page_title: Material selection
section_title: Performance index during bending
hierarchical_section_title: Material selection / Example of using an Ashby chart / Performance index during bending
context_page_description: 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.
  Systematic selection for applications requiring multiple criteria is more complex.
context_section_description: Next, suppose that the material is also subjected to bending forces. The max tensile stress equation of bending is , where  is the bending moment,  is the distance from the neutral axis, and  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 , where  is the length and  is the height of the beam. Assuming that , , and  are fixed design variables, the performance index for bending becomes .
caption: Figure 2. Beam under bending stress. Trying to minimize weight