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1 Engineering materials
1.1 Introduction
I found this old nail (Figure 1.1)- some 135 mm long - whilst poking about among the debris from restoration work in the medieval hill-top village of San Gimignano, Tuscany. In the Middle Ages, such nails were made by hand, a blacksmith using a hammer and an anvil, in this case containing a suitable slot so that he could form the nail head. His only additional equipment would be a charcoal fire assisted by goat-skin bellows to enable him to heat the piece of iron from which he forged and pointed the nail. Because of the way in which they were made, medieval nails were roughly square in cross-section and differed little from those with which Christ was crucified more than a thousand years earlier.
Such products were of course very labour intensive and, even allowing for the relatively low wages and long hours worked by craftsmen in those days, nails would have been quite costly items. Producing the necessary lumps of iron from the original ore was also an expensive process, adding further to the cost of the nail.
Following the Industrial Revolution, methods used to manufacture nails became increasingly mechanised. In modern processes, steel rod - or wire - is fed into a machine where a die automatically forges the head whilst almost simultaneously cutting through the stock rod at suitable angles to form the point of the resulting nail. The process is completed in a fraction of a second and the rod, or wire, immediately travels forward to produce the next nail. Since a single operator may tend several machines, the immediate labour cost of producing a nail is very low compared with medieval times. However, the costing is not quite as simple as that - things rarely are! A modern nail-making machine is a complex and fairly expensive piece of equipment which must be serviced frequently if it is to show a suitable profit before it finally becomes 'clapped out' (how often has one purchased nails whose points proved to be blunt due to lack of maintenance of the shearing blades?). There are of course many other overheads as well as running costs which must be taken into account in assessing the cost of producing a modem nail. One thing is certain: technological advances during and since the Industrial Revolution have reduced the unit cost reckoned in man-hours in producing a nail.
The above illustrates how a material, in this case iron, has been made use of for the production of a product for which there is a demand (how would structures such as wooden houses and ships have been made in ancient times without nails?). In this chapter, we take a look at the relationship between materials, their properties and the uses of the products made from them.
1.2 The requirements
The selection of a material from which a product can be manufactured is determined by:
- The conditions under which the product is to be used, i.e. the service requirements. These dictate the properties required of a material. For example, if a product is to be subject to forces then it might need strength and toughness and/or if subject to a corrosive environment it might require corrosive resistance.
- The methods proposed for the manufacture of the product. For example, if a material has to be bent as part of its processing then it must be ductile enough to be bent without breaking. A brittle material could not be used.
- The price of the material and its availability and the cost of making the product.
1.2.1 Properties of materials
Materials selection for a product is based upon a consideration of the properties required. These include:
- Mechanical properties: these include such properties as density, and the properties displayed when a force is applied to a material, e.g. yield strength, strength, stiffness, hardness, toughness, fatigue strength (how many times can it be flexed back-and-forth before it breaks?), creep strength (how will it change in length with time when subject to a constant force?).
- Electrical properties: these are the properties displayed when the material is used in electrical circuits or electrical components and include resistivity, conductivity and resistance to electrical breakdown.
- Magnetic properties: these are relevant when the material is used as, for example, a magnet or part of an electrical component such as an inductor which relies on such properties.
- Thermal properties: these are the properties displayed when there is a heat input to a material and include expansivity and heat capacity.
- Optical properties: these include transparency.
- Surface properties: these are, for example, relevant in con siderations of abrasion and wear, corrosion and solvent resistance.
- Aesthetic properties: these include appearance, texture and the feel of a material.
The properties of materials are often markedly changed by the treatments they undergo. Thus, the properties of a material might be changed as a result of working, e.g. if carbon steel is permanently deformed then it will have different mechanical properties to those existing before that deformation, or heat-treatment, e.g. steels can have their properties changed by a heat-treatment such as annealing, in which the steel is heated to some temperature and then slowly cooled, or quenching, which involves heating to some temperature and then immersing the material in cold water.
1.3 The materials
The history of the human race can be divided into periods according to the materials that were predominantly used (though modern times have a multiplicity of materials in widespread use, in olden times there was no such great multiplicity):
- The Stone Age (about 10000 BC-3000 BC). People could only use the materials they found around them such as stone, wood, clay, animal hides, bone, etc. The products they made were limited to what they could fashion out of these materials, thus they had tools made from stone, flint, bone and horn, with weapons - always at the forefront of technology at any time - of wood and flint.
- The Bronze Age (3000 BC-1000 BC). By about 3000 bc, people were able to extract copper from its ore. Copper is a ductile material which can be hammered into shapes, thus enabling a greater variety of items to be fashioned than was possible with stone. The copper ores contained impurities that were not completely removed by the smelting and so copper alloys were produced. It was found that when tin was added to copper, an alloy, bronze, was produced that had an attractive colour, was easy to form and harder than copper alone.
- The Iron Age (1000 BC-1620 AD). About 1000 BC the extraction of iron from its ores signalled another major development. Iron in its pure form was, however, inferior to bronze but by heating items fashioned from iron in charcoal and hammering them, a tougher material, called steel, was produced. Plunging the hot metal into cold water, i.e. quenching, was found to improve the hardness. Then reheating and cooling the metal slowly produced a less hard but tougher and less brittle material, this process now being termed tempering. Thus heat-treatment processes were developed.
- The Cast Iron Age (1620 AD-1850 AD). Large-scale iron production with the first coke-fuelled blast-furnace started in 1709. The use of cast iron for structures and machine parts grew rapidly after 1750, including its use for casting cannon. In 1777, the first cast iron bridge was built over the River Severn near Coalbrookdale. Cast iron established the dominance of metals in engineering. The term Industrial Revolution is used for the period that followed as the pace of developments of materials and machines increased rapidly and resulted in major changes in the industrial environment and the products generally available. During this period, England led the world in the production of iron.
- The Steel Age (1860 AD onwards). Steel was a special-purpose material during the first half of the nineteenth century. However, the year 1860 saw the development of the Bessemer and open hearth processes for the production of steel, and this date may be considered to mark the general use of steel as a constructional material. This development reinforced the dominance of metals in engineering.
- The Light Alloys Age (such alloys only widely used from 1940 onwards). Although aluminium was first produced, in minute quantities, by H. C. Oersted in 1825, it was not until 1886 that it was produced commercially. The high strength aluminium alloy duralumin was developed in 1909, high strength mckel-chromium alloys for high temperature use m 1931, Titanium was first produced commercially in 1948.
- The Plastics Age (1930 onwards). The first manufactured plastic, celluloid, was developed in 1862; in 1906, Bakelite was developed. The period after about 1930 saw a major development of plastics and their use in a wide range of products. In 1933, a Dutch scientist, A. Michels, was carrying out research into the effects of high pressure on chemical reactions when he obtained a surprise result - the chemical reaction between ethylene and benzaldehyde was being studied at a pressure of 2000 times the atmospheric pressure and a temperature of 170°C when a waxy solid was found to form. This is the material we call polyethylene. The commercial production of polyethylene started in England in 1941. The development of polyvinyl chloride was, unlike the accidental discovery of polythene, an investigation where a new material was sought. In 1936, there was no readily available material that could replace natural rubber and since, in the event of a war Britain's natural rubber supply from the Far East would be at risk, a substitute was required. In July 1940, a small amount of PVC was produced with commercial production of PVC starting in 1945.
- The Composites Age (from about 1950s onwards). Though composites are not new, bricks and concrete being very old examples, it is only in the second half of the twentieth century that synthetic composites became widely used. Reinforced plastics are now very widely used and carbon-fibre reinforced composites are, from their initial development in the 1960s, now becoming widely enough used to become known to the general public.
When tools and weapons were limited to those that could be fashioned out of stone, there were severe limitations on what could be achieved with them. The development of metals enabled finer products to be fashioned, e.g. bronze swords which were far superior weapons to stone weapons. The development of cast iron can be considered one of the significant developments which ushered in the Industrial Age. The development of plastics enabled a great range of products to be produced cheaply and in large numbers - what would the world be like today if plastics had not been developed? As a consequence of the evolution of materials over the years, our lifestyles have changed.
1.3.1 Materials classification
Materials can be classified into four mam groups, these being determined by their internal structure and consequential properties.
- Metals: these are based on metallic chemical elements...
