Design tools and materials data

To implement the steps of Figure 2.1, use is made of design tools. They are shown as inputs, attached to the left of the main backbone of the design methodology in Figure 2.4. The tools enable the modelling and optimization of a design, easing the routine aspects of each phase. Function modellers suggest viable function structures. Geometric and 3-D solid modelling packages allow visualization and create files which can be downloaded to numerically controlled forming processes.Optimization, DFM, DFA* and cost-estimation software allow details to be refined. Finite element packages allow precise mechanical and thermal analysis even when the geometry is complex. There is a natural progression in the use of the tools as the design evolves: approximate analysis and modelling at the conceptual stage; more sophisticated modelling and optimization at the embodiment stage; and precise (‘exact’ - but nothing is ever that) analysis at the detailed design stage.

Materials selection enters each stage of the design. The nature of the data needed in the early stages differs greatly in its level of precision and breadth from that needed later on (Figure 2.4, right-hand side). At the concept stage, the designer requires 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, even though the function is the same. The problem at this stage is not precision; it is breadth and access: how can the vast range of data be presented to give the designer the greatest freedom in considering alternatives? Selection systems exist which achieve this.

Embodiment design needs data for a subset of materials, but at a higher level of precision and detail. They are found in more specialized handbooks and software which deal with a single class of materials - metals, for instance - and allow choice at a level of detail not possible from the broader compilations which include all materials.

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. A given material (polyethylene, for instance) has a range of properties which derive from differences in the way 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.

It's all a bit like choosing a bicycle. You first decide which concept best suits your requirements (street bike, mountain bike, racing, folding, shopping. ..), limiting the choice to one subset. Then comes the next level of detail: how many gears you need, what shape of handlebars, which sort of brakes, further limiting the choice. At this point you consider the trade-off between weight and cost, identifying (usually with some compromise) a small subset which meet both your desires and your budget. Finally, if your bicycle is important to you, you seek further information in bike magazines, manufacturers' literature or the views of enthusiasts, and try the candidate bikes out yourself. Only then do you make a final selection.


The materials input into design does not end with the establishment of production. Products fail in service, and failures contain information. It is an imprudent manufacture who does not collect and analyse data on failures. Often this points to the misuse of a material, one which re-design or re-selection can eliminate.






MATERIALS SELECTION IN MECHANICAL DESIGN
SECOND EDITION
MICHAEL F. ASHBY
Department of Engineering, Cambridge University, England
OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

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