Production I- Production I-
Chapter 3 Part B
4.6. Material Selection in Mechanical Design 1. Overview Of Material Selection
4.6.2. Factors Affecting Material Selection
4.6. Material Selection in Mechanical Design
great benefit to the design engmeer as they will manifest themselves as the selection terms of reference.
The questions are:
What is the components general shape and size?
Is the mass critical? Is a high density or low density material better?
Does the component need to have good thennal or electrical conductivity?
Does the component need to have anti corrosive properties?
What dimensional accuracy is required of the component?
What are the quantities in which the product will be produced ?
If the operating temperature of the product will vary - Will expansion or contraction of the material present problems?
Does the component need to be hardened to prevent wear or indentation ?
Is the component subject to static and or fatigue loads?
What mechanical strength is required of the component ? Is high tensile / compressive strength required?
What are the intended methods of fabrication ?
What are the time scales of the project ? Is the candidate material available within this constraint?
What are the project economic issues? How much or a constraint is this ?
These questions have arisen from four basic sub headings, namely, properties, availability, economics and miscellaneous issues. The author has opted to brieny discuss each of these in turn.
4.6.2.1. Availability
It is important that candidate materials are available within the time frame of the project. Whether the material is available on hand or it must be obtained within a stipulated period is certainly worth considering. The number of suppliers from whom the product is available is also important. Taking into account that a product/s may be manufactured over a period to follow, obtaining a material that is exclusive to one supplier, will place the designer at the mercy of the supplier for cost and possibly delivery.
It is therefore prudent to use materials that are readily available, if the design permits, as this would help to ensure efficient time to market scales. It is a little known fact in the commercial world that the producer who has introduced his product to the market before his competitors will have the initial sales burst necessary to balance the usually high cost of development.
4.6.2.2. Economics
Ln order to discuss this topic, one must understand that the relative price of raw materials varies from country to country. This is because different countries have
different materials in abundance and different amounts in which the 'shortages' occur. Taking the aforementioned statement into account and thc variability of the economy both on a world wide scalc and a intra govemmcntal scale, it is quite a difficult task to compile a price list that would be of any value over even a short period of time. What the author recommends is that the designer make frequent contact with suppliers via either personal calls or through design publications pertinent to a specific country. The electronic media available on the subject is of great help in this regard.
There may be cases where the product requires to function in a particular way that may only be fulfilled by a material with a particular . special or rare quality. In this case the designer must be able to justify that the usual added cost associated with this special material will be ultimately of benefit to the consumer and that this added functionality will add value to the product that the customer is willing
(0 pay for.
The other points worth discussing is the quantities in which the material will be required and the time for which the product will be in service. If only a few products are required, the design of the project may therefore be considered to be oppex sensitive as opposed to being cappex sensitive. If a substantial quantity is required the selection of a material should be influenced by a comprehensive cost study using current material costs.
Cost is allied also to the cost of treatment processes associated with some materials. It may be the case that a cheaper material that has to be treated will ultimately end in up being more costly than selecting a more expensive material which has the property that the cheaper one was treated for. A cost analysis in this respect is not just useful but necessary.
4.6.2.3. Material Properties
Fundamentally. the properties that a material possesses detennine its viability for a specific application. A materials properties may be broken down into mechanical properties. physical properties, chemical properties and dimensional properties. Each of these headings may be even further subdivided. Some of the latter sub headings are explored but again the list is by no means exhaustive.
4.6.2.3.1. Mechanical Properties
Strength
Hardness
Elasticity
Stiffness
Plasticity
Malleability
Ductility
Is the ability of a material to resist fracture under load.
Is the ability of a material to resist scratching and indentation
Is the ability to return to its original shape and dimension after being subjected to a load that caused or tended to cause, its defonnalion.
Is the measure of how well a malerial resists defonnation.
The ability of a solid material to undergo permanent deforn13tion without rupture.
Is the ability of a material to be defomled by predominantly compressive stresses.
The ability of a material to be plastically deformed by predominately tensile stresses.
4.6.2.3.2. Physical Properties
Density
Conductivity -
Conductivity -
Melting Point -
Is defined, serving all useful needs, as the ratio of a matcrials mass to its volulllc.
Is the affinity of a material to provide [or the process of (electrical) charge transfer through it.
Is the affinity of a material to provide [or the internal (thennal) transfer of energy frol11 particle to particle.
the temperature at which the material state changes.
4.6.2.3.3. Chemical Properties
Environmental Resistance, Composition, Bonding and Structure:
The elemcntal, atomic and sub atomic makeup and behavior oflhe material that provides for its suitability to a particular environmcnl.
4.6.2.3.4. Dimensional Properlies
Flatness, Surface Finish, Stability, Tolerances
These tern1S refer either to a materials 'raw property' or its susceptibility to undergo fabrication or treatment to attain such a state.
4.6.2.4. Miscellaneous Properties
This heading should be reserved for those Issues which cannot be categorized above but nonetheless must be bome in mind during the material selection process.
Recyclability
Heallh hazards (Carcinogens, Toxic, Flammable)
Emissions (Heavy metals, Volatile solvents, Ozone depletion) Waste Disposal (Bum, Landoll)
Product Liability Code Compliance