Selection of a proper material for the machine component is one of the most important steps in the process of machine design. The best material is one which will serve the designed objective at minimum cost.Selection of material by trial and error method. While selecting the material to follow the factors
Availability
Cost
Mechanical properties
Manufacturing considerations.
Availability:
The material should be readily available in the market, in large enough quantities to meet the requirement.
Cost:
For every application, there is a limiting cost beyond which the designer cannot go.When this limit is exceeded the designer has to consider other alternative materials. In cost analysis, there are two factors, namely cost of material and the cost of processing the material into finished goods. It is likely that the cost of material might be low, but the processing may involve costly machining operations.
Mechanical properties:
These properties govern the selection of materials. Depending upon the service conditions and the functional requirement, different mechanical properties are considered and a suitable material is selected.
Ex: Connecting rod of I .C-Withstand fluctuating stresses due to a combination of fuel-
Endurance strength criterion of design.
Piston rings resist wear- Surface hardness criterion of design.
Bearing material – have a low coefficient of friction.
Clutch or brake lining -has a high coefficient of friction.
Manufacturing considerations:
The manufacturing processes such as casting, rolling, forging, extrusion, welding and machining govern the selection of the material. Machine ability of material is an important consideration in selection. Sometimes, an expensive material is more economical than a low priced one. Which is difficult to machine. Where the product is of a complex shape, casting properties are important.Past experience is a good guide for the selection of material. Iron and its alloys, Cu, Zn, Mg, Ni, Silica, Ag,Cast iron, carbon steels, Alloy steels.
Various Properties of materials are classified as follows:
Physical properties: They include density, porosity, structure, Fusibility, Shape and size.
Mechanical properties: They include strength, stiffness, elasticity, plasticity, ductility, brittleness, creep, fatigue, hardness etc.
Magnetic properties: They include thermal permeability and hysterics.
Thermal properties: They include thermal conductivity, specific heat, latent heat and thermal stresses.
Electrical properties: They include dielectric strength, conductivity and restively.
Chemical properties: They include chemical composition, corrosion resistance, acidity and alkalinity.
Mechanical properties of materials:
The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. Which undergo any changes in shape and structure during the application of force on these elements.
Ex: if a rod is subjected to a tensile load, its length can be increased and soon.
Strength: It is the ability of a material to resist the externally applied forces without breaking or yielding.Static load: Ultimate Tensile Strength or tensile yield strength. Fluctuating load: Endurance strength.
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: It is the property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forging, in stamping images on coins and in or nonmetal work.
Ductility: It is the property of a material enabling it to be drawn into a wire with the application of a tensile force. A 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, aluminium, nickel, zinc, tin and lead.
Brittleness: It is the property of a material opposite to ductility it is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads snap off without giving any sensible elongation. Cast iron is a brittle material.
Toughness: It is the property o a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. This property is desirable in parts subjected to shock and impact loads.
Malleability: It is a special case of ductility which permits materials to be rolled or hammered into thin sheets due to compressive force. 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 aluminium.
Creep: When a part is subjected to a constant stress at high temp for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing I.C engine, boilers and turbines.
Fatigue: When a material is subjected to repeated stresses it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of progressive crack formations which are usually fine and microscopic size. This property is considered in designing shafts, connecting rod, springs, gears etc.
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