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Properties for Material Selection |
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Strength : It refers to the material's ability to resist an applied force. It is measured by the tensile strength.
The elastic limit is considered to guard material against permanent deformation. Rather than elastic limit, yield
point is used for ductile materials.
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Rigidity : It refers to inflexibility or resistance to change. It is important where deflections are limited by
service requirements. It is also known as the material property of shear modulus which is a measure of force per unit
area needed to change the shape of a material.
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Resistance to Fatigue : It is measured by means of endurance limit and is important for members subjected to cyclic loading.
Harmful surface effects must be carefully controlled during heat treatment.
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Damping Capacity : It refers to the material's capacity of absorbing vibrations.
It is the energy which is dissipated as heat by a unit volume of the material during a completely
reversed cycle of stress. It is represented by a mechanical-hysteresis diagram. It is related to internal friction in
the material. High damping capacity is desirable to decrease vibration, chatter and noise.
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Resilience : It refers to the property of a material to absorb energy when it is deformed
elastically and then, upon unloading to have this energy recovered. In other words, it is the maximum
energy per volume that can be elastically stored.
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Hardness : It refers to various properties of matter in the solid phase which give it high resistance
to various kinds of shape change when force is applied. Ductility is desirable to relieve stress concentration in
parts subjected to static loading.
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Corrosion Resistance : It is important in members subjected to corrosive environment. In the presence of
stress concentration its effect is particularly serious in cyclic loading. Other such considerations which are
depending on application are weight, electrical properties, thermal properties, resistance to wear, casting and
forging characteristics, machinability, low friction etc.
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Cast Iron : It identifies a large group of alloys. It has low melting point, good fluidity, castability,
excellent machinability and wear resistance. It can be easily cast into any size and form. It is the cheapest of
the cast metals. Grey iron is easily machinable but is not as abrasive and resistant as chilled iron. Cast iron
usually refers to grey iron. White iron is hard and difficult to machine.
Cast iron tends to be brittle, except for malleable cast irons.
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Chilled Cast Iron : It is produced by rapid cooling to provide hard wear-resistant surface. It has high damping
capacity, making it suitable for beds, bases and frame of machinery subjected to vibrations.
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Malleable Iron : It is white cast iron which has been rendered malleable by heat treatment. That heat treatment
converts it into a matrix of ferrite which contains nodules of temper carbon and thus it becomes tough and ductile,
resistant to impact and easily machinable.
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Wrought Iron : It is an easily welded and forged iron which is a mechanical mixture of refined metallic iron
having a small amount siliceous slag. It welds readily. It is resistant to corrosion in ordinary atmospheres.
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Cast Steel : It can be used for castings of improved properties. It is stronger and tougher. It is weighs less
for the same strength or stiffness and has a higher endurance limit than cast iron corrosion resistance can improved
by alloying.
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Wrought Steel : The composition and heat treatment can control mechanical properties. It may be forged,
swaged, drawn, welded and machined. It finds many applications in engineering.
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Brass and Bronze : Both are used in machinery parts in both cast and wrought form. Brass is any alloy of
copper and zinc. It is malleable and acoustic. It is used where moderate strength and ductility, resistance to
corrosion or good wearing qualities are desired. Bronze is principally an alloy of copper and tin. Bronze is superior
in these properties but it is more expensive. Phosphor bronze is having relatively high tensile, yield point, endurance
limit and shock-resistance properties. It is used for springs, bolts and various other items used in situations where
resistance to fatigue, wear and chemical corrosion are needed.
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Aluminium : It is ductile member of the boron group of chemical elements and the most abundant metal.
It is a soft, durable, lightweight, malleable metal. Pure aluminium resists oxidation and it has good forming
properties. It is the most widely used non-ferrous metal. Aluminium and its alloys have variety of uses. But it has
poor casting characteristics and mechanical properties.
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Magnesium : It is fairly strong and light weight metal. It is corrosion-resistant in salt free atmosphere and
galvanic corrosion can be prevented by coatings. In its purest form it can be compared with aluminium, and is strong
and light, so it is used in several high volume part manufacturing applications. It is a highly flammable metal.
It is used in structural alloys, pyrotechnics, flash photography and incendiary bombs.
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Titanium Alloys : It is a light, strong, lustrous, corrosion-resistant transition metal.
The two most useful properties of the metal form are corrosion resistance and the highest strength-to-weight ratio
of any metal. It is used to alloy aircraft metals for low weight, strength and high-temperature stability.
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Rubber : Natural rubber is an elastic hydrocarbon polymer that naturally occurs as a milky colloidal
suspension, or latex, in the sap of some plants. It is used extensively in many applications and products. It can
also be synthesized. It is an excellent material for seals and diagrams for water-lubricated bearings used in parts
which are subjected to vibratory forces. It should be protected from oil, sunlight and high temperature.
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Non-metallic Materials : Plastics refer to the polymer material. These have low cost, light weight, self-lubrication
and ease of lubrication. These are having good resistance to shock and vibration. But these have low strength, high thermal
expansion, low heat resistance, high creep and deformation under load and embrittlement at low temperature.
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Fiberglass : It is made from extremely fine fibers of glass. It has high tensile strength and corrosion
resistance. It is used as a reinforcing agent for many polymer products. It has high ratio of surface area to weight.
But the increased surface area makes it much more susceptible to chemical attack.
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