3.3 New materials
Many problems have been solved by the application of a new material or process. The unique properties of a new material can make possible products or product forms that were previously unconsidered. This is a theme that I will explore further in introducing the steps of invention and product development.
The development of the carbon filament seemed a logical step because it had been shown very early in the Victorian period by Davy and Faraday that the illumination from a candle is supplied by brightly glowing, heated carbon particles. (The experiment is easily carried out: simply play a flame against a cold surface and the surface will quickly blacken with the deposit of carbon particles from the flame.) Inventors already had some idea of what direction to proceed since it was known that carbon could conduct electricity (hence its use in the carbon-arc light). Thus Edison performed experiments on thousands of carbonaceous (carbon-rich) materials before arriving at bamboo fibres, which could be charred to create a carbon filament that would conduct electricity.
This trial-and-error approach to solving a problem has always been one of the principal ways in which inventions are made, and it is still an important tool in the inventive method. It is not a step entirely into the unknown, however, because the inventor will have a good idea of the properties needed, and the classes of material that will meet those needs: the inventor always builds on what has gone before.
For simple products, many alternative materials may be acceptable, especially where the desired main property criteria are mechanical (like strength or stiffness) rather than for more specialised applications (like optical or magnetic properties). Another important mechanical property is Toughness. Thus a chair can be made in steel, wood, plastic, composites, textiles, and even inflated rubber. Each material has its own characteristic set of properties, and within each of the very large classes of material, there are numerous types and grades that can show enormous variation of individual properties. Iron, for example, can be obtained in several forms: as wrought iron (soft and ductile), steel (tough and stiff), or cast iron (stiff but brittle). Wood can vary from a stiff and heavy hardwood like oak to softwood, and even includes a light foam (balsa wood). Aluminium is much lighter than steel but can meet and achieve equivalent functions, although at greater raw material cost. Modern plastic materials can offer a wide variation in properties from a tough polymer like polyethylene to a brittle material like polystyrene.
Put simply, toughness is just the opposite of brittleness. A material is said to be brittle if it is easily broken by an impact. Most ceramics are brittle: you will be aware that a ceramic mug will shatter if dropped on a hard surface. Metals, however, are almost always tough materials: a dropped metal saucepan might suffer a dent, but will not shatter.
This is essentially the difference between a brittle material and a tough one: tough materials will tend to absorb damage by denting or changing their shape permanently in some other way, whilst brittle ones will break.
You can demonstrate the difference between tough and brittle to yourself using a metal paperclip and a piece of chalk. You will find that bending the paperclip doesn't cause it to snap even though a lot of deformation is involved. (If you bend it back and forth repeatedly you can break it, but this shows a different phenomenon – metal fatigue.) The chalk, on the other hand, snaps in two very easily, despite being thicker. The chalk (which is essentially a ceramic material) is much less tough than the steel used to make the paperclip.
Toughness is not as easy to calculate for a material as strength. Strength just needs a measure of the force needed to break the material and the area of the sample over which the force was acting. Calculating toughness requires knowledge of the force required to break a specimen of the material that has a crack of known length in it. Figure 41 shows a typical specimen used to test the toughness of metal samples, and how it is loaded. A crack is grown into the sample by imposing metal fatigue on it, which involves loading and unloading the sample many tens of thousands of times at a load much below that at which the sample would normally fail. This causes a crack to grow in the sample, and the geometry and loading of the test piece are designed so the crack grows in a controlled manner.
You may ask yourself: how did my simple paperclip and chalk test for toughness manage to show anything, as there were no cracks visible in my samples? The answer is that all materials contain small flaws or cracks, even if they are not apparent to the eye: they may be only fractions of a millimetre in size. Toughness is a measure of how susceptible a material is to fracture, even if obvious large flaws are not present.
Toughness is very useful for designers because it can be used to predict whether small flaws in a structure are safe or unsafe. Good toughness is important if you're designing a bicycle, which must be reasonably resistant to heavy bumps, or a knife that won't splinter in the washing-up bowl.
As the property specification becomes more severe for a specific product, the choice of materials diminishes. Let's look at two examples of how materials are used to meet a product specification.
A domestic kitchen table has to meet several functions: act as a level work top for preparation of food for cooking, support moderate loads when in use either as a work bench or as a surface from which meals can be eaten; and be resistant to impact blows from users. It should also resist the moderate heat and humidity that arises during cooking and related tasks, and it might have to withstand the heat of a hot pan from the cooker being placed upon it. An additional requirement might include wear resistance for the worktop itself so that knife blades do not scar or harm the material of construction. Such scars could act as a reservoir for food debris, which could decay there and form a breeding ground for harmful bacteria. These are all functional requirements of the design. There are also constraints on the form: the dimensions should conform to the space available in the kitchen and the height to the normal working height of the cook.
The materials used must therefore be capable of withstanding loads applied to the working surface without noticeable distortion, be impact and wear resistant, and resist the effects of moisture and heat. Many woods, such as oak and pine, are well capable of meeting these demands, and are the 'traditional' materials of choice. Modern equivalents include combinations of steel legs and plastic-faced wooden worktops, where the plastic finish may offer greater wear resistance and hence be less likely to act as a source of bacterial contamination, but may not be suitable for the placing of hot objects.
The outer case of an electric light bulb presents a much tighter property specification than the kitchen table, since all the materials of its construction must be able to withstand the high working temperatures produced by the incandescent filament. The exterior shell (or envelope) of the bulb, which is the prime container for the glowing filament, must also resist atmospheric pressure since it encloses a vacuum around the filament itself. It must be translucent or transparent to allow light from the filament to escape. The base to which the envelope is attached needs to provide safe electrical contacts for the filament. The material used in the bulb must also be corrosion resistant.
Glass is the only material capable of meeting the stringent requirements of mechanical and thermal stability needed for the envelope. Its thickness can be controlled to provide varying levels of mechanical resistance (to external impacts for example), although a domestic bulb is normally fragile because glass is brittle. Metal contacts are usually made from aluminium sealed in an insulating material, and the cap is also normally made from aluminium, a tough metal.
Activity 18 (self-assessment)
- a.What is the function of a lightweight ladder with 15 rungs for use around the home?
- b.What materials properties should be included in its specification?
- c.Describe from your own knowledge what materials are used in such a product and how it meets the specification.
(The answer to this activity mentions Angles , so it would be helpful to read the box below before looking at it.)
- a.The main function of a ladder is to provide access to the upper parts of a wall or building that are normally inaccessible. In normal use it will be leant at an angle to a wall so the user can climb to a given position and gain that access. It must support the user in a stable and safe way, so the materials of construction must resist deformation under the weight of the user (plus any additional load carried by the user, such as paint or tools). The ladder is normally used at an angle of about 75 degrees to the horizontal (as shown by a warning notice posted on all new ladders), and the ladder will normally be fitted with an anti-slip device. Since it is certainly used externally, and may be left outside for some time, it should be resistant to the weather (rain, frost, sunlight, etc.) It should also be light enough in weight to be moved easily.
- b.The material used for the ladder should be strong enough to not fail under the user's weight. It should also be stiff so it does not flex unduly during use. It should be resistant to degradation if left exposed to the elements.
- c.The main materials of construction include wood and aluminium for both the stiles and rungs. Wooden ladders usually rely on unprotected feet and tips, while aluminium ones are normally equipped with rubber feet and plastic tips to prevent slip during use. While both wood and aluminium are intrinsically stiff enough to provide stable ladders (they have reasonably high Young's moduli), problems could arise if the thickness of the component parts is too low. Both materials offer acceptable resistance to deformation caused by the user, but there are differences in environmental resistance. Wood in particular is susceptible to rot from fungi, algae and bacteria, which can cause major structural weakness in the long term. Aluminium is much less susceptible to such degradation.
Quite often the relative position of two objects is defined in terms of the angle between them. A ladder leaning against a wall might carry a warning that it should be used at an angle of around 75º (75 degrees) to the horizontal (Figure 42). Note that it is important to say what the angle is relative to: the angle that the ladder makes with the wall is much smaller: 15º.
By definition, a complete circle sweeps out 360º. A ladder standing vertically on a flat planar surface would be at 90º (a quarter of 360º). You will probably already have a familiarity with this notation for describing angles.