Choice of materials for a machine element depends very much on its properties, cost, availability and such other factors. It is therefore important to have some idea of the common engineering materials and their properties before learning the details of design procedure. Common engineering materials are normally classified as metals and nonmetals. Metals may conveniently be divided into ferrous and non-ferrous metals. Important ferrous metals for the present purpose are: (i) Cast iron (ii) wrought iron (iii) steel.
Some of the important non-ferrous metals used in engineering design are: (a) Light metal group such as aluminum and its alloys, magnesium and Manganese alloys.
(b) Copper based alloys such as brass (Cu-Zn), bronze (Cu-Sn). (c) White metal group such as nickel, silver, white bearing metals e.g. Selection criteria for engineering materials
The selection of material, for engineering purposes, is one of the most difficult problem for designer. The best material is one which serves the desired objective at the minimum cost. The following factors should be considered while selecting the material: 1. Availability of the materials.
2. Suitability of the materials for the working conditions in service. 3. The cost of the material.
4. Its susceptibility to corrosion.
5. Its physical, chemical as well as thermal stability.
6. Material must withstand service demands. Such as dimensional stability, adequate strength, toughness, thermal conductivity etc. 7. The extent of the stresses induced.
8. Factor of safety desired.
9. The initial stresses during the material processing.
10. Its density, melting point, boiling point at the working conditions. 11. The extent of surface finish required.
12. Fabrication requirement.
13. Ease of joining, repair by welding etc.
14. Disposability and recyclability.
15. The aesthetics of the material.
16. Chemical nature of the material.
17. Environmental conditions.
Mechanical properties of engineering materials
This is the property of a material to regain its original shape after deformation when the external forces are removed. All materials are plastic to some extent but the degree varies, for example, both mild steel and rubber are elastic materials but steel is more elastic than rubber.
This is associated with the permanent deformation of material when the stress level exceeds the yield point. Under plastic conditions materials ideally deform without any increase in stress Strength
It is the ability of a material to resist deformation. The strength of a component is usually considered based on the maximum load that can be borne before failure is apparent. If under simple tension the permanent deformation (plastic strain) that takes place in a component before failure, the load-carrying capacity, at the instant of final rupture, will probably be less than the maximum load supported at a lower strain because the load is being applied over significantly smaller cross-sectional area. Under simple compression, the load at fracture will be the maximum applicable over a significantly enlarged area compared with the cross-sectional area under no load. Ductility
It is more commonly defined as the ability of a material to deform easily upon the application of a tensile force, or as the ability of a material to withstand plastic deformation without rupture. Ductility may also be thought of in terms of bend ability and crushability. This is the property of the material that enables it to be drawn-out or elongated to an appreciable extent before rupture occurs. The percentage elongation or percentage reduction in area before rupture of a test specimen is the measure of ductility. Normally if percentage elongation exceeds 15% the material is ductile and if it is less than 5%the material is brittle. Lead, copper, aluminium, mild steel are typical ductile...
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