Materials selection enters each stage of the design (Figure 3.1, left-hand side). The nature of the data needed in the early stages differs greatly in its level of precision and breadth from that needed later on. At the concept stage, the designer requires only approximate property values, but for the widest possible range of materials. All options are open: a polymer may be the best choice for one concept, a metal for another. The problem, at this stage, is not precision and detail, it is breadth and speed of access: how can the vast range of data be presented to give the designer the greatest freedom in considering alternatives? At the embodiment stage the landscape has narrowed. Here we need data for a subset of materials, but at a higher level of precision and detail. These are found in more specialized handbooks and software that deal with a single class or subclass of materials—metals, or just aluminum alloys, for instance. The risk now is that of losing sight of the bigger spread of materials to which we must return if the details don’t work out; it is easy to get trapped in a single line of thinking when others have potential to offer better solutions. The final stage of detailed design requires a still higher level of precision and detail, but for only one or a very few materials. Such information is best found in the data sheets issued by the material producers themselves and in detailed databases for restricted material classes. A given material (polyethylene, for instance) has a range of properties that derive from differences in the ways different producers make it. At the detailed design stage, a supplier must be identified and the properties of his product used in the design calculations; that from another supplier may have slightly different properties. And sometimes even this is not good enough. If the component is a critical one (meaning that its failure could, in some sense or another, be disastrous) then it may be prudent to conduct in-house tests to measure the critical properties, using a sample of the material that will be used to make the product itself. The process is one of narrowing the materials search space by screening out materials that cannot meet the design requirements, ranking those that remain and identifying the most promising choice (Figure 3.4). The materials input does not end with the establishment of production. Products fail in service and failures contain information. It is an imprudent manufacturer who does not collect and analyze data on failures. Often this points to the misuse of a material, one that redesign or re-selection can eliminate. The selection of a material cannot be separated from that of process and of shape. To make a shape, a material is subjected to processes that, collectively, we shall call manufacture. Figure 2.5 of Chapter 2 introduced them. The selection of process follows a route that runs parallel to that of material (Figure 3.1, right-hand side). The starting point is a catalog of all processes, which is then narrowed by screening out those that fail to make the desired shape or are incompatible with the choice of material. Material, shape and process interact (Figure 3.5). Process choice is influenced by the material: by its formability, machinability, weldability, heat treatability and so on. Process choice is influenced by the requirements for shape—the process determines the shape, the size, the precision and, to a large extent, the cost of a component. The interactions are twoway: specification of shape restricts the choice of material and process, but equally the specification of process limits the materials you can use and the shapes they can take. The more sophisticated the design, the tighter the specifications and the greater the interactions. The interaction between material, shape and process lies at the heart of the selection process. To tackle it we need a strategy.
Processing and Design
Michael Ashby, Hugh Shercliff and David Cebon
University of Cambridge,
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