Engineers design and manufacture products from various materials. The materials used in the design must meet some constraints or threshold.
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The following are some of the requirements these materials must meet; withstand stress and strain or loads, be insulators or conductors of heat and electricity, be magnetic or non magnetic, light transmitters or reflectors, resistant to harsh environmental conditions, cost less, able to serve the purposes and impact less on the environment.
Designing things not only requires materials but it also demands the application of correct process. The process should not just be any—the chosen process must be compatible with the material that is used (Ashby & Johnson, 2002).
Today, almost all the materials used were developed over a hundred years ago Over 200 000 materials are available to be chosen by engineers, leaving manufacturers, designers, engineers with a challenge that our forefathers experienced in decisively choosing from this long list of options.
Selecting the right material and process is driven by factors including material performance, cost-cutting measures and efficiency, and minimizing environmental damage. Innovative design implies envisaging the use of the properties presented to us by different materials.
Today, these properties can be found from well documented engineering books. Nonetheless, advances in computer technology have even made things much easier in terms of storage manipulation, thanks to computer aided design.
Thinking strategically is imperative when it comes to matching material to design safety rolling. The shape of the final product made determines the choice of the process and vice versa. The relation is two- way traffic —the process too affects the shape, the size, the accuracy and, the price of a component.
Specifying the shape limits the option of material and process, similarly, specifying the process restricts the materials to be used and the types of shapes they form. The more complicated the design is the more you will be limited to the specifications.
This explains the close relationship between the two. The relationship between material, shape and process is at the core of the selection process. Material selection strategy is needed to deal with the problem of designing material of safety rolling. The processes steps to be followed include:
Translation: Design constraints, objective and free variables
Translation is ‘‘the process of adapting the design attributes for a component into a statement of function, constraints, objectives and free variables’’. Function means the work that the component can perform. A constraint is an indispensable requirement that must be achieved and is presented as a limit on a material or process quality.
An objective is the quantity for which limits (maximum or minimum values) are identified, usually cost, mass or volume, among others. Constraints are applied in the process of screening to isolate candidates that are able to perform the function. Objectives are employed in the ranking process to isolate the materials available that can perform the function.
Free variables are the limits or constraints of the material that can be changed by the designer. Material of safety rolling must meet some constraints, but at the same time attain the objective of safety on the environment.
The constraints of materials of safety rolling include; material of less toxic elements, Maximum use temperature > 600 C, corrosive resistant, maximum strength to withstand breakage, stress, strain; and capacity to be manufactured cheaply.
Objective: Maximize the strength of safety rolling; hence maximize safety of the material on the environment at the same time making it strong withstand any strain and stress to avoid breakage when rolling. Free variables are the parameters that can be changed to maximize the objective.
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The designer is at liberty to alter measurements that are limited to the design requirements. The designer is free to select any material for the component. Free variables for material for safety rolling can be the choice of the material and the cross- sectional area (Callister, 2003).
Screening: Five feasible materials for preliminary consideration
This step gets rid of materials that cannot perform the specified function. It is either because one or more of their characteristics does not fall in the extreme values- maximum and minimum of the constraints.
For example, the constraint that “the constituent must function in boiling water” or that “the constituent must not be opaque” sets clear maximum value on the aspect of maximum performance heat and optical properties that qualified candidates must fulfill. Based on the design objective of material of safety rolling, materials that can be preliminarily considered include; metals, polymers, elastomers, ceramics, and glasses.
Materials includes the family ‘Metals’, which is further classified into smaller classes like ‘Aluminum alloys’, and other sub-classes. Every material is classified based on a set of characteristics, design properties, the ecological impacts and the applications of the material.
This is commonly known as property profile. Selection process comprises, marrying the right property profiles of the materials in the world and that which is needed by the design. The materials from a particular family have similar properties. Metals are fairly stiff, calculated by the modulus, E. Nearly all metals are soft and can be deformed easily in their natural state; this implies that σy is low.
Metals can be strengthened by adding alloys and by mechanical and heat treatment, raising σy, however they retain their ductility, making it possible to be made by the process deformation. Generally speaking metals are tough, with a high fracture toughness of K1c.
They conduct both heat and electricity. Ceramics are non-metallic, inorganic solids, common ones are porcelain or alumina—the material widely used in spark-plug insulators. They have numerous desirable characteristics.
Their properties include; stiffness, hard and resistant to abrasive force, maintains their strength when exposed to extreme heat, and they are corrosion resistant. Large portions are used as perfect insulators. But they have disadvantages: unlike metals, they can easily break, and have low K1c.
This makes it almost impossible for ceramics to withstand low stress levels (such as holes or cracks) or for extreme joint stresses (like a clamp point). As result of this major drawback, it is easier to design with metals than ceramics.
The third group of material are the glasses, these materials are solid, non-crystalline (‘amorphous’). Commonly occurring glasses are the soda-lime and borosilicate glasses widely called bottles and Pyrex ovenware. Other materials are abundant.
The absence of crystalline structures stifles their plasticity, thus, similar to ceramics, glasses are hard and extremely resistant to corrosion. They are non conductors and are used as conductors, indeed, they allow light to pass through. Nevertheless, ceramics are brittle and susceptible to stress and strain conditions.
Polymers are organic solid compounds with long strands of carbon (or, in several, silicon) atoms. Polymers are not heavy—their densities ρ are low compared with other lighter metals. Their modulus E is approximately 50 times less compared with other materials like those of metals.
They are floppy, strong, and because they are less dense, the strength per unit weight of polymers can be equated to that of metals. Their characteristics are varied based on the changes in temperature hence a polymer that is tough and flexible at room temperature might become brittle at the lower temperatures.
But can become tender at boiling point.. Beyond 100 degrees centigrade the strength of polymers becomes useless. You can use these properties in the design. They can be easily shaped (thus the name plastics) thus sophisticated products doing different job are produced from one polymer in just one process.
Their characteristics are perfectly suitable for components that stick together, necessitating rapid assembly and less expensive. By precisely determining the size of the mold and prior application of color on the polymer, finishing operations are not required. A well designed process maximizes all the above properties.
Elastomers are mostly materials of rubber bands and sports shoes. These are polymers with exceptional properties. The stiffness, determined by E, is very low.
Compared with metals, they are 500–5000 times more. The other unique property is that they are capable of being stretched several times their original length but retain their original shape after the stretch. In spite of low stiffness, elastomers are strong and tough. This makes them suitable for car tires (Callister, 2003).
Ranking: Comparing the advantages and disadvantages of the five materials
After screening, the materials are then ranked using material indices. Material property cannot rule out those that are retained. It only aids in ranking those that remain. In order to achieve it an optimization criterion is required.
This is embodied in the material indices developed, which measures how perfectly the material has survived the screening process and hence can perform the function. The ability to perform the job is to some extent constrained by either one or more properties. The property or property class that optimizes the capacity to perform a given function for a particular design is known as material index.
There are several indices, with every index designed to maximize specific attribute of performance. They present standards of excellence that permit ranking of materials by their capacity to perform well in certain applications.
To sum it up whereas screening eliminates materials that have the ability to perform the work, ranking isolates those materials presented that can perform the job better. Re-examining our materials; metals, polymers, elastomers, ceramics, and glasses it is possible to rank them from the worst suited to the best suited for the job.
Starting with metals, they have several advantages. Metals are stiffer, easily deformed in their pure state, can be strengthened by alloying, retain ductility after treatment, and finally they are good conductors of heat and electricity.
They have only disadvantages namely; most are corrosive, some are toxic, cancerous and react with other elements. The ceramics have the following advantages; they are stiff, hard and resistant to abrasive force, retain their strength when exposed to extreme heat, and they are resist corrosion.
But ceramics have disadvantages too: unlike metals, they are fragile, and have a low K1c. The Advantages of glasses include: easily available, the materials are plenty, hard, resistant to corrosion, and transparent. The disadvantages of glasses can identified as, poor conductors of electricity and heat, vulnerable to stress concentrations and lastly they are brittle.
Advantages of polymers: can be easily shaped, hence sophisticated products doing different job are produced from one polymer in just a single operation and stick together, require no finishing operations. Advantages of Elastomers: stiff, easily stretched and return to original shapes and length, tough and strong.
The main disadvantage of elastomers is that they are not stiff. The merits and demerits of the five materials leave us with two materials at the top to be considered. The materials can be ranked from the most preferred based on the number of advantages and design limits and properties.
Ranked at the top (most preferred) are metals followed by elastomers, polymers, ceramics and glasses (the least preferred) material to be considered. As was aforementioned, metals have other subclasses like alloys- steel, iron, zinc Lead, aluminium, tin and copper.
Documentations: Detailed evidence of the key material related to the design constrictions, objectives and free variables
Documentation is different from the ordered property information employed in the screening process. Normally, it is a description, graphical presentation or pictures and charts of the case studies of earlier functions of the material, success analysis and facts on the corrosion, data about availability and costs, among others.
This kind of information can be retrieved from various sources like handbooks, manufacture’s data sheets, case studies of use, and success or failure studies. Documentation aids in narrowing down the selection choice to a final option, permitting final selection to be made between design constraints and material characteristics.
Screening and ranking help to reduce or simply eliminate the materials that are not suitable for the job. Without these two steps, then the choices from which to select is vast hence the volume of documentation becomes cumbersome. Closing eyes and blindly choosing any material is an exercise in futility.
However, after a reasonable number of materials have been isolated by the screening process and ranking step, an in depth analysis (documentation) can be done for the few remaining materials, and the exercise becomes feasible.
Figure 1: A flow diagram of selection a process, which is similar to material selection.
Metals: metals have various properties- chemical properties, physical properties, thermal properties and mechanical properties.
A steel ruler can ‘elastically’ bend with ease—‘elastically’ implies that it returns back after being released. The elastic stiffness is because of two factors- shape and property of steel itself. Thin size enables it to bend easily and the elastic modulus E is also high.
The point here is the steel ruler can bend elastically, although if it is a good one, it is not possible to make it permanently bend. Permanent deformation is linked to the strength of the material but not stiffness.
The ease with which a ruler bends permanently is determined by the shape and the different properties of the steel—namely the yield strength, σy. Materials with huge σy, such as titanium alloys, are difficult to bend permanently even if their stiffness, emanating from E, might be low; metals with low σy, for example lead, can bend without any difficulty.
When metals bend they become even much stronger. A concept commonly referred to as ‘work hardening’), of course the extreme limits must not be exceeded, called the tensile strength, σts, exceeding this limit the material deforms. This property of steel makes it a perfect choice of materials of safety rolling.
Metal properties change with changes in temperature, generally for the worse. Metals lose their strength at different heat levels. They can oxidize, degrade or worse of are decomposed.
It goes without saying that metals have a limiting temperature known as the maximum service temperature, Tmax, beyond this their application is not viable. Stainless steel has a highest Tmax—it may be applied up to 800°C. But most polymers have relatively low Tmax and are rarely used beyond 150°C. This difference in properties makes it easy to isolate materials.
Electrical, magnetic and optical properties
Without electrical conductivity man would not have easy access to light, heat, power, control and communication. Metals are good conductors. Copper and aluminum top the list of cheap conductors.
Although conductivity to some extent is undesirable fuse boxes, switch casings, the suspensions for power transmission lines all need to be insulted. Materials with high resistivity, ρe, are required the reverse the electrical conductivity κe.
Many plastics and glass have high resistance thus are used as insulators—although, with special treatment, they can become conductors. There is a close relationship between electricity and magnetism. Electric currents generate magnetic fields; a rotating magnet creates an electric current to any close conductor (Bralla 1998).
Most of the time metals operate in antagonistic environments, subjected to corrosive substances like fluids, to hot gases or to even radioactive radiation. Conditions that corrode metals include damp air, water, the sweat on people’s hands and other corrosive environments.
Therefore, for the material or metal to withstand the corrosion, then it must be designed with materials that are less corrosive or at times can be coated with materials which can withstand the environments where they are used. Common surroundings include fresh and salt water, acids and bases, organic solvents, and oxidizing flames (Ashby & Johnson, 2002).
The choice of process is limited by the choice of the material selected. For instance, compared with other materials it is easy to mould polymers. Elastic materials can be forged, rolled and drawn as opposed to materials that are delicate and hence must be shaped in other ways.
Materials that thaw at relatively low temperatures and low-viscosity liquids can be cast, while others can be processed by other methods. The shape of the material and product also determines the choice of process. Slender shapes can be designed easily by rolling or drawing but not by casting. It is hard to make hollow shapes by forging, but they can be made by casting or molding.
There are different classifications of processes. Manufacturing processes are categorized under the following heading; Primary processes which creates shapes. Primary forming processes include: casting, molding, deformation, powder methods, methods for forming composites, special methods including rapid prototyping.
Next are the secondary processes which aim at modifying the shapes or properties. In most cases they are depicted as ‘machining’, which adds some features to an already shaped product, and ‘heat treatment’, which improves surface or bulkiness of the properties. Under these we have joining and, lastly, surface treatment (Bralla, 1998).
Just like the flow diagram of the selection process of material, a step by step of a manufacturing process can be drawn. Nevertheless, the arrangement of the steps can vary based on the constraints of the design. Basically, three process families have been identified moving from shaping to joining and finally to finishing.
It is critical to revisit our major goal of safety rolling i.e. the material being friendly on the environment and at the same time meets some constraints, of safe rolling, material of less toxic elements, maximum use temperature > 600 C, corrosive resistant, maximum strength to withstand breakage, stress, strain, and the capacity to be manufactured cheaply.
But of course the Objective remains: to maximize the strength of safety rolling; hence maximize safety of the material on the environment at the same time making it strong to withstand any strain and stress to avoid breakage when rolling. Comparing lead and Aluminium, the latter can be the most preferred metal with such properties which is less hazardous both to the environment and human beings.
Moreover, it can be safely rolled because it is less ductile. But coating as a way of finishing, increases the cost of the manufacturing process. On the contrary, coating as a finishing process is justified because it hardens, safeguards, or makes the surface more attractive thereby increases its value.
In a nutshell, materials have properties like density, strength, cost, and the ability to resist corrosion. Proper design demands a particular profile. It is imperative to begin with the whole set of choices of materials as options. Without going through the steps selection process, the entire process is doomed from the start.
The process of engineering design is a multifaceted process for which there is hardly ever a precise right accurate solution. An in depth knowledge of the loop connecting or linking the function, materials, processes and the shape of materials is inevitable. The right choice of material can minimize costs, and damage on the environment.
Ashby, M.F. and Johnson, K. (2002). Materials and Design the Art and Science of Material Selection in Product Design, Butterworth-Heinemann, Oxford, UK.
Bralla, J.G. (1998) Design for Manufacturability Handbook, 2nd edition, McGraw- Hill, New York, USA
Callister, W.D. (2003) Materials Science and Engineering, An Introduction, 6th edition, John Wiley, New York, USA.