Introduction
Selection of materials is an important part of engineering design. Material selection is aimed at increasing the performance as well as the life of the engineering component and hence, very important for success of engineering component. The nature of loading, expected life, working environment amongst other factors determines the material to be selected for a particular application.
Therefore, engineers and designers must determine the properties of materials before they incorporate them in their design. Mechanical properties of materials such as ultimate tensile strength, Modulus of Elasticity, proof stress, specific strength, toughness and density amongst others are some of the important properties of materials. Understanding trends in properties variation between different engineering materials (metals, polymers and ceramics) in the periodic table is very important. In this work, materials data base software known as CES Edupack has been used to study trends in properties variations in different materials.
Specific strength is an important mechanical property that engineers considers when the design requirements call for a higher strength to weight ratio. For instance, in the transportation equipment, aircraft industry high strength to weight ration helps to achieve fuel economy. Specific strength is given by the gradient in a graph of tensile strength versus density and thus the Tensile Strength versus Density graph shown in Fig. 2 only gives estimated values and not the exact values.
Figure 1: Tensile strength versus Density
Young’s Modulus (E) or modulus of elasticity (E) is a constant of proportionality that defines the ratio of stress to strain in uniaxial loading within the elastic region where hooke's law holds. It defines the stiffness of elastic materials. It thus, defines the tendency of a material or an object to undergo axial deformation upon application of opposing forces.
Yield strength refers to the strength at yield point or the strength at which materials starts to deform plastically. The yield point represents the end of elastic deformation and the onset of plastic deformation
Strain is a dimensionless quantity given by the ratio of the change in length to the original length of a material subjected to tensile loading.
Elongation is the difference between the final length is greater that the initial length because of plastic deformation. The percentage elongation of the specimen is given by
Young's modulus (E) is one of the most important factors that various engineers consider in selection of materials. For instance, in civil designs engineers are concerned with specific young's modulus (E/density). In addition, in the design of an aircraft it is important that the strength to weight ratio of materials considered should be very high in order to reduce the weight of the aircraft. Thus when designing, engineers are concerned by maintain the balance between young's modulus (E) and the density f the materials. Other engineering designs also calls for strong and tough materials. Thus, to use Young's modulus as a material selection criteria engineers must determines a merit index of a material and compares it with those of other materials. The merit indices determined are used for instance with Young's modulus versus toughness or Young's modulus versus density selection map. The figure below shows a Young's modulus versus density selection map commonly used by civil engineers.
Figure 2: Young modulus versus density map
The data of atomic radius versus cohesive energy shown in figure 3 and Table 2 shows that cohesive energies are greater in simple metals such as aluminum. We can also note that cohesive energy variation in transitional metals possibly follows a parabolic relationship.
The Young's modulus versus density graph shows that the Young's modulus of metals increases with increase in metal density and vice versa.
Figure 6. Electrical resistance vs thermal conductivity
Electrical resistance is directly proportion to thermal conductivity. This happens because increase in temperature increases the kinetic energy of the atoms and consequently increased vibration of the atoms in the lattice structure. These vibrations rand random movement of atoms interferes with movement of free (delocalised) electron in a material.
The melting point of Titunium (Ti) with a cohesive enrgy if 470 kJ/mol can be approximated as follows:
Crystal structure is a unique arrangement of molecules or atoms in a crystalline solid or liquid. Crystal structure therefore reflects the periodic pattern of the molecules or atoms that compose the solid or liquid. Common crystal structures include body centered cubic and face centered cubic structures.
Cohesive energy refers to the forces of attraction between molecules of the same kind. In fact, cohesive energy is the energy required to separate atoms or molecules in a crystalline solid to an infinite separation distance. Cohesive energy is also referred to as the binding energy of the atom .
Melting point refers to the temperature at a change of phase occurs from solid to liquid at standard temperature and pressure. At this temperature both the solid phase and solid phase coexist at equilibrium.
The data of heat of fusion versus cohesive energy listed in Table 3 shows that cohesive energy is directly proportional to the heat of fusion of a material . This is consistent with our expectations since the forces of attraction been molecules of the same kind must be proportional the energy required to separate them. The data is very accurate with liquid (mercury) having lowest cohesive energy and heat of fusion as expected.
Conclusion
References
Ashby, M. & Hugh, D., 2007. Materials: Engineering, Science, Processing and Design. Butterworth-Heinemann.
Askeland, D. & Phulé, P., 2006. The science and engineering of materials. 5th ed. Cengage Learning.
Haynes, W., 2011. CRC Handbook of Chemistry and Physics. 92nd ed. CRC Press.
Higdon, A., 2009. Mechanics of Materials. 2nd ed. John Wiley & Son.
Kittle, C., 1997. An Introduction to Solid State Physics. 6th ed. John Wiley.
Tonkov, Y. & Ponyatovsky, E., 2005. Phase Transformations of Elements Under High Pressure. 3rd ed. Boca Raton: CRC Press.
