Material Selection

Material Selection
During recent years the selection of engineering materials has assumed great importance.Moreover,the process should be one of continual reevaluation. New materials often become available and there may be a decreasing availability of others. Concerns regarding environmental pollution, recycling and worker health and safety often impose new constraints. The desire for weight reduction or energy savings may dictate the use of different materials. Pressures from domestic and foreign competition, increased serviceability requirements, and customer feedback may all promote materials reevaluation. The extent of product liability actions, often the result of improper material use, has had a marked impact. In addition, the interdependence between materials and their processing has become better recognized. The development of new processes often forces reevaluation of the materials being processed.Therefore,it is imperative that design and manufacturing engineers exercise considerable care in selecting,specifying,and utilizing materials if they are to achieve satisfactory results at reasonable cost and still assure quality.
The first step in the manufacture of any product is design,which usually takes place in several distinct stages:a)conceptual;(b)functional;(c)production design.During the conceptual-design stage,the designer is concerned primarily with the functions the product is to fuilill. Usually several concepts are visualized and considered,and a decision is made either that the idea is not practical or that the idea is sound and one or more of the conceptual designs should be developed further.Here,the only concern for materials is that materials exist that can provide the desired properties.If no such materials are available,consideration is given as to whether there is a reasonable prospect that new one could be developed within cost and time limitations.
At the functional or engineering-design stage,a practical,workable design is developed.Fairly complete drawing are made,and materials are selected and specified for the various components.Often a prototype or working model is made that can be tested to permit evaluation of the product as to function,reliability,appearance,serviceability,and so on.Although it is expected that such testing might show that some changes may have to be made in materials before the product is advanced to the production-design stage,this should not be taken as an excuse for not doing a thorough job of materials selection.Appearance,cost,and reliability factors should be considered in detail,together with the functional factors.There is much merit to the practice of one very successful company which requires that all prototypes be built with the same materials that will be used in production and,insofar as possible,with the same manufacturing techniques.It is of little value to have a perfectly functioning prototype that cannot be manufactured economically in the expected salesvolume,or one that is substantially different from what the production units will be in regard to quality and reliability.Also,it is much better for design engineers to do a complete job of material analysis,selection,and specification at the development stage of design rather than to leave it to the production-design stage,where changes may be made by others,possibly less knowledgeable about all of the functional aspects of the product.
At the production-design stage,the primary concern relative to materials should be that they are specified fully,that they are compatible with,and can be processed economically by,existing equipment,and that they are readily available in the needed quantities.
As manufacturing progresses,it is inevitable that situations will arise that may require modifications of the materials being used.Experience may reveal that substitution of cheaper materials can be made.In most cases,however,change are much more costly to make after
manufacturing is in progress than before it starts.Good selection during the production-design phase will eliminate the necessity for most of this type of change.The more common type of change that occurs after manufacturing starts is the result of the availability of new materials.These,of course,present possibilities for cost reduction and improved performance.However,new materials must be evaluated very carefully to make sure that all their characteristics are well established.One should always remember that it is indeed rare that as much is know about the proportion of product failure and product liability cases have resulted from new materials being substituted before their long-term properties were really known.
Product liability actions have made it imperative that designers and companies employ the very best procedures in selecting materials.The five most common faults in material selection have been : (a)failure to know and use the latest and best information available about the materials utilized;(b)failure to foresee,and take into account the reasonable uses for the product(where possible,the designer is further advise to foresee and account for misuse of the product,as there have been many product liability cases in recent years where the claimant,injured during misuse of the product ,has sued the manufacturer and won);(c)the use of materials about which there was insufficient or uncertain data,particularly as to its long-term properties;(d)inadequate,and unverified,quality control procedures;and(e) material selection made by people who are completely unqualified to do so.
An examination of the faults above will lead one to conclude that there is no good reason why they should exist.Consideration of them provides guidance as to know they can be eliminated.While following the very best methods in material selection may not eliminate all product-liability claims,the use of proper procedures by designers and industries can greatly reduce their numbers.From the previous discussion,it is apparent that those who select materials should have a broad,basic understangding of the nature and properties of materials and their processing.。