Civil Engineering Design Process
Step 7: Construction Administration Phase
3.4 Material Selection
As design engineers, whether you are designing a machine part, a toy, or a frame for a car or a structure, the selection of material is an important design decision. There are a number of fac- tors that engineers consider when selecting a material for a specific application. For example, major part of your engineering education and any engineering design. Let us now define the key sustainability concepts, methods, and tools. These terms are self explanatory, think about them for a while and then using your own words explain what they mean to you.
Key sustainability concepts—understanding Earth’s finite resources and environmental issues;
socioeconomic issues related to sustainability; ethical aspects of sustainability; sustainable development.
Key sustainability methods—life-cycle based analysis; resource and waste management (material, energy); environmental impact analysis.
Key sustainability tools— life-cycle assessment; environmental assessment; use of sustainable- development indicators; U.S. Green Building Council (USGBC) Leadership in Energy and Environmental Design (LEED) rating system.
As stated on their website, “LEED is an internationally recognized green building certification system, providing third-party verification that a building or community was designed and built using strategies aimed at improving performance across all the metrics that matter most: energy savings, water efficiency, CO2emissions reduction, improved indoor environmental quality, and stewardship of resources and sensitivity to their impacts. Developed by the U.S. Green Building Council (USGBC), LEED provides building owners and operators a concise framework for iden- tifying and implementing practical and measurable green building design, construction, operations and maintenance solutions.” You can learn more about LEED, by visiting www.usgbc.org/LEED.
As you take additional courses in engineering and design, gradually, you will learn in more detail about these concepts, methods, and tools, and will apply them to the solutions of engi- neering problems and design.
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3.4 Material Selection
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they consider the properties of material such as density, ultimate strength, flexibility, machin- ability, durability, thermal expansion, electrical and thermal conductivity, and resistance to cor- rosion. They also consider the cost of the material and how easily it can be repaired. Engineers are always searching for ways to use advanced materials to make products lighter and stronger for different applications.
In Chapter 17, we will look more closely at materials that commonly are used in various engineering applications. We will also discuss some of the basic physical characteristics of materials that are considered in design. We will examine the application and properties of common solid materials; such as metals and their alloys, plastics, glass, and wood and those that solidify over time; such as concrete. We will also investigate in more detail basic fluids;
such as air and water.
By now, it should be clear that material properties and material cost are important design factors. In general, the properties of a material may be divided into three groups:
electrical, mechanical, and thermal. In electrical and electronic applications, for example, the electrical resistivity of materials is important. How much resistance to flow of elec- tricity does the material offer? In many mechanical, civil, and aerospace engineering appli- cations, the mechanical properties of materials are important. These properties include the modulus of elasticity, modulus of rigidity, tensile strength, compression strength, strength- to-weight ratio, modulus of resilience, and modulus of toughness. In applications dealing with fluids (liquids and gases), thermophysical properties such as density, thermal con- ductivity, heat capacity, viscosity, vapor pressure, and compressibility are important prop- erties. Thermal expansion of a material, whether solid or fluid, is also an important design factor. Resistance to corrosion is another important factor that must be considered when selecting materials.
Material properties depend on many factors, including how the material was processed, its age, its exact chemical composition, and any nonhomogenity or defect within the mate- rial. Material properties also change with temperature and time as the material ages. Most companies that sell materials will provide, upon request, information on the important prop- erties of their manufactured materials. Keep in mind that when practicing as an engineer, you should use the manufacturers’ material property values in your design calculations. The property values given in this and other textbooks should be used as typical values — not as exact values.
In the upcoming chapters, we will explain in detail the properties of materials and what they mean. For the sake of continuity of presentation, a summary of important material prop- erties follows.
Electrical Resistivity— The value of electrical resistivity is a measure of resistance of material to flow of electricity. For example, plastics and ceramics typically have high resistivity, whereas metals typically have low resistivity, and among the best conductors of electricity are silver and copper.
Density—Density is defined as mass per unit volume; it is a measure of how compact the material is for a given volume. For example, the average density of aluminum alloys is 2700 kg/m3; when compared to steel density of 7850 kg/m3, aluminum has a density that is approximately one third of the density of steel.
Modulus of Elasticity (Young’s Modulus)—Modulus of elasticity is a measure of how eas- ily a material will stretch when pulled (subject to a tensile force) or how well the material
will shorten when pushed (subject to a compressive force). The larger the value of the modulus of elasticity is, the larger the required force would be to stretch or shorten the material. For example, the modulus of elasticity of aluminum alloy is in the range of 70 to 79 GPa (giga Pascal, giga109), whereas steel has a modulus of elasticity in the range of 190 to 210 GPa; therefore, steel is approximately three times stiffer than aluminum alloys.
Modulus of Rigidity (Shear Modulus)—Modulus of rigidity is a measure of how easily a material can be twisted or sheared. The value of the modulus of rigidity, also called shear modulus, shows the resistance of a given material to shear deformation. Engineers con- sider the value of shear modulus when selecting materials for shafts, which are rods that are subjected to twisting torques. For example, the modulus of rigidity or shear modulus for aluminum alloys is in the range of 26 to 36 GPa, whereas the shear modulus for steel is in the range of 75 to 80 GPa. Therefore, steel is approximately three times more rigid in shear than aluminum is.
Tensile Strength— The tensile strength of a piece of material is determined by measuring the maximum tensile load a material specimen in the shape of a rectangular bar or cylinder can carry without failure. The tensile strength or ultimate strength of a material is expressed as the maximum tensile force per unit cross-sectional area of the specimen. When a material specimen is tested for its strength, the applied tensile load is increased slowly. In the very beginning of the test, the material will deform elastically, meaning that if the load is removed, the material will return to its original size and shape without any permanent deformation. The point to which the material exhibits this elastic behavior is called the yield point. The yield strength represents the maximum load that the material can carry without any permanent deformation. In certain engineering design applications, the yield strength is used as the tensile strength.
Compression Strength— Some materials are stronger in compression than they are in ten- sion; concrete is a good example. The compression strength of a piece of material is deter- mined by measuring the maximum compressive load a material specimen in the shape of rectangular bar, cylinder, or cube can carry without failure. The ultimate compressive strength of a material is expressed as the maximum compressive force per unit cross-sec- tional area of the specimen. Concrete has a compressive strength in the range of 10 to 70 MPa (mega Pascal, mega106).
Modulus of Resilience—Modulus of resilience is a mechanical property of a material that shows how effective the material is in absorbing mechanical energy without going through any permanent damage.
Modulus of Toughness—Modulus of toughness is a mechanical property of a material that indicates the ability of the material to handle overloading before it fractures.
Strength-to-Weight Ratio—As the term implies, it is the ratio of strength of the material to its specific weight (weight of the material per unit volume). Based on the application, engineers use either the yield or the ultimate strength of the material when determining the strength-to-weight ratio of a material.
Thermal Expansion— The coefficient of linear expansion can be used to determine the change in the length (per original length) of a material that would occur if the temperature of the material were changed. This is an important material property to consider when designing
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