Materials are like humans – they have their strengths and weaknesses. Some are hard others soft, some are flexible, other will not even budge a centimeter. Next to the more classic materials there are new high-tech materials. The topic is far too complex to discuss in full. Therefore, on the following pages I will only describe certain material properties that play an important role in industrial design.
In simple words: Strength is the ability of a material to withstand external forces without being damaged or irreversibly deformed. While elastic deformations are reversible, plastic deformations are irreversible and the material doesn‘t return back to its initial state after the force has been removed. The consistency of a material can be measured by how much force is necessary to deform it irreversibly. Depending on the direction of a force, one speaks of tensile, compressive or bending strength.
One could think that all three forms of strength and firmness are the same in one material: That tensile, compressive and bending strength all have the same effect and are responded to with the same resistance. But that is not true. One simple example can prove the different effects the direction of acting forces can have on a material. A piece of chalk has a very meagre bending strength. Only a slight bend is enough and - crack! Such a case is called a failure of material. While being bent, chalk traction forces are working on the top and compression on the lower part resulting in the fracture. Chalk cannot resist traction forces and easily breaks down in the upper part. The chalk ruptures and breaks in two. There is no repairing this piece of chalk even if it withstands compression rather well. Have you ever tried to break a piece of chalk only by pressing it with your hands? It is nearly impossible. Chalk is very strong when it comes to compression. To achieve a high level of resistance to bending it is therefore important that the material is able to withstand traction and compression equally. If one of the two components is less highly developed the material becomes easy to break, since bending forces are always a combination of tensile and compressive forces.
Comparable features can be found in concrete – but in bigger dimensions. Concrete cannot resist traction for long. Houses made of cement would not exist had builders not found a way of combining it with steel. At those points where traction forces are at work reinforcing steel is integrated into the structure to protect it. Steel combined with cement has the ultimate tensile strength. This shows that combining different materials often optimizes the result. Both materials compensate for their weaknesses and combine their strengths.
Of course, one has to consider whether both materials also work together on other scales. It is like in a marriage: Not only do the strengths and weaknesses have to be considered but the construction also has to work well in daily life. With cement and steel this is nearly ideal. Steel does have one big weakness: In the short or long term it corrodes. But this is no longer a problem since the alkaline properties of cement prevent any corrosion. Another aspect is that cement and steel have nearly the same warmth specific elongation, which means that when getting warm they stretch nearly in the same way and shrink when it is getting cold. If this were not the case steel cement constructions would be damaged as a result of fluctuating tempera-tures – houses made of those materials would then be a rather bad idea. But since both like each other and work jointly they make a fruitful marriage in good and bad times.
Keeping up appearances.
Hardness and strength sound as if they should mean the same thing. What both material properties have in common is that both are meant to resist forces from the outside. But there are differences between these two. Hardness among other things describes the degree to which a substance withstands wear and tear. Hard glasses and watch glass resist scratches and stay beautiful far longer. There are different testing procedures to examine a material’s hardness. The two mostly used and best known are Rockwell and Vickers.
The Rockwell Assay Method
determines the Rockwell hardness of metallic materials. For that an indenter is put upon a material’s surface with a certain amount of pressure. The lesser the penetration depth the harder the material. Rockwell Hardness is measured in HR (Hardness after Rockwell). According to the tested material different indenters are used. This can directly be seen by means of the Rockwell Hardness unit. When for example a cone shaped test probe with a diamond tip is used the unit is called HRC. A Nirosta© stainless steel knife blade has a hardness of about 53 HRC, a gear shaft in comparison has a hardness of “only” 48 HRC. A diamond with 100 HRC is the measure of all things.
The Vickers Assay Method
determines the Vickers Hardness for an amount of materials ranging from aluminium to hard materials like titanium carbide. A pyramid-shaped indenter made of diamond puts a certain pressure upon a material. The remaining imprint is measured under a microscope; the hardness is calculated based upon the imprint-length (d1, d2) of the diagonal. The same indenter is used for soft and hard materials. The hardness is measured in Hardness after Vickers HV. To give two examples: A Japanese Katana sword has a Vickers Hardness of about 600 HV, titanium carbide has a slightly higher scale of 3,200 HV. That makes titanium carbide a valuable material for coating a surface.
Elastomer materials and plastics on average are softer than other materials like metal or minerals. Therefore a far softer method is needed to measure their hardness. In 1915 the US-American Albert Shore developed a new procedure for measuring the hardness of elastomers and rubbery-elastic polymers. His name gave it its title. The device for measuring Shore Hardness consists of a spring-loaded hardened steel pin. The penetration depth (h) into the tested material reveals its hardness. According to the form of the pin and the used spring force procedure Shore a, b, c and d have to be differentiated. Most times tests use Shore a or Shore d. With Shore a the force of 12.5 Newton is used to put pressure on the material, Shore d uses 50 Newton. With Shore a the steel pin has a flattened tip, with Shore d the steel pin has a conical tip. The Shore hardness a is used for soft rubber, Shore d for harder plastics like elastomers. Since synthetic materials are heavily influenced by temperature, every measurement takes place at a temperature of 23° C +/- 2K. The scale lies between 0 and 100 Shore. The higher the figure the harder the material.
The casing was forged from rust-free steel and covered with titanium carbide. Together it builds a surface hardness of approx. 3200 HV
A constant back and forth
A metallic scroll spring symbolizes elasticity like nothing else. Under pressure it changes form, but the moment that force ceases the scroll spring turns back to its original form. Elasticity is an important factor in materials. Many elastic materials – like the synthetic material ABS – is used in nearly every product we encounter in our everyday life. A material’s elasticity, its ability to compensate small deformations, is used in industrial design for many purposes. Sometimes it can replace mechanic constructions that would be far more expensive and complex, for example the snap lock of a shampoo bottle. Thanks to elasticity the latch can be opened and closed easily. The use of plastic therefore is of value. Another plus point is plastic’s ability to deform when confronted with a heavy impact. Through deformation the time frame of the impact is increased as well as the contact surface. This reduces the power spike during a crash and therefore lessens the damage. The effect is not only used for rubber rooms in psychiatric wards but also for many other products.
An easy way to stability.
Material b has the same volume as material a but weighs more, therefore we can conclude that b has a higher density than a. Density is an important sign of efficiency, it defines the relation between weight and volume. In many areas materials with a high strength and at the same time minor weight are required. The monocoque of a Formula 1 racer for example has a very stable chassis that can resist speeds up to 200 miles per hour. Its construction consists of a carbon strengthened synthetic material and is not only far more lightweight than the formerly favoured aluminium but also has very special features concerning rigidity.
Often enough the density of a material relies on its use. Density defines how efficient a material is compared to other material properties. Just imagine a bicycle: The frame should be robust and resist tensions. At the same time it should be as light as possible. A bicycle made of Steel 316L would be extraordinarily stable but would weigh a bit too much. Certainly many cyclists would lose interest in using it after a very short time. Modern alternatives are Kevlar and carbon – they offer maximum stability and minimum weight.
As with any other substance materials have a lifecycle. They change according to environmental impact like ultraviolet rays, heat, and humidity as much as due to constant stress through vibrations, compression or tractive forces. These can impair function over the long term. Therefore when choosing a material it is important to consider chemical features as well.
The most essential chemical features are:
• UV resistance
• Humidity resistance
• Corrosion resistance
Let us take wood for an example. Wood is nature’s wonder material: solid and workable, naturally degradable, aesthetic, non toxic. But sadly enough it is not perfect and deforms in time. Hygroscopic features are to blame: Wood absorbs the humidity of its surroundings resulting in fluctuation in form due to swelling and shrinkage. The result is that a beautiful wooden table becomes wobbly after a few years use and solid wood windows can no longer be shut properly. Many synthetic materials like PP and APS change as well during time. Plastic cases become brittle and acquire an unhealthy looking yellow tone. That is why plastic is often considered to be something cheap. To prevent changes caused by our natural environment they can be pre-treated. Additives can change the features of synthetic materials. Plasticizer prevents brittleness and hardness, stabilizers increase the lifespan and protect the product from other influences like oxidation and UV light. Even the features of wood can be improved when it is pre-treated, for example with the wood modifier TMT, or in letting it be stacked for a longer time or pre-treating it with UV radiation.
Besides hardness and elasticity there are other material features essential for a product. Equally important is the possibility to shape it into a desired form in serial production. Only after a material can be handled easily and precisely as a product can it then be fabricated in bigger quantities and for a reasonable price.
Important criteria for material processing are:
Can a material be made highly fluid and will there be bubbles or cracks after casting?
Will it shrink?
Can a material be re-shaped durably?
Are procedures like rolling, forging or other possible?
Can a material be worked with machining techniques like drilling, turning, milling or cutting?
Composite materials like Kevlar or carbon are good (or bad) examples for how important the processability is when choosing a material. They have a good stability and hardness and are light at the same time. That is why they are used in aeronautics. When in the 1980s these materials were used for the first time for Formula 1 cars, the automotive manufactures could not fabricate the parts themselves. Because of the difficult fabrication process they were produced by the aerospace industry. The fabrications of these high-tech materials were complex and expensive because they had to be fabricated under high temperatures and extremely high pressure in a special furnace.
Every product is constantly interacting with the world. Trees have to be cut if you want to build a wooden table. For a plastic computer casing oil has to be dispersed into its particles to get the ingredients needed for the production process. If – after only a few years use – a computer is thrown into the trash this has an impact on nature as well. Energy is needed for recycling which creates new pollutant emissions. Even more bizarre is the export of wealthier nations‘ waste to poorer countries. The ecological features of a material can help to protect our natural resources and environment. The use of renewable energies is a start. Wooden houses are not only built from a renewable resource but also have excellent features to assure a quality in living. Organic clothes do not only create a good figure but a good conscience as well. It is important that products are made from renewable and regionally produced materials. That reduces transport routes and supports regional business. Sounds too good to be true? Okay, what to do when in order to create a product a designer cannot relinquish a certain material – because of its features, the costs or the client who desperately wants to have it? Even then it is necessary to look more closely, as alternatives exist for special materials as well. Toxic substances should better to be avoided. Many IT manufacturers, for example, have pledged to reduce the use of bromine based flame-retardants in their products. The use of lead today is almost completely forbidden in Germany. These are only two tiny little stones in a big mosaic. A designer can actively protect the environment by thinking a bit further out of the box and not just settling for the status quo.
These factors are essential for ecological features:
Do the resources exist in large quantities? Or is it necessary to save resources by using alternative materials?
How much energy is needed for the fabrication? Are there energy efficient alternatives in materials?
Does a material consist of toxic ingredients or are they superseded by fabrication? Can these substances be replaced by non-toxic material?
Can the material be recycled well enough? Does this process require as little effort as possible?
The magic word is “combination”
An industrial designer has a position similar to that of a soccer trainer: A trainer has to put the right person with the right abilities in the right position. Only then can a team become successful. As much as the football players complete each other, materials in design should be used in a way that allows them to contribute their strengths and compensate for each other’s weaknesses.
The mortar Milli is a good example of successful teamwork: Ceramic materials possess an extremely hard surface but are difficult to process precisely. They are also brittle and fragile. Synthetic materials like plastic work the exact opposite way. They lack hardness but can be shaped quite easily. Therefore it is an option to combine both materials when fabricating a mortar in a way that they can rely on each other’s strengths. The areas exposed to friction are made from ceramic while the pestle’s handle and the bowl’s bottom are made from silicon. The silicon parts shield the mortar from damage when accidently dropped and by doing so increase the practical value.
Milli – an intelligent kitchen helper
Milli creates the necessary friction and at the same time maintains as much contact as possible between mortar and pestle so that nothing remains stuck on the sides. The uneven dimples are concave, have a big surface and are not very deep. The dimple’s margins are sharpened to heighten the friction. Ceramic is very fragile and is therefore used only at the bottom of the pestle and for the mortar’s core areas. All other parts are replaced by silicon.