SEED Guides Selecting Structural Configuration < >

The main questions of material and manufacturing choice will be dealt with in other Guides. The prime consideration here is their interaction with the structural design criteria. Most theoretical structural work assumes the linearly elastic behaviour of an idealised metal. Stiffness is a constant up to a known elastic limit and deflections are insignificant in relation to overall geometry. Ceramics, plastics, and even composites are treated as elastic by finding some equivalence relationship to a simpler ideal structure. While this may be convenient for analytical purposes, and usually adequate for most cases, it may lead to a distorted view of how the structure actually behaves and ignore some potentially significant issues. Different manufacturing methods for producing the same shape even from the same material may confer different structural properties. This may be deliberately used to gain an advantage where the Œgrain flow' in a forging, for example, will increase the strength in certain directions compared to a machined or cast shape. For most purposes the range of acceptable materials and processes is limited to those generally available in the industry concerned. Departures from these norms may be expensive and have to be strongly justified to be acceptable, both to the industry and its customers.

Considerations of material choice and the methods of manufacture and assembly or construction play an important role in structural design. The choice of materials depends not only on the desired structural properties but also the available means of construction and a variety of other factors. Internally configured materials, such as reinforced and pre- or post-stressed concrete, locally hardened metals, or the growing range of fibre composites, are becoming more common. Access to joints may be required during assembly or to exposed surfaces to apply protective coatings during its service life. Temporary additional structures may be needed during assembly; jigs for smaller structures to ensure accuracy or Œfalse workı to support parts of larger assemblies before they can support themselves. It may be necessary to avoid closed volumes if the assembly is to be heat treated, dip coated, or subjected to pressure loads. These are examples of the sort of material and manufacturing factors which must be born in mind during the design process.