Professor Gordon died in 1998. In its obituary The Times wrote of him that he was ‘one of the founders of materials science’
Why do things break? Why do materials have any strength at all? Why are some solids stronger than others? Why is steel tough and why is glass brittle? Why does wood split? What do we really mean by ‘strength’ and ‘toughness’ and ‘brittleness’? Are materials as strong as we ought to expect them to be? How far can we improve existing types of materials? Could we make altogether different kinds of materials which would be much stronger? If so, how, and what would they be like? If we really could make better materials then how and where should we make use of them?
Metals, however, do not have a monopoly of strength. Some of the best combinations of lightness and strength are afforded by non-metals and the strongest substances in existence are the recently discovered ‘whisker’ crystals of carbon and of ceramics.
jump backwards and forwards from the ideas of chemistry to those of engineering. In the current phrase materials science is ‘interdisciplinary’.
while we have some idea of ‘how’ materials behave, we have really very little idea of ‘why’
materials receive, transmit and resist their loads
In the past, of course, instinct and experience were the only guides to the choice of materials and to the design of structures and devices.
the smallest particle one can see with the naked eye is about 500,000 atoms across and the smallest particle one can see with the optical microscope is about 2,000 atoms across. With the electron microscope one can see arrays of atoms in crystals, like soldiers on parade, quite easily and with a device called the field emission microscope one can see individual atoms
the wind, blowing where it listeth, pushes on my chimney pots but the chimney pots, bless them, push back at the wind just as hard, and that is why they don’t fall off. All this is merely a restatement of Newton’s third law of motion which says, roughly speaking, that if the status quo is to be maintained then all the forces on an object must cancel each other out.
anthropomorphic
If I stretch out my hand and you put a weight on it such as a pint of beer, then I have to increase the tensions in certain muscles so as to sustain the load.
The atoms in a solid are held together by chemical forces or bonds (see Appendix 1) which may perhaps be thought of as electrical springs since there is nothing ‘solid’ in any crude sense to make any other kind of spring.
The only thing, therefore, which can ‘give’ is the interatomic bond. These bonds or springs vary a good deal in stiffness or springiness (or, as the layman might put it, in ‘strength’)
The standard way of measuring the distance between the atoms in a crystal is to study the way in which an X-ray beam is deflected when it passes through the crystal. This method has been used now for sixty years or more and it is nowadays capable of considerable accuracy. It is found that the atoms in a metal, for instance, move apart or together exactly in proportion to the amount by which the metal as a whole is stretched or compressed.
apt
keel
Stress is simply load per unit area. (pounds per square inch)
stress is proportional to strain and vice versa. So, if an elastic body such as a wire is stretched one inch under a load of 100 pounds it will stretch two inches under 200 pounds and so on, pro rata. This is known as Hooke’s law and is regarded as one of the pillars of engineering.
for small strains the whole process of extension and recovery is reversible and can usually be repeated many thousands or millions of times with identical results; the hairspring of a watch which is coiled and uncoiled 18,000 times each hour is a familiar example. This type of behaviour by solids under loading is called ‘elastic’ and is widespread. Elastic behaviour, which is shown by the majority of engineering materials, contrasts with ‘plastic’ behaviour, shown to the extreme by putty and Plasticine, where the material does not obey Hooke’s law
The Young’s modulus of steel, for example, is about 30,000,000 pounds per square inch.
Steel is about the stiffestreasonably cheap material, which is one of the reasons why it isused so widely.
if the material is at allsoft or ductile like mild steel or copper, it will simply squish outsideways, like Plasticine. If the material is brittle, like stone or glass,it will explode sideways (and very dangerous it can be) into dust andsplinters.
architrave or lintel
Roman arches, such as the aqueducts. The wedgeshaped pieces which make up the arch-ring are called ‘voussoirs’.
auxiliary
modern railway station which is roofed with steel trusses