Cost per Unit Property

Cost per unit property can be initially used to group the materials. This method of initial selection is applicable if one property is critical in the application. If the strength of the material is the critical property, then we can calculate the cost per unit strength in the following manner  

F is the force applied in the application and A is the cross-sectional area of the part. The material is supposed to possess at least this much strength, though the actual strength requirement is higher after incorporating the factor of safety. Let us say that the length of the part is L. Now, the cost of the material, let us say, is C per unit mass. Then the cost of the part becomes  

L is the length of the part and p is the density of the material. Substituting for A from Equation 2.1, we get  

For a given application, both F and L are fixed. Hence, the different materials can be compared witii respect to their pC/S values. Fix an upper limit for this value, and make a group of materials having this upper limit value or lower. Materials from this group are taken for further analysis in the selection process. 

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To avoid complete chaos, we badly need to convert to computerized searching in the very near future. Such a search system would start first with the absolute property requirements, Pj, and select only those materials which passed this first screening test. Second, it would balance the relative importance f of other properties pj, preferably on the basis of cost per unit property. And third it would present the design engineer with the one or several materials whose balance of properties 2P would be best suited to his needs. 

Cost puts a constraint on material selection. In simple cases, the cost per unit property can be used as a criterion for selecting the optimum material. Because of the changing cost of materials in the market, inflation, supply and scarcity, the cost of materials is measured as a relative cost (Cr) where the cost of material is normalized to the cost of mild steel. 

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