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WELDER - CITS

3 Stiffness It is the ability of a material to resist deformation under stress. Modulus of elasticity is the measure
of stiffness. Material which suffers slight deformation under load has a high degree of stiffness or rigidity. Steel
beam is stiffer or more rigid than aluminium beam. Finally, it means that the ability of material to resist elastic
deflection is known as stiffness.
4 Elasticity It is the property of materials to regain its original shape after deformation when the external forces
are removed. Example is the extension or compression of a spring. This property is desirable for materials
used in tools and machines. Steel is more elastic than rubber. Elasticity is a tensile property of the material.
Proportional limit and elastic limit indicate elasticity. It is also known as Nonpermanent deformation. It consists
of two sub properties within this elastic region. They are proportional limit and elastic limit. Proportional
limit is the maximum stress under which a material will maintain a perfectly uniform rate of strain to stress.
Proportional limit applications are precision instruments, springs etc... The greatest stress that a material can
endure without taking up some permanent set is called elastic limit. Beyond the elastic limit, material does not
regain its original form and permanent set occurs.

5 Plasticity It is the ability of material to undergo some degree of permanent deformation without rupture
or failure. That means, this is the property of a material to deform permanently under the application of a
load. Plastic deformation will take place only after the elastic range has been exceeded by the process of
slipping when the shear stress on the slip plane reaches a critical value. Displacement caused by slipping
is permanent and the crystal planes do not return to their original positions even after the removal of the
stresses. Applications are forming, shaping, extruding, hot & cold working process, forging, ornamental work,
stamping, rolling, drawing, pressing, etc.. Aluminium is a good plasticity material.
6 Ductility It is the property of a material which enables it to draw out into thin wire with the application of
a tensile force. Ductile material must be both strong and plastic. Ductile materials are gold (most ductile
material), mild steel, copper, aluminium, nickel, zinc, tin. Ductility usually measured by the terms, percentage
elongation and percentage reduction in area. Ductility is thought of as a tensile quality. Ductile material
combines the properties of plasticity and tensile strength. It is also mentioned as a capacity of a material to
undergo deformation under tension without rupture or the ability of amaterial to withstand cold deformation
without fracture. Ductility of a material is to stretch under the application of tensile load and retain the deformed
shape on the removal of the load. If subjected to a shock load the material would yield and become deformed.
Ductile material can be worked into a shape without loss of strength. All materials which are formed by drawing
are required to be ductile, e.g.- drawing into wire form. Brittleness Breaking of a material with little permanent
distortion simply states the property of brittleness. Brittle materials when subjected to tensile loads snap off
without giving any sensible elongation. Usually the tensile strength of brittle materials is only a fraction of their
compressive strength. Examples of brittle materials are glass, bricks, cast iron etc… It is also a tendency of a
material to fracture when subjected to shock loading or a blow. Material that shatters is also a brittle material.
7 Malleability It is the ability of materials to be rolled, flattened or hammered into thin sheets without cracking by
hot or cold working. Malleable material should be plastic but it is not essential to be strong and malleability is
considered as a compressive quality. Examples for malleability Al, Cu, Sn, Pb, soft steel, wrought iron. This is
the property of a material to deform permanently under the application of a compressive load. A material which
is forged to its final shape is required to be malleable. Forging, Rolling processes are malleability.
8 Toughness and Testing It is the ability of a material to withstand bending without fracture due to high impact
loads. Toughness of material decreases when it is heated. It is also measured by the amount of energy that a
unit volume of the material has absorbed after being stressed up to failure point and is the area under stress
strain curve. For example, if a load is suddenly applied to a piece of mild steel and then to a piece of glass, the
mild steel will absorb much more energy before failure occurs. Thus mild steel is said to be much tougher than a
glass. This property is desirable in parts subjected to shock and impact loads. Notch toughness is the measure
of the metal‘s resistance to brittle fracture in presence of flaw or notch and fast loading conditions. Examples
are Mn-steel, wrought iron, MS, etc…it can be also defined as property of absorbing energy before fracture.
To the opposite of brittleness, the ability of a material is to resist fracture under shock loading. Basically, two
main impact tests for measuring the toughness of material in Joule are available namely Izod and Charpy test.
Fig 2 shows the three types of Notches used for fracture study. U type notch specimens can also be used for
testing. In case of ductile materials, when the material is stressed, it plastically deforms by absorbing high
energy and then the material fractures. But in the case of brittle materials, the cohesive strength of the material
exceeds before getting plastically deformed and hence absorbs less energy before getting fractured. There are
factors responsible for brittle behaviour; they are notch, low temperature, thickness and microstructure. When
temperature falling, the failure mode of certain materials changes from ductile to brittle. For FCC materials,

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CITS : C G & M - Welder - Lesson 83 - 97