34 HANDBOOK OF MECHANICAL DESIGN 



SELECTION OF MATERIALS • 



The universal problem in engineering design is the selection of the materials from which the 

 various parts of the device, machine, or product are to be made. It is also the first problem because 

 the material selected will govern the allowable stresses, the types of construction that might be 

 adopted, the manufacturing methods employed, the assembly operations, the finishes that might be 

 applied, and, of greatest importance, the cost and sales appeal of the product. In many designs, 

 the commercial success or failure will be determined definitely by the materials selected. 



In practically every design, the physical and other properties required will determine which 

 materials might be used. But the relative importance of the different properties will vary consider- 

 ably for different types of design. The unit strength of the material is practically always a factor 

 though often a minor one. 



For constructions subjected to only a steady tension, the yield point on the stress-strain curve 

 or the yield strength of the material, i.e., the unit tension it can withstand with a specified elongation, 

 will be the first consideration. But for a compression-loaded column, both the tensile strength and 

 the elastic modulus must be considered. For vibratory or repeated stresses, the endurance limit of 

 the material becomes the governing strength consideration, whereas for low-temperature service and 

 shock loads the impact values are of great importance. And, of course, there is also to be considered 

 the compressive strength or the shear strength, according to the type of stresses to which the mem- 

 ber will be subjected. 



In addition to the unit strength considerations, any one or a group of almost innumerable other 

 properties must be considered. If, as in most machine tools, it is important to have little or no 

 vibration, a material with a high vibration damping capacity, such as cast iron, might be considered 

 first. Hardness, wear resistance, porosity, and ductility are some of the other properties that may 

 be of major importance. 



In addition to physical properties; corrosion resistance, heat conductivity, electrical conduc- 

 tivity, dielectric strength, frictional properties, and many others may enter into the problem. 



There is no formula or equation by which the most suitable material from the standpoint of 

 properties can be selected. Nor is il always advisable to use the material that has the highest values 

 for the properties desired. Invariably the final selection must be a compromise largely because two 

 other important factors enter into the problem, namely, the workability of the material and its cost. 



When a number of different materials have been selected, each of which possesses the desired 

 properties to a satisfactory degree, the next step toward the final selection is the determination of 

 the manufacturing methods that might be employed. Aluminum, zinc, and many of the non- 

 ferrous alloys naturally suggest die-casting, stamping, and forging. Iron, steel, aluminum, and some 

 other metals offer great possibilities bj^ virtue of their weldability. Casting is suitable for almost 

 all metals and alloys. Plastics are mostly molded; some are sheet-laminated or are in the form of 

 sheets; a few are extruded. To mention only a few other manufacturing processes, we have impact 

 extrusion, die extrusion, drawn shapes and rolled shapes, and roll-formed sheet sections. 



After it has been determined what types of construction might be used, the design must be 

 analyzed with reference to such things as the use of inserts, consolidating different parts into one 

 piece, use of standard purchased parts, and similar possibilities. 



Hand in hand with the types of construction that might be employed are the costs of machining, 

 grinding, and other operations, which will vary greatly. Included in this category may be pimch- 

 ing, hand reaming, riveting, buffing, and polishing. 



Not until all the factors discussed above have been studied closely and analyzed should any 

 consideration be given to the cost per pound of the material. A complete analysis may often reveal 

 that aluminum at 30 cts. per lb. or zinc at 10 cts. per lb. is cheaper to use than gray iron at 5 cts. 

 per lb. 



A complete analysis of all the items to be considered in the selection of materials and the associ- 

 ated problems of types of constructions and workability considerations would require volumes and 

 even then would obscure the problem rather than clarify it. In the final analysis, nothing can be 

 substituted for clear engineering thinking based on broad experience and knowledge. 



