(a) Describe the steps usually considered in the process of material selection.
(b) How material selection is done using decision theory.
Ans. (a) Like any other aspect of engineering design, material selection is also equally important aspect. Material of part depends on the requirements of the part, number of parts needed, and manufacturing process to be adopted to make the part. The material selection primarily depends upon its properties and several other factors like availability, economic aspects, etc.
As regards properties of materials, all the following properties need to be considered,
(i) chemical (characteristics that relate to the structure of a material and its formation from other elements),
(ii) physical (characteristics that are determined by nature, like electrical, magnetic, thermal, etc.),
(iii) mechanical (strength, ductility, toughness, rigidity and behaviour under application of force).
(iv) dimensional (relating to available size, shape, microtopography and tolerances),
(v) technological (machinability, weldability, castability, forgability, bendability, malleability, ductility, hardenability, etc. all relating to manufacturing process).
It is essential to establish the performance requirements that the material must meet; then select the appropriate materials for evaluation, and finally select the material that best meets the performance requirements and economic constraints.
It is best to consider the alternatives from a wide gambut of materials, viz. ferrous materials (cast iron, cast steel, mild steel, alloy steel, carbon steel, tool steel etc.), non-ferrous materials and their alloys, plastics, ceramics, elastomers, composite materials, etc.
Having selected the material from consideration of properties, next step is to check availability, i.e. whether material is readily available, lead time for procurement, quantity required, nature of supplies, whether any special method required to manufacture.
The final step is to examine economics, i.e. the cost should be minimum. Cost of a material consists of the initial cost, processing cost and maintenance cost