1. ENGINEERING MATERIALS Introduction Since the earliest days of the evolution of mankind , the main distinguishing features between human begins and other mammals has been the ability to use and develop materials to satisfy our human requirements. Nowadays we use many types of materials, fashioned in many different ways, to satisfy our requirements for housing, heating, furniture, clothes, transportation, entertainment, medical care, defense and all the other trappings of a modern, civilised society. Most materials doesn't exist in its pure shape , it is always exist as a ores . During the present century the scope of metallurgical science has expanded enormously , so that the subject can now be studied under the following headings : a) Physical metallurgy b) Extraction metallurgy c) Process metallurgy Classification of Engineering materials: Almost every substance known to man has found its way into the engineering workshop at some time or other. The most convenient way to study the properties and uses of engineering materials is to classify them into ‘families’ as shown in figure below: 1. Metals 1.1 Ferrous metals • These are metals and alloys containing a high proportion of the element iron. • They are the strongest materials available and are used for applications where high strength is required at relatively low cost and where weight is not of primary importance. • As an example of ferrous metals such as : bridge building, the structure of large buildings, railway lines, locomotives and rolling stock and the bodies and highly stressed engine parts of road vehicles. • The ferrous metals themselves can also be classified into "families', and these are shown in figure
Classification of ferrous metals. 1.2 Non – ferrous metals • These materials refer to the remaining metals known to mankind. • The pure metals are rarely used as structural materials as they lack mechanical strength. • They are used where their special properties such as corrosion resistance, electrical conductivity and thermal conductivity are required. Copper and aluminum are used as electrical conductors and, together with sheet zinc and sheet lead, are use as roofing materials. • They are mainly used with other metals to improve their strength. • Some widely used non-ferrous metals and alloys are classified as shown in figure Composite materials (composites) These are materials made up from, or composed of, a combination of different materials to take overall advantage of their different properties. In man-made composites, the advantages of deliberately combining materials in order to obtain improved or modified properties was understood by ancient civilizations. An example of this was the reinforcement of air-dried bricks by mixing the clay with straw. This helped to reduce cracking caused by shrinkage stresses as the clay dried out. In more recent times, horse hair was used to reinforce the plaster used on the walls and ceiling of buildings. Again, this was to reduce the onset of drying cracks Various mechanical properties of engineering materials A mechanical property deals with the behavior of materials or metals when they are subjected to the external forces or loads. It is the characteristic that indicates the variations taking place in the metal. These mechanical properties are considered while designing machine components. The component will perform well during its use only when it is designed by considering all mechanical properties. The behavior of materials under external loads is called Mechanical Properties of Materials.
The most important and useful mechanical properties are; 1. Stress. 2. Strain. 3. Elongation. 4. Contraction. 5. Elasticity. 6. Plasticity. 7. Strength. 8. Impact strength. 9. Yield strength. 10. Stiffness. 11. Toughness. 12. Hardness. 13. Brittleness. 14. Malleability. 15. Ductility. 16. Fatigue. 17. Creep. 1. Strength. It is the mechanical property of a metal, which provides resistance to an external force or it is the capacity or ability to withstand various loads without deformation or breaking. Hence, it is the highest resistance offered by the material when it is subjected to an external load. Stronger the material, greater is the load it can withstand. In the case of metals, strength is very important, because the metals should tolerate heavy loads. It means that metals should not be induced with heavy stress and deform. If the metals have high strength, they can withstand various loads. The various loads which may act on the metal components of the machine tool are; 1. Tensile, 2. Compressive, 3. Shear,
4. Bending, 5. Torsion etc., and their respective strengths include; 1. Tensile strength, 2. Compressive strength, 3. Shear strength. 4. Bending strength, 5. Torsion strength, etc. Some metals and their alloys possess high strength per unit mass, making them useful materials for carrying heavy loads or resisting any damages due to impact loads. Depending upon the type of load applied the strength can be tensile, compressive, shear or torsional. The material can be loaded by means of heating, internal structure, loading type, etc. The maximum stress that any material will withstand before destruction is called its ultimate strength. 2. Impact Strength. It is that property of the metal which gives its ability to withstand shock or impact or sudden loads. When impact load is applied within the elastic limit of the material, that energy is absorbed by the material and given out when the load is removed, as in case of spring materials. This property within an elastic limit is known as resilience. However, the impact strength is its load withstanding capacity up to its rupture. Some times the impact load leads to failure of the metal component. The impact loads may exist in shear, compressive, or tensile. Impact strength can be measured by Charpy or Izod test. The Charpy test measures the ability of a metal to withstand the applied impact load, while, Izod test measures the load required to rupture the metal component. 3. Elasticity. The property of metal and its ability to return to its shape and size after removal of load or to regain its initial position or shape and size when the applied load is removed is called elasticity. Most of the components are designed with a suitable elasticity; otherwise, the machine components will be deformed when it is subjected to loads. Most of the metals have better elasticity such as heat treated springs and coils made up of steel, copper, aluminum, etc. However, some of the metals are not elastic; they have properties like brittleness and hardness. Elasticity is a tensile property of its material.