amount under a given load possesses a high degree of stiffness. For identical cross-sections, the stiffness is proportional to the modulus of elasticity. Mechanical properties of materials define the behaviour of materials under the action of applied forces, called loads. These are usually expressed in terms of stress, strain or both. The knowledge of mechanical properties of materials is very essential to construct a mechanically sound, stable and durable structure. These properties can be determined by conducting laboratory tests on the material specimen.