Assignment 2.6.1
1. Define the following mechanical properties of solids:
(i) elasticity, (ii) plasticity, (iii) strength, (iv) ductility, (v) hardness, (vi) brittleness, (vii) toughness, (viii) stiffness, (ix) resilience, (x) malleability, (xi) fatigue, and (xii)creep
Answers
Strength
It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress.
Stiffness
Stiffness is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.
Elasticity
It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines.
It may be noted that steel is more elastic than rubber.
Plasticity
Plasticity is a property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.
Ductility
Ductility is the property of a material enabling it to be drawn into a wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice are mild steel, copper, aluminum, nickel, zinc, tin and lead.
Brittleness
It is the property of breaking of a material with little permanent distortion. Brittleness of a material is opposite to ductility property.
Brittle materials are withstanding compression load. When subjected to tensile loads snap off without giving any sensible elongation. Cast iron is a brittle material.
Malleability
It is a special case of ductility which permits materials to be rolled or hammered into thin sheets, making wire. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice are lead, soft steel, wrought iron, copper, and aluminum.
Toughness
Toughness is the property of a material to resist fracture due to high impact. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture.
This property is desirable in parts subjected to shock and impact loads. Normally the toughness of the material decreases when it is subjected heat.
Machinability
It is the property of a material which refers to a relative ease with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. For example, that brass can be easily machined than steel. That means the machinability property of brass is high when compare to steel.
Resilience
It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for designing the spring materials.
Creep
When a material is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers, and turbines.
Fatigue
Fatigue is the repeated loading and unloading of metal due to direct load variation, eccentricity in a rotating shaft and differential thermal expansion of a structure. Even substantially below the yield point (elastic limit) of a metal or alloy this repeated loading can lead to failure, usually measured in terms of the number of cycles (repeated load applications) to failure.
Some studies have suggested that well over 80% of all mechanical failures of metal are attributable to fatigue.
Hardness
Hardness is a very important property of the metals and has a wide variety of meanings. It also embraces many different properties such as resistance to wear, scratching, deformation and machinability etc.
Also, it is the property of a metal, which gives it the ability to resist being permanent, deformed (bent, broken, or have its shape changed) when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.
It also means that the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the test.
They are four types of tests are used to determine the hardness of metals, they are
Brinell hardness test,
Rockwell hardness test,
Vickers hardness test,
Shore scleroscope.
Conclusion
By understanding these basic 13 Mechanical Properties Of Material one can able to select a correct material for the specific application.