3.4 Limitations of new materials
Quite often a new invention is found to have deficiencies when it is tested in use. For example, when the properties of rubber were examined in more detail, it was found to deform slowly under an applied load, a phenomenon known as Creep. Another drawback to the use of rubber in products was its tendency to melt and become sticky at even moderate temperatures. This is now known to be related not so much to the purely physical attribute of melting, but rather to chemical degradation. The material, in other words, oxidised slowly in air.
Of course this was just another set of problems waiting to be solved. In 1846, it was discovered – almost accidentally, by Goodyear – that rubber could be stabilised by heating with a small amount of sulphur in a process called vulcanisation.
Creep is a phenomenon whereby a material will deform very slowly as a result of a stress. Application of a load, or stress, causes a strain to occur. When creep occurs, the strain continues to increase over time, and will no longer return to zero when the load is removed.
The effect of creep can be observed in lead pipes (used in older buildings) that have gradually sagged over the years. In more modern materials, plastic guttering can also show sagging between supports. Creep is accelerated as the temperature rises, so it is a very important factor in engineering applications where the temperatures are high, and for metals that are being used close to their melting point. For example, materials for jet engine combustors need to be creep-resistant. Alloys used for soldering electronic components on circuit boards have low melting points for ease of manufacture, but they can be susceptible to creep at ordinary room temperature.
As with any fundamental advance in technology, the initial discovery of a new material is followed by a sequence of further discoveries that widen the scope of the original invention or discovery, and any associated patents are known as 'improvement' patents. It is a stepwise sequence, each further step relying on the previous development. What came before a particular invention is known as the 'prior art' – the accepted knowledge base – of this particular area of invention.
Problems in exploitation were also encountered with cast iron: it is brittle in tension, like stone. The design of structures using cast iron must allow for this deficiency, as with stone. That is why the original iron bridge was built as an arch with the imposed loads placing the arches in compression. But one important advantage of cast iron is that it can be made into large beams, rather like wood: it does not have to be used in blocks like stone. So the designer of the Coalbrookdale Iron Bridge made the cast beams to fit together like wooden beams: the ends were shaped so that they could mesh together just like the joints on a wooden structure, and the joints were pinned with dowels (Figure 46).