Engineering materials 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.
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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. . 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. 4. Disposability and recyclability. 15. The aesthetics of the material. 16. Chemical nature of the material. 17. Environmental conditions. Mechanical properties of engineering materials Elasticity 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. Plasticity 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 materials. Ductile materials show large deformation before fracture. The lack of ductility is often termed brittleness. Usually, if two materials have the same strength and hardness, the one that has the higher ductility is more desirable. The ductility of many metals can change if conditions are altered.
An increase in temperature will increase ductility. A decrease in temperature will cause decrease inductility and a change from ductile to brittle behavior Malleability Where ductility is the ability of a material to deform easily upon the application of a tensile force, malleability is the ability of a metal to exhibit large deformation or plastic response when being subjected to compressive force. It is a special case of ductility where it can be rolled into thin sheets but it is not necessary to be so strong. Lead, soft steel, wrought iron, copper and aluminium are some materials in order Of diminishing malleability.
Uniform compressive force causes deformation in the manner shown in Figure 7. The material contracts axially with the force and expands laterally. Restraint due to friction at the contact faces induces axial tension on the outside. Tensile forces operate around the circumference with the lateral expansion or increasing girth. Plastic flow at the center of the material also induces tension. Therefore, the criterion of fracture (that is, the limit of plastic deformation) for a plastic material is likely to depend on tensile rather than compressive stress.
Temperature change may modify both the plastic flow mode and the fracture mode. Toughness The quality known as toughness describes the way a material reacts under sudden impacts. This is the property which enables a material to be twisted, bent or stretched under impact load or high stress before rupture. It may be considered to be the ability of the material to absorb energy in the plastic zone. The measure of toughness is the amount of energy absorbed after being stressed upto the point of fracture. It is defined as The work required to deform one cubic inch of metal until it fractures.
Toughness is measured by the Charpy test or the Izod test. Both of these tests use a notched sample. The location and shape of the notch are standard. The points of support of the sample, as well as the impact of the hammer, must bear a constant relationship to the location of the notch. Hardness Hardness is the property of a material that enables it to resist plastic deformation, penetration, indentation, and scratching. Therefore, hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Several methods have been developed for hardness testing. Those most often used are Brinell, Rockwell, Vickers, Tukon, Sclerscope, and the files test. The first four are based on indentation tests and the fifth on the rebound height of a diamond-tipped metallic hammer. The file test establishes the characteristics of how well a file takes a bite on the material. Creep When a member is subjected to a constant load over a long period of time it undergoes a slow permanent deformation and this is termed as “creep”. This is dependent on temperature. Usually at elevated temperatures creep is high.
Resilience This is the property of the material that enables it to resist shock and impact by storing energy. The measure of resilience is the strain energy absorbed per unit volume. For a rod of length L subjected to tensile load P, a linear load-deflection plot is shown in figure- Brittleness- This is opposite to ductility. Brittle materials show little deformation before fracture and failure occur suddenly without any warning. Normally if the elongation is less than 5% the material is considered to be brittle. E. g. cast iron, glass, ceramics are typical brittle materials.
Fatigue Fatigue is a phenomenon associated with variable loading or more precisely to cyclic stressing or straining of a material. Just as we human beings get fatigue when a specific task is repeatedly performed, in a similar manner metallic components subjected to variable loading get fatigue, which leads to their premature failure under specific conditions. Fatigue loading is primarily the type of loading which causes cyclic variations in the applied stress or strain on a component. Thus any variable loading is basically a fatigue loading. Stress Concentration
In developing a machine it is impossible to avoid changes in cross-section, holes,notches, shoulders etc. Some examples are shown in figure Any such discontinuity in a member affects the stress distribution in the neighbourhood and the discontinuity acts as a stress raiser. whenever a machine component changes the shape of its cross section, the simple stress distribution does not holds good and the neighbourhood of discontinuity is different this irregularity in the stress distribution caused by abrupt changes of form is called stress concentration.
It occurs for all kinds of stresses in the prescence of fillets, notches, holes, keyways, splines, surface roughness or scratches etc. the nominalstress in the right and left sides, of the above mentioned components, will be uniform but in the region where the cross section is changing, a re-distribution of the force whithin the member must take plac. The material near the edges is stressed considerably higher than the average value. The maximum stress occurs at some point on the fillet and is directed parallel to the boundry at that point.
Theoratical or form stress concentration factor The theoratical or form stress concentration factor is defined as the ratio of the maximum stress in a member (at a notch or fillet) to the nominal stress at the same section based upon net area. Mathematically, theoratical or form stress concentration factor. Kt = Maximum stress Nominal stress Methods of reducing stress concentration A number of methods are available to reduce stress concentration in machineparts.
Some of them are as follows: 1. Provide a fillet radius so that the cross-section may change gradually. 2. Sometimes an elliptical fillet is also used. 3. If a notch is unavoidable it is better to provide a number of small notchesrather than a long one. This reduces the stress concentration to a large extent. 4. If a projection is unavoidable from design considerations it is preferable toprovide a narrow notch than a wide notch. 5. Stress relieving groove are sometimes provided. These are demonstrated in figure