Q.1 What do you understand by properties of material.
Ans. Properties Of Material: -
As we know that different materials possess different properties varying degrees and, therefore, behave in different ways under given conditions. These properties include mechanical properties, electrical properties, thermal properties, chemical properties, magnetic properties and physical properties. Ans. Properties Of Material: -
(i) Physical Properties : This property include shape, size, colour, lusture, porosity, finish, specific gravity, structure etc.
(ii) Chemical Properties : This property include atomic weight, molecular weight, atomic number, acidity, alkalinity, chemical composition etc.
(iii) Thermal Properties : This property include specific heat, thermal conductivity, thermal expansion, latent heat, thermal stresses, thermal shock etc.
(iv) Mechanical Properties : This property include elasticity, plasticity, ductility, brittleness, malleability, hardness, toughness, stiffness, fatigue, creep, strength etc .
(v) Magnetic Properties : This property includes magnetic dipole, hysteresis loop etc.
(vi) Electrical Properties : This property include conductivity, resistivity, dielectric, strength, capacitance etc.
Q.2 What do you mean by mechanical properties? Explain in brief their importance in the design of a machine or structure?
Related Questions-
Q. Compare between Hardness and toughness. (AKTU - 2011 - 12)
Q. Differentiate between creep and fatigue failures. (AKTU - 2012 - 13)
Ans. Mechanical properties of materials define the behaviour of materials under the action of external loads or forces. Mechanical properties are related to elastic and plastic behaviour and also to overall strength and fracture phenomenon of materials. These properties are structure sensitive in the sense that they largely depend on the crystal structure and its bonding forces. Common mechanical properties of metals are strength, stiffness, malleability, elasticity, ductility, brittleness, plasticity, machinability, toughness, creep, fatigue, etc.
(a) Strength: -
Strength of a material is a measure of its capacity to withstand destruction under the effect of external loads or forces. It determines the ability of a material to withstand external forces or loads without rupture. Strength varies according to type of loading. It can be classified as tensile strength, compressive strength, shearing or torsional strength,
It is defined as the maximum stress that any material can withstand before rupture or destruction.
(c) Elasticity: -
It is defined as that property of material by virtue of which deformation caused by externally applied load disappear after removal of the load. In other words, elasticity of a material is its capacity to regain its original shape and size after removal of external load. Elasticity is a tensile property of the material.
(d) Stiffness: -
The resistance of a material to elastic deformation is called rigidity or stiffness. A material which is slightly deformed under external load has a high degree of stiffness and vice-versa. E.g. steel beam is more stiff than aluminium beam, because aluminium beam will sag or deflect more than steel beam under same load. In tensile or compressive stress, it is called “Modules of stiffness” or “Modulus of elasticity”. In shear it is called “Modulus of rigidly”. The opposite of stiffness is flexibility.
(e) Plasticity: - (AKTU - 2010 - 11)
Plasticity of a material is the property by virtue of which it undergoes permanent deformation without failure or destruction. It takes place only after elastic range has been exceeded.
Plasticity is important in shaping, forming, extruding and hot and cold working process. Clay, lead, etc are plastic at room temperature. Steel is plastic at high temperature. Plasticity, generally increases with temperature.
(f) Malleability: - (AKTU - 2010 - 11)
Malleability of a material is its ability to be deformed into thin sheets without cracking either by hot working or cold working.
Aluminium, Copper, Gold, Tin, Steel have this property. Malleability is considered as a compressive quality.
(g) Ductility: -
It is defined as the property of a material by virtue of which it can be drawn into thin wire without rupture. Mild steel is a ductile material. Percent elongation and reduction in area in tension are used as measures of ductility. Ductility is considered as a tensile quality. Silver, copper, gold, aluminium, tin are some common ductile materials.
(h) Resilience: -
Resilience is the capacity of a material to absorb energy elastically under the action of external load. When the load is removed, stored energy is given off. It is measured by the amount of energy that can be stored per unit volume when material is stressed up to its elastic limit.
Resilience gives capacity to the material to withstand shocks and vibration.
Maximum energy which can be stored in a body up to elastic limit is called “Modulus of resilience”.
(i) Toughness: - (AKTU - 2010 - 11)
It is defined as that property of a material by virtue of which it can absorb maximum amount of energy before actual fracture or failure takes place. Toughness of a material is its ability to withstand both elastic and plastic deformations. Toughness is related to Impact Strength i.e. resistance to shock loading. Manganese steel, wrought iron are tough materials.
(j) Hardness: - (AKTU - 2010 - 11)
It is the ability of a material to resist scratching, abrasion, cutting, indentation or penetration. It is closely related to strength. Tests which are performed to determine hardness of a material are Brinell, Rockwell and Vickers hardness tests.
(k) Creep: -
The slow and progressive deformation of a material under steady load (stress) with time is called creep. In other words, it is a permanent deformation under static loading of materials over long period of time. It is also defined as time-dependent strain occurring under steady load (stress). Lead, rubber, plastics, leather, zinc exhibit creep at high temperatures. There are three stages of creep :
Stage 1 : Primary Stage : It takes place as very low temperatures. It is, therefore, also called cold creep.
Stage 2 : Secondary Stage : It takes place at high temperature and rate of elongation is some-what linear (constant). It is also called hot creep.
Stage 3 : Tertiary Stage : Rate of elongation increases rapidly until the material fails. In this stages straining is very fast because of neck formation in the material.
(1) Fatigue: - (AKTU - 2010 - 11)
The behavior of a material under cyclic (fluctuating) and reverse loading or stresses is called fatigue. Fatigue properties of a material determine its behavior when subjected to cyclic loads in which maximum stress developed in each cycle is within the elastic range of the material. Nearly 90% mechanical fractures are due to fatigue.
Aeroplane wings, leaf springs, turbine engines, connecting rod in internal combustion engines, rubber tyres, etc are subjected to fluctuating or cyclic loads.
(m) Fatigue Strength: -
Maximum stress that a material can withstand without failure for a specific large number of cycles of stress is called its fatigue or endurance strength (limit). This strength is 0.3 to 0.5 times the ultimate strength of materials.
(n) Brittleness: -
The property of fracturing of a material without perceptible warning or without appreciable deformation is called brittleness. This property is opposite to ductility. Materials having percent elongation less than 5% may be considered to be brittle. Brittleness is an undesirable property of materials, therefore it has no practical use in the design of machine elements; rather to some extent, it is desirable in powder matallurgy.
(o) Machinability: -
Machinability is used to mean the ease with which a material can be machined under a given set of cutting condition. Following are some machinability criteria.
(i) Cutting force: A machinability operation requiring large cutting forces indicates poor machiniability. When strength of the tool is a matter of concern, cutting forces become an important consideration.
(ii) Surface Finish: In some situations the major concern could be over the quality of finish. Depending on the quality of finish the machiniability could be good or poor.
(iii) Tool Life: Tool life is the length of time for which a tool show satisfactory performance.
(p) Formability: -
Formability is a measure to which a material extends can be deformed in specific process without formation of cracks (surface or internal). In some processes the limit is necking rather than fracture.
Formability is not only a material property, it also depends on process parameters (reduction, friction, temperature, strain rate etc.)
(q) Weldability: -
Weldability is the capacity of a material to be welded under the imposed fabrication condition into specific suitably designed structure and to perform satisfactory in the intended service. Weldability can be considered from different points.
¨ Melting and cooling of the material
¨ Metallurgical and thermal changes
¨ Cracking and brittleness.
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