Material selection

Material selection is a step in the process of designing any physical object. In the context of product design, the main goal of material selection is to minimize cost while meeting product performance goals. Systematic selection of the best material for a given application begins with properties and costs of candidate materials. Material selection is often benefited by the use of material index or performance index relevant to the desired material properties. For example, a thermal blanket must have poor thermal conductivity in order to minimize heat transfer for a given temperature difference. It is essential that a designer should have a thorough knowledge of the properties of the materials and their behavior under working conditions. Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc. In contemporary design, sustainability is a key consideration in material selection. Growing environmental consciousness prompts professionals to prioritize factors such as ecological impact, recyclability, and life cycle analysis in their decision-making process. Systematic selection for applications requiring multiple criteria is more complex.…

Material selection is a step in the process of designing any physical object. In the context of product design, the main goal of material selection is to minimize cost while meeting product performance goals. Systematic selection of the best material for a given application begins with properties and costs of candidate materials. Material selection is often benefited by the use of material index or performance index relevant to the desired material properties. For example, a thermal blanket must have poor thermal conductivity in order to minimize heat transfer for a given temperature difference. It is essential that a designer should have a thorough knowledge of the properties of the materials and their behavior under working conditions. Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc. In contemporary design, sustainability is a key consideration in material selection. Growing environmental consciousness prompts professionals to prioritize factors such as ecological impact, recyclability, and life cycle analysis in their decision-making process. Systematic selection for applications requiring multiple criteria is more complex. For example, when the material should be both stiff and light, for a rod a combination of high Young's modulus and low density indicates the best material, whereas for a plate the cube root of stiffness divided by density \sqrt[3]{E}/\rho is the best indicator, since a plate's bending stiffness scales by its thickness cubed. Similarly, again considering both stiffness and lightness, for a rod that will be pulled in tension the specific modulus, or modulus divided by density E/\rho should be considered, whereas for a beam that will be subject to bending, the material index \sqrt[2 ]{E}/\rho is the best indicator. Reality often presents limitations, and the utilitarian factor must be taken in consideration. The cost of the ideal material, depending on shape, size and composition, may be prohibitive, and the demand, the commonality of frequently utilized and known items, its characteristics and even the region of the market dictate its availability. The stress in the beam is measured as P/A whereas weight is described by w = \rho AL . Deriving a performance index requires that all free variables are removed, leaving only design variables and material variables. In this case that means that A must be removed. The axial stress equation can be rearranged to give A = P/\sigma . Substituting this into the weight equation gives w=\rho (P/\sigma) L = \rho L P/\sigma . Next, the material variables and design variables are grouped separately, giving w = (\rho/\sigma)LP . Since both L and P are fixed, and since the goal is to minimize w , then the ratio \rho/\sigma should be minimized. By convention, however, the performance index is always a quantity which should be maximized. Therefore, the resulting equation is \text{Performance index} = P_{cr} = \sigma/\rho Next, suppose that the material is also subjected to bending forces. The max tensile stress equation of bending is \sigma = (-My)/I , where M is the bending moment, y is the distance from the neutral axis, and I is the moment of inertia. This is shown in Figure 2. Using the weight equation above and solving for the free variables, the solution arrived at is w=\sqrt{6MbL^2}(\rho/\sqrt{\sigma}) , where L is the length and b is the height of the beam. Assuming that b , L , and M are fixed design variables, the performance index for bending becomes P_{CR}=\sqrt{\sigma}/\rho . For the tension performance equation P_{CR}=\sigma/\rho , the first step is to take the log of both sides. The resulting equation can be rearranged to give \log(\sigma) = \log(\rho) + \log(P_{CR}) . Note that this follows the format of y = x + b , making it linear on a log-log graph. Similarly, the y-intercept is the log of P_{CR} . Thus, the fixed value of P_{CR} for tension in Figure 3 is 0.1. The bending performance equation P_{CR}=\sqrt{\sigma}/\rho can be treated similarly. Using the power property of logarithms it can be derived that \log(\sigma) = 2 \times (\log(\rho) + \log(P_{CR})) . The value for P_{CR} for bending is ≈ 0.0316 in Figure 3. Finally, both lines are plotted on the Ashby chart. First, the best bending materials can be found by examining which regions are higher on the graph than the \sqrt{\sigma}/\rho bending line. In this case, some of the foams (blue) and technical ceramics (pink) are higher than the line. Therefore those would be the best bending materials. In contrast, materials which are far below the line (like metals in the bottom-right of the gray region) would be the worst materials. Lastly, the \sigma/\rho tension line can be used to "break the tie" between foams and technical ceramics. Since technical ceramics are the only material which is located higher than the tension line, then the best-performing tension materials are technical ceramics. Therefore, the overall best material is a technical ceramics in the top-left of the pink region such as boron carbide. ==References== ==External links==

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